Francis Le-Gouevec

Julien Georges

Alain Chauvin

Guillaume Poupeau

Mickaël Genissel

Fabien Guerin

Amandine Cauchi

Julien Fortin

Deux

Jean-François Rouaud

Simon Duteil

Bruno Duteil

Josselin Delamarre

Patrice Roger

Ludovic Brossard

Eric B Pour

Merci Aussi

Angélique C Pour

Les Pauses-Café Je Voudrais Remercier Aussi

Francoise T Special Thanks To Demi

A M Serviento

M Castex

D Renaudeau

E Labussière
email: etienne.labussiere@inrae.fr

Effect of live yeast supplementation and feeding frequency in

Keywords: heat stress, pigs, energy metabolism, thermoregulation feeding behaviour, live yeast heat stress, energy metabolism, live yeast, meal frequency, pig, thermoregulation Results 87 heat stress, pigs, live yeast, sex, growth Annex Prenatal heat stress, pig, growth, carcass composition

Lucile Montagne, Sophie Tesseraud, and Sophie Blat d'avoir accepté d'évaluer mon travail et de faire partie de mon jury. Un grand merci à mon comité de thèse Olivier Zemb, Anne Collin, et Annick Bernalier. Merci d'avoir pris le temps de suivre mon travail et merci pour vos précieux conseils

1, sept mois pour l'essai 2, huit mois pour l'essai 3, et encore deux mois pour l'essai 4 ! 28-34°C n'est certainement pas l'environnement de travail idéal à gérer au quotidien, en particulier les jours où on a dû rester dans des salles chaudes pendant de nombreuses heures. Merci pour le suivi quotidien des expérimentations ! Francis, merci pour ta gentillesse et pour tes nombreuses histoires qui ont fait les longues journées des essais 1 et 2 plus faciles et plus agréables. Julien, Alain, et Francis, merci en particulier pour les implantations de pilules, les poses de cathéters, les mesures des températures chez les porcs, et pour beaucoup d'autres choses que vous avez faites pour les essais dans le bâtiment 32. Guillaume, merci pour l'aide dans l'essai 3 (suivi, broyage, lyophilisation, collecte, etc.) et pour suivre fidèlement le planning sur la chaîne de papier collée sur la porte que je mettais à jour semaine après semaine. À l'équipe : Ce n'était pas toujours facile avec la rigueur et les longues périodes des essais, notamment avec le nombre de mesures dans les protocoles donc merci à vous tous pour votre énorme contribution à ce projet ! Je voudrais aussi dire merci à Herve Demay et à Josselin Delamarre qui étaient toujours présents dans tous les essais en partageant leur expertise dans tous les prélèvements de sang que ce soit en conditions thermoneutres ou chaudes, et, avec Renan Deleunay et Georges Guillemos, pour les suivis des essais en garde et/ou les mutations.

Merci aussi à l'équipe abattage (Jérôme Liger, Maurice Alix, et Jean-François Rouaud) qui ont participé dans les deux premiers essais particulièrement pour les longues journées d'abattage avec les milliers de prélèvements prévus. Je ne peux pas parler de prélèvements (et des analyses subséquents) sans dire merci aux gens de l'unité PEGASE dans l'équipe Alinut (Cécile Perrier, Colette Mustière, Agathe Costentin, Xenia Arondel-Piekarz, Philippe Ganier, Gwennola Robin, Gwenola Le-Roy, Maryline Lemarchand) à qui nous demandons souvent de l'aide (parfois de manière très urgente) dans tous les essais. Je voudrais remercier aussi Jean-Yves Dourmad pour les conseils et les idées surtout quand j'étais déjà en train d'écrire ma discussion.

L'étude sur le stress thermique prénatal (essai 1) a été un grand projet et je n'ai joué qu'un petit rôle. Je voudrais remercier tous ceux qui en faisaient partie, en particulier ceux avec qui je travaillais directement : l'équipe maternité et post-sevrage (Daniel Boutin, Yannick Surel, Fabien Guerin, Patrick Touanel) pour le suivi des expérimentations avec les truies et les porcelets, et Agathe Costentin et Christine Trefeu pour les analyses de laboratoire. Merci aussi à Benedicte Lebret pour son aide et ses idées en tant que l'un de nos co-auteurs dans l'un des articles de la thèse et à Elodie Merlot, Armelle Prunier, Helene Quesnel, Isabelle Louveau pour la mise en place de cette étude. Je voudrais aussi dire merci à Olivier Zemb et à Mathilde Le Table 1. Examples of factors that affect energy partitioning of growing pigs such as previous feeding level, sex, diet composition, and high ambient temperature. 1 3. Summary of gut microbiota alterations induced by heat stress in monogastric animals. ....... Table 4. Potential effects of in utero heat stress on the swine industry. ............................................... Table 5. Examples of nutritional and management strategies to mitigate heat stress in pigs............... Table 6. Summary of the experiments included in this thesis project 1 ................................................. Table 7. Basal composition of the experimental growing and finishing diets. 1,2 ................................. Table 8. Summary of the measurements taken during the experiments in this thesis project 1 ............. Table 9. Plasma hormones and metabolites analyzed in Experiments 1,2, and3

Table of contents

List of tables

List of figures

Figure 1. Simplified representation of where heat is generated in animal cells: ATP production and ATP utilization. ATP is the chemical link between catabolism and anabolism and is the energy currency of the living cell. (Adapted from [START_REF] Lehninger | Lehninger Principles of biochemistry[END_REF] Example of heat production partitioning between components due to the basal metabolic rate (fasting heat production; FHP), the short-and long-term thermic effect of feeding (TEFst and TEFlt), and the physical activity (AHP). MHP (or RHP; minimal or resting HP) is the sum of the FHP and the TEFlt. (Adapted from Labussière et al., 2013) Model of thermoregulation as a negative feedback system for maintaining a homeothermic body temperature. If there is a deviation sensed in the body temperature, the system makes alterations to restore it toward its set point value. BAT: brown adipose tissue. (Adapted from [START_REF] Sawka | Physiological responses to acute exercise-heat stress[END_REF][START_REF] Weller | Body temperature and its regulation[END_REF][START_REF] Modell | A physiologist's view of homeostasis[END_REF][START_REF] Fossion | A physicist's view of homeostasis: how time series of continuous monitoring reflect the function of physiological variables in regulatory mechanisms[END_REF][START_REF] Tan | Regulation of body temperature by the nervous system[END_REF]. ................ 25 Figure 7. General relationship of the core body temperature, heat production, and heat losses according to the ambient temperature in growing pigs. LCT: Lower critical temperature, ECT: evaporative critical temperature, UCT: upper critical temperature. Adapted from [START_REF] Mount | Adaptation to thermal environment: man and his productive animals[END_REF], [START_REF] Renaudeau | Atténuer les effets de la chaleur sur les performances des porcs : la voie génétique[END_REF], Renaudeau et al. (2012), and [START_REF] Gourdine | The genetics of thermoregulation in pigs: A review[END_REF] . Body (intestinal) temperature of pigs with a high (400 g/meal) or low (320 g/meal) feed intake recorded at 5-min intervals. Pigs received their meals (0600, 1400, and 2200 h) and were exposed to a natural ambient temperature from around 29 to 36°C. (Adapted from Cervantes et al., 2018). ........... 30 Figure 10. Postprandial plasma kinetics of insulin and glucose exposed to different ambient temperatures after 17 d at 24°C or 30°C (Adapted from [START_REF] Campos | Postprandial insulin and nutrient concentrations in lipopolysaccharide-challenged growing pigs reared in thermoneutral and high ambient temperatures[END_REF] Effect of temperature and ME intake on protein and lipid deposition in 24 to 65 kg live weight barrows. Predicted data for protein deposition (PD, g/day) at 23°C (-), at 30°C (-) and lipid deposition (LD, g/day) at 23°C (----), at 30°C (----). (Adapted from Le Bellego et al., 2002b). ..... 34 (21.9°C) and cyclical HS (31.8°C average) HS environments for three pig lines subjected thrice to a 4-day heat exposure (HS1, HS2, and HS3). HS1 was preceded by a 9-day TN period and HS2 and HS3 by a 7-day TN period. (Adapted from [START_REF] Rauw | Effects of diet and genetics on growth performance of pigs in response to repeated exposure to heat stress[END_REF] (T at 22°C vs. H at 32°C) in the first (P1), second (P2), and third (P3) experimental periods on internal body temperature (Tcore, °C) profiles throughout the experiment. The challenge, which was repeated in P1, P2, and P3, consisted of a 3-d prechallenge period (22°C), a 5-d HS challenge (32°C), and a 7-d recovery period (22°C) (Adapted from [START_REF] Renaudeau | Impact of single or repeated short-term heat challenges mimicking summer heat waves on thermoregulatory responses and performances in finishing pigs[END_REF].......... 37 Figure 17. Relationship between ambient temperature and respiration rate (a), rectal temperature (b), feed intake (c), and water to feed ratio (d) according different relative humidities (50% RH, 65% RH, and 80% RH) (Adapted from Huynh et al., 2005)

List of tables and figures: Results section Chapter 1

Tables Table 1. Effect of the prenatal and postnatal (growing) climatic environment on the growth performance of growing pigs………………………………………………………………………………………...61 Table 2. Effect of the prenatal and postnatal (growing) climatic environments on the carcass and organ traits of growing pigs……………………………………………..……………………………………63 Figures Figure 1. Framework of the experimental study from gestation to growing period. …………………57 Figure 2. Actual average hourly ambient temperature (mean ± SE) of the gestation rooms (a) from day 9 to 109 of gestation and of the growing rooms (b) from 83 to 140 d of age during the growth period.58 Figure 3. Effect of the climatic environment on the skin (a) and rectal (b) temperatures of mixed parity sows from gestation to lactation (LSmeans ± SEM)……...……………………………………………60 Figure 4. Effect of climatic environment (TN or HS) during prenatal development (PE; PTN vs. PHS) and during growing (GE; GTN vs. GHS) on the skin (a) and rectal (b) temperatures of growing pigs.61 Figure 5. Effect of climatic environment (TN or HS) during prenatal development (PE; PTN vs. PHS) and during growing (GE; GTN vs. GHS) on ADFI per metabolic BW (a) and ADG (b) in growing pigs using TN performance as covariate (LSMeans ± SEM). ………………………………………………62 Figure 6. Effect of climatic environment (TN or HS) during prenatal development (PE; PTN vs. PHS) and during growing (GE; GTN vs. GHS) on plasma parameters of growing pigs (LSMeans ± SEM).64

Chapter 2

Tables Table 1. Composition of the experimental diet……………………………………………………..….72 Table 2. Effect of HS and feeding management on growth performance and feeding behavior of finishing pigs…………………………………………………………………………..………………74 Table 3. Effect of HS and feeding management on slaughter traits of finishing pigs…………………75 Table 4. Effect of HS and feeding management on the blood gas parameters of finishing pigs……….78

Figures Figure 1.
Graphical illustration of how the area under the curve was calculated for a specific meal using the Tmuscle (°C) of the PF8 pigs and the time passed (min) after each meal provision………………74 Figure 2. Effect of the climatic environment and feeding management on acute response of finishing pigs in terms of Tskin (a), respiration rate (b), Trectal (c), and Tmuscle (d) (means ± SE)………………….76 Figure 3. Effect of the climatic environment and feeding management on the Tskin (a), respiration rate (b), Trectal (c), and Tmuscle (d) of finishing pigs (means ± SE). …………..………………………..…77 Figure 4. Effect of the climatic environment and feeding management on plasma metabolites [creatinine (a), NEFA (b), glucose (c)] and plasma hormones [insulin (d), T3 (e), T4 (f), T3:T4 (g), and insulin:glucose (h)] of finishing pigs: (means ± SE).…………..…………………………….……..…77 Figure 5. Effect of the environmental treatment and feeding management on the nycthemeral pattern of Tmuscle (°C) and presence at the feeder (min) of finishing pigs…………………………….…………79 Figure 6. Effect of the environmental treatment and order of meal in terms of Tmuscle [baseline (a) and maximum (b)], total presence at the feeder (c), and area under the Tmuscle curve (calculated based on Figure 1) of PF8 finishing pigs………………………………..…………………………….…………80

Chapter 3

Tables Table 1. Composition of the experimental diets…………………………………………….………...89 Table 2. Effect of live yeast supplementation and feeding window on feed and water intake, growth, and feeding behavior of male finishing pigs exposed to high ambient temperature…………………………………………………………………….……………………... 95 Table 3. Effect of feed access and live yeast supplementation on energy and nitrogen metabolism of male finishing pigs exposed to high ambient temperature. …………………………………….……..96 Table 4. Effect of feed access and live yeast supplementation on thermoregulation responses of male finishing pigs exposed to high ambient temperature………………………………………………….. 97 ) vs. 100 (Y) g/ton LY supplementation] and feeding window (Unli vs. 2FW vs. 8FW) on the postprandial plasma concentrations of (a) insulin, (b) glucose, (c) lactate, and (d) α-amino nitrogen of heat-stressed entire male finishing pigs (means ± SE)…………..………………100 Figure 5. Effect of diet [0 (N) vs. 100 (Y) g/ton LY supplementation] and feeding window (Unli vs. 2FW vs. 8FW) on the postprandial plasma concentrations of (a) urea, (b) creatinine, and (c) NEFA of heat-stressed entire male finishing pigs (means ± SE)…………………………………….....……….101 Figure 6. Effect of diet [0 (N) vs. 100 (Y) g/ton LY supplementation] and feeding window (Unli vs. 2FW vs. 8FW) on the plasma concentrations of (a) T3 and (b) T4 of heat-stressed entire male finishing pigs (means ± SE). ……………………………………………………………………………….......102

Figures

Chapter 4

Table Table 1. Effect of live yeast supplementation and sex on growth and slaughter performance of finishing pigs exposed to high ambient temperature……………………………….....……….…………….....115

List of abbreviations

ADFI: average daily feed intake ADG: average daily gain AHP: activity heat production AL: ad libitum ATP: adenosine triphosphate BMR: basal metabolic rate BW: body weight ECT: evaporative critical temperature EM: entire male F: female FCR: feed conversion ratio FHP: fasting heat production FW: feeding window HL: heat loss HP: heat production HPA axis: hypothalamic-pituitary-adrenal axis HS: heat stress / heat stressed LCT: lower critical temperature LD: lipid deposition LTHA: long-term heat acclimation LY: live yeast ME: metabolizable energy MHP: minimal heat production NEFA: non esterified fatty acids PCA: principal component analysis PD: protein deposition PDmax: maximum protein deposition PF : pair-fed PF2: pair-fed with 2 feeding provisions PF8: pair-fed with 8 feeding provisions RFI: residual feed intake RH: relative humidity RHP: resting heat production RMR: resting metabolic rate STHA: short-term heat acclimation TEF: thermic effect of feeding TN: thermoneutral TNZ: thermoneutral zone UCT: upper critical temperature List of journal publications 1. Serviento, A.M., B. Lebret, and D. Renaudeau. 2020. Chronic prenatal heat stress alters growth, carcass composition, and physiological response of growing pigs subjected to postnatal heat stress. Journal of Animal Science. doi:10.1093/jas/skaa161. 2. Serviento, A. M., E. [START_REF] Serviento | Effect of heat stress and of feed provision modification on growth and thermoregulation in pigs[END_REF]. Effect of heat stress and feeding management on growth performance and physiological responses of finishing pigs.

Journal of Animal Science. doi: 10.1093/jas/skaa387. 3. Serviento, A. M., M. Castex, D. Renaudeau, and E. [START_REF] Serviento | O164 Effect of live yeast supplementation and feeding frequency in heat stressed pigs[END_REF]. Effect of live yeast supplementation and feeding frequency in male finishing pigs subjected to heat stress. British Journal of Nutrition. doi:10.1017/S0007114522002513.

General Introduction

General introduction

Climate change remains to be one of the major challenges of the 21 st century especially with more frequent heat waves and with global temperature expected to increase by at least 1.5°C between 20301.5°C between and 20501.5°C between (IPCC, 2018)). This poses a threat to livestock production in many countries due to its consequences on animal health and welfare and on the quantity and quality of resources such as crops used for animal feeding [START_REF] Rojas-Downing | Climate change and livestock: Impacts, adaptation, and mitigation[END_REF][START_REF] Lacetera | Impact of climate change on animal health and welfare[END_REF]Renaudeau and Dourmad, 2021). This is especially true for pig production due to its increasing demand and growth in subtropical and tropical counties (OECD-FAO, 2021).

Like most livestock animals, pigs are homeothermic animals that regulate their core body temperature within a narrow range by maintaining a balance between heat production and heat loss. Pigs are particularly susceptible to heat stress not only because of their inability to sweat [START_REF] Ingram | Stimulation of cutaneous glands in the pig[END_REF] but also because selection for higher feed intake and faster lean growth resulted to higher metabolic heat production in modern pigs [START_REF] Brown-Brandl | Heat and moisture production of modern swine[END_REF]. In hot conditions, maintenance of body temperature through physiological, and/or behavioral adjustments usually reduce economically important traits related to reproduction and production (DeShazer et al., 2009;Renaudeau et al., 2012). This makes heat stress one of the main limiting factor in pig production especially with associated economic losses related to reduced growth, feed efficiency, and carcass quality in growing pigs and reduced reproduction performance in sows, among others [START_REF] Mayorga | Heat stress adaptations in pigs[END_REF][START_REF] Liu | Review: What have we learned about the effects of heat stress on the pig industry?[END_REF]. While the most widely reported effect of heat stress in growing pigs is the reduction in feed intake that translates to reduced growth (Renaudeau et al., 2011), studies in the past decade have demonstrated that there are still other consequences to be considered such as direct effects of heat exposure and even prenatal thermal environment of the pigs [START_REF] Ross | Physiological mechanisms through which heat stress compromises reproduction in pigs[END_REF][START_REF] Johnson | Physiology symposium: Postnatal consequences of in utero heat stress in pigs[END_REF][START_REF] Liu | Review: What have we learned about the effects of heat stress on the pig industry?[END_REF]. Indeed, in order to develop nutritional and management strategies to mitigate its stress effects, mechanisms by which pigs maintain heat balance and by which heat stress deteriorates animal performance must be further understood.

Therefore, the general objective of this thesis project is to contribute to the growing body of knowledge on heat stress in pigs. Thus, the thesis aims to determine how a prenatal to a postnatal heat challenge and to distinguish direct effects of heat exposure (i.e., those that are unrelated to both feed intake and behavior).

Moreover, the project also aims to understand how pigs maintain heat balance during the day especially that which related to the feeding behavior. Finally, feeding strategies (i.e., feeding frequency modification and live yeast supplementation) are evaluated that aims to mitigate heat stress effects in pigs. This thesis dissertation is divided into six sections. After this general introduction, the literature review presents how the pigs balances heat production and loss to maintain body temperature, and how failing to maintain this heat balance can compromise pig performance.

The main objectives of the thesis are then presented in the conclusion of the literature review.

Afterwards, the general experimental strategies section discusses the experimental methods employed in the thesis project and the rationale behind them. The results are then presented in four chapters according to the four experiments conducted in the context of the thesis. The general discussion then takes into account the highlights of the thesis and considers the possible implications of these findings in future research of heat stress in pigs. Conclusions and perspectives are then presented in the final part of the manuscript.

Literature Review

Figure 1. Simplified representation of where heat is generated in animal cells: ATP production and ATP utilization. ATP is the chemical link between catabolism and anabolism and is the energy currency of the living cell. (Adapted from [START_REF] Lehninger | Lehninger Principles of biochemistry[END_REF] ATP production in the three steps of cellular respiration (in yellow). 1) Oxidation of fatty acids, glucose, and some amino acids yields acetyl-CoA. 2) Oxidation of acetyl groups in the Krebs cycle includes four steps in which H+ ions are abstracted. Stage 3: H+ ions carried by NADH and FADH2 are funneled into a chain of mitochondrial electron carriers-the respiratory chain-ultimately reducing O2 to H2O. This electron flow drives the production of ATP. For simplicity's sake, H+ ions shown are only those involved in the cellular respiration although these are also generated from other reactions. ATP utilization or the exergonic conversion of ATP to ADP and Pi, or to AMP and PPi, is coupled to many endergonic reactions and processes.

Literature Review

The pig and its environment As all living organisms, animals follow the law of thermodynamics and are in constant energy exchange with their environment [START_REF] Wiegert | Thermodynamic considerations in animal nutrition[END_REF][START_REF] Ingram | Heat exchange between animal and environment BTman and animals in hot environments[END_REF]. This erent factors affecting it) and exchanges with the environment (with particular focus on heat loss pathways), and how the pig balances them in order to maintain a thermal equilibrium and a core body temperature in a stable range.

Heat production

In order to grow and reproduce, maintain their structures, and respond to their environment, living organisms need energy provided mostly in the form of adenosine triphosphate (ATP; 30.5 kJ/mole) but also in reduced electron carriers 1 . In animals, this energy comes from catabolism of different substrates [START_REF] Lehninger | Lehninger Principles of biochemistry[END_REF], e.g. oxidation of nutrients such as carbohydrates, lipids, or proteins. ATP is mostly generated from cellular respiration processes occurring in the mitochondria in the presence of oxygen particularly from oxidative phosphorylation in the electronic transport chain (Figure 1). However, this process is only ~40% efficient [START_REF] Nath | The thermodynamic efficiency of ATP synthesis in oxidative phosphorylation[END_REF] and the rest of the energy is thus released as heat. A certain amount of heat is therefore generated as a by-product from the inefficiency of energy transformation from oxidation of consumed nutrients to the cellular energy needed by the animals [START_REF] Lehninger | Lehninger Principles of biochemistry[END_REF]. Catabolism of different substrates for ATP production can also have different thermogenic costs which is particularly higher when protein is used as energy source compared to carbohydrates and lipid. This is not only because of the additional ATP required for amino acid deamination but also for the subsequent urea synthesis and excretion [START_REF] Bender | The metabolism of "surplus" amino acids[END_REF][START_REF] Van Milgen | The role of energy, serine, glycine, and 1-carbon units in the cost of nitrogen excretion in mammals and birds[END_REF].

Aside from ATP production, heat is also produced when ATP is hydrolyzed to provide energy for vital functions such as muscle contraction [START_REF] Barclay | Energetics of contraction[END_REF]. Deposition of nutrients needed for production and reproduction purposes also requires energy. For example, lipid deposition requires the expenditure of one mole of ATP and two moles of NADPH from the pentose phosphate pathway2 for the addition of two-carbon units in the hydrocarbon chain for fatty acid synthesis [START_REF] Cooper | The cell: a molecular approach[END_REF]. Meanwhile, protein synthesis is a much more energetically costly process as it can take at least 5 moles of ATP only for the incorporation of one amino acid into protein [START_REF] Stouthamer | A theoretical study on the amount of ATP required for synthesis of microbial cell material[END_REF], not even taking account the cost of protein turnover which is the continuous renewal and replacement of protein in the body [START_REF] Schutz | Protein turnover, ureagenesis and gluconeogenesis[END_REF]. Pigs like most mammals have evolved to be homeothermic animals, characterized by their ability to regulate their core body temperature in a narrow range (which is generally higher than the environment, i.e. from 37-39°C in swine depending on the physiological stage) through complex neural controls and mechanisms [START_REF] Bligh | Mammalian homeothermy: an integrative thesis[END_REF][START_REF] Morrison | Central mechanisms for thermoregulation[END_REF]. However, maintaining a constant and elevated internal temperature is energetically expensive and requires large amounts of heat [START_REF] Bennet | Structural and functional determinates of metabolic rate[END_REF][START_REF] Levesque | Increased homeothermy during reproduction in a basal placental mammal[END_REF].

From the methodological point of view, one method for assessing energy expenditure is the direct calorimetry technique [START_REF] Lavoisier | Memoir on Heat Read to the Royal Academy of Sciences[END_REF]. This method is considered the gold standard for heat production (HP) measurement as it allows direct quantification of heat produced by the organism (whether coming from aerobic and anaerobic metabolism) via the temperature gradient between the body and the environment [START_REF] Blaxter | Techniques in energy metabolism studies and their limitations[END_REF][START_REF] Kaiyala | Direct animal calorimetry, the underused gold standard for quantifying the fire of life[END_REF][START_REF] Kenny | Direct calorimetry: A brief historical review of its use in the study of human metabolism and thermoregulation[END_REF]. However, direct calorimetry method requires expensive facilities and does not give an idea about the substrate being oxidized [START_REF] Johnson | Nutrition in the prevention and treatment of disease[END_REF]. In livestock animals, rodents, and humans, a well-known alternative technique for HP measurement is the indirect calorimetry method which allows for continuous HP calculation via gas exchanges of animals placed in respiration chambers (O2 consumption, and CO2 and CH4 production; Text box 1). Water evaporation at the level of the lungs and skin can also be considered in these methods which is important since the change of water from liquid to gaseous state consumes about 44.0 kJ/mole or 2.4 kJ/g of water [START_REF] Lehninger | Lehninger Principles of biochemistry[END_REF]. This is in contrast to the comparative slaughter technique that can only measure retained energy in the body over a rather long duration [START_REF] Blaxter | Techniques in energy metabolism studies and their limitations[END_REF]. The indirect calorimetry technique has some limitations such as the assumption that O2 and CO2 pools in the body does not change although some intermediates might be temporarily stored in the body [START_REF] Burnett | Direct calorimetry identifies deficiencies in respirometry for the determination of resting metabolic rate in C57Bl/6 and FVB mice[END_REF][START_REF] Gerrits | Deriving heat production from gaseous exchange: Validity of the approach[END_REF]. Nevertheless, this method is widely used and has been Text box 2. Examples of measurements in the respiratory chamber.

i. Principle of the partitioning of total heat production from kinetics of O2 consumption and CO2 production as described by van Milgen et al. (1997) with modifications described by Quemeneur et al. (2020). Example shown for the force signals and smooth parameters using spline comes from the data of a pig with 2 feeding windows.

ii. Principle of the calculation of evaporative water loss described by Renaudeau et al. (2013) and of latent heat as the required amount of heat to evaporate 1 g of liquid water based on the equation of [START_REF] Curtis | Environmental management in animal agriculture[END_REF].

reported to be a close match with other techniques [START_REF] Burnett | Direct calorimetry identifies deficiencies in respirometry for the determination of resting metabolic rate in C57Bl/6 and FVB mice[END_REF][START_REF] Gerrits | Deriving heat production from gaseous exchange: Validity of the approach[END_REF].

Indirect calorimetry studies in growing pigs has demonstrated that HP is not constant during the day [START_REF] Verstegen | Diurnal variation in the thermal demand of growing pigs[END_REF]van Milgen et al., 1997;[START_REF] Van Milgen | Energetic efficiency of starch, protein and lipid utilization in growing pigs[END_REF][START_REF] Schrama | Dietary betaine supplementation affects energy metabolism of pigs[END_REF][START_REF] Bolhuis | Effects of fermentable starch and straw-enriched housing on energy partitioning of growing pigs[END_REF]Labussière et al., 2013;[START_REF] Brown-Brandl | Heat and moisture production of modern swine[END_REF][START_REF] Pedersen | Influence of diurnal variation in animal activity and digestion on animal heat production[END_REF][START_REF] Van Erp | Circadian misalignment imposed by nocturnal feeding tends to increase fat deposition in pigs[END_REF]and Quemeneur et al., 2020 among others). The amount of heat produced differs according to the metabolic processes involved and mathematical modelling methods have thus been used to understand the dynamic changes in HP and its different components (van Milgen et al., 1997;[START_REF] Van Klinken | Estimation of activity related energy expenditure and resting metabolic rate in freely moving mice from indirect calorimetry data[END_REF][START_REF] Van Klinken | Practical aspects of estimating energy components in rodents[END_REF]Labussière et al., 2013;Quemeneur et al., 2020). Text box 2 shows the principle of total HP partitioning from the kinetics of O2 consumption and CO2 production. Regardless of the mathematical method used, total HP is generally partitioned in the aforementioned studies into different components (example shown in Figure 2): fasting HP (FHP), thermic effect of feeding (TEF), and activity related HP (AHP).

Fasting HP can be used to evaluate an BMR) and estimate the corresponding energy requirements for maintenance of vital body processes.

Based on the concept of BMR [START_REF] Bois | The basal energy requirement of man[END_REF][START_REF] Lusk | On the constancy of the basal metabolism[END_REF], it is assumed to be the minimum energy requirement for vital organs such as the heart, lungs, liver, kidneys, intestine, nervous system, sex organs, muscles, and skin and is measured in adult, inactive, and non-productive individual who is in a post-absorptive fasting state in a thermoneutral (TN) environment [START_REF] Mcnab | On the Utility of Uniformity in the Definition of Basal Rate of Metabolism[END_REF][START_REF] Kumagai | Basal metabolic rate BT -Encyclopedia of behavioral medicine[END_REF]. The FHP is generally expressed according to metabolic body weight (BW) with an exponent that varies according to the age, i.e. BW 0.75 for sows and BW 0.60 for growing pigs [START_REF] Kleiber | The fire of life. An introduction to animal energetics. fire life[END_REF][START_REF] Brown | The metabolic body size of the growing pig[END_REF][START_REF] Noblet | Metabolic utilization of energy and maintenance requirements in growing pigs: effects of sex and genotype[END_REF]. As previously mentioned, BMR is generally measured in adults and can be difficult to estimate in growing pigs as they can still gain weight and deposit protein even with no positive energy retention as a result of fat mobilization [START_REF] Le Dividich | Effects of environmental temperature on heat production, energy retention, protein and fat gain in early weaned piglets[END_REF].

Nevertheless, the concept of maintenance requirements has been accepted even in growing animals to estimate energy needs wherein there is zero net energy gained or lost and no change in body pools [START_REF] Van Milgen | Partitioning of energy intake to heat, protein, and fat in growing pigs[END_REF].

The other two components (AHP and TEF) are collectively referred to as the heat increment in pig nutrition [START_REF] Noblet | Utilisation of metabolisable energy of feeds in pigs and poultry: interest of net energy systems[END_REF][START_REF] Labussière | Modelling gas exchanges to partition heat production between fasting, thermic effect of feeding and physical activity in farm animals[END_REF]. Heat from physical activity (AHP) comes from the discussed ATP hydrolysis during muscle contraction (Loiselle with different parameters for O2 and CO2 (Labussière et al., 2013). The last component of total HP is the TEF which is also referred to as dietary induced thermogenesis and is commonly split into components, i.e. the short-term and the long-term TEF [START_REF] Rothwell | Regulation of energy balance[END_REF][START_REF] Van Milgen | Partitioning of energy intake to heat, protein, and fat in growing pigs[END_REF][START_REF] Armbruszt | The short-and long-term effects of food intake on thermogenesis[END_REF][START_REF] Calcagno | The thermic effect of food: a review[END_REF]. In modelling energy partitioning, short-term TEF is assumed to represent the dynamic variations in HP after meal ingestion that are related to the act of ingestion, the digestion and absorption of food, and a part of intermediary metabolism [START_REF] Labussière | Modelling gas exchanges to partition heat production between fasting, thermic effect of feeding and physical activity in farm animals[END_REF]. Meanwhile, long-term TEF is generally associated with nutrient deposition and hindgut fermentation [START_REF] Van Milgen | Partitioning of energy intake to heat, protein, and fat in growing pigs[END_REF][START_REF] Labussière | Modelling gas exchanges to partition heat production between fasting, thermic effect of feeding and physical activity in farm animals[END_REF]. While BMR is generally estimated after one day of fasting (FHP), there is also the so-called resting metabolism rate (RMR) that is similar to the BMR except that it is measured in non-fasted individuals that is not in post-absorptive state.

The RMR is thus the BMR plus the heat increment from a previous feeding activity [START_REF] Brody | Bioenergetics and growth[END_REF]. The resting (RHP) or minimal HP (MHP) can be considered as the RMR of the pigs measured during a fed state and is therefore calculated as the sum of the FHP and the long-term TEF [START_REF] Labussière | Modelling gas exchanges to partition heat production between fasting, thermic effect of feeding and physical activity in farm animals[END_REF]. In estimating energy partitioning, the basal metabolism is generally assumed to be stable and the observed variations in HP during the day are assumed to be associated to the instantaneous peaks of AHP and the short-term TEF (van Milgen et al., 1997;Labussière et al., 2013;Quemeneur et al., 2020).

While FHP and MHP are usually reported as singular values, it must be noted that they only serve as estimates especially since BMR and RMR varies throughout the day and has been reported to follow a circadian rhythm in pigs and in other mammalian species [START_REF] Benedict | The role of the gut barrier function in health and disease[END_REF][START_REF] Ingram | Circadian rhythms in the pig[END_REF][START_REF] Nagashima | Effects of fasting on thermoregulatory processes and the daily oscillations in rats[END_REF][START_REF] Mortola | Correlations between the circadian patterns of body temperature, metabolism and breathing in rats[END_REF][START_REF] Bailey | Circadian regulation of metabolism[END_REF][START_REF] Mchill | Role of sleep and circadian disruption on energy expenditure and in metabolic predisposition to human obesity and metabolic disease[END_REF]. Indeed, RMR is reported to be higher in the morning than at night which is associated to the endogenous circadian effect on energy expenditure [START_REF] Romon | Circadian variation of diet-induced thermogenesis[END_REF][START_REF] Bo | Is the timing of caloric intake associated with variation in diet-induced thermogenesis and in the metabolic pattern? A randomized cross-over study[END_REF][START_REF] Morris | The human circadian system has a dominating role in causing the morning/evening difference in diet-induced thermogenesis[END_REF][START_REF] Richter | Twice as high diet-induced thermogenesis after breakfast vs dinner on high-calorie as well as low-calorie meals[END_REF][START_REF] Ruddick-Collins | Circadian rhythms in resting metabolic rate account for apparent daily rhythms in the thermic effect of food[END_REF]. Thermal demand was also found to be lower in pigs at night than during the day [START_REF] Verstegen | Diurnal variation in the thermal demand of growing pigs[END_REF] suggesting a diurnal difference in metabolic rate. Nevertheless, mathematical modeling of the dynamic variations in HP helps understand the factors that contribute and that affect the total HP, examples of which are shown in Table 1. In pigs, FHP generally represents 50 to 65% of the total HP depending on different factors. For example, FHP can increase with a higher previous feeding level and plane of nutrition [START_REF] Koong | Effects of plane of nutrition on organ size and fasting heat production in genetically obese and lean pigs[END_REF][START_REF] De Lange | Previous feeding level influences plateau heat production following a 24 h fast in growing pigs[END_REF] due to the weight of metabolically active organs [START_REF] Koong | Effects of plane of nutrition on organ size and fasting heat production in genetically obese and lean pigs[END_REF][START_REF] Koong | Assessment of interrelationships among levels of intake and production, organ size and fasting heat production in growing animals[END_REF][START_REF] Pekas | Principal gastrointestinal variables associated with metabolic heat production in pigs: statistical cluster analyses[END_REF] as these are vital organs needed to sustain life. A higher FHP in entire males compared to surgically castrated males (Labussière et al., 2013) has been associated, among other factors, to a higher visceral weight [START_REF] Quiniou | Prediction of tissular body composition from protein and lipid deposition in growing pigs[END_REF] as visceral organs largely contribute to FHP [START_REF] Van Milgen | Major determinants of fasting heat production and energetic cost of activity in growing pigs of different body weight and breed/castration combination[END_REF] and can constitute up to 40-50% of animal energy expenditure [START_REF] Ferrell | Contribution of visceral organs to animal energy expenditures[END_REF]. Leaner pigs can have higher FHP due to the contribution of muscle energy expenditure [START_REF] Tess | The effects of body composition on fasting heat production in pigs[END_REF][START_REF] Van Milgen | Major determinants of fasting heat production and energetic cost of activity in growing pigs of different body weight and breed/castration combination[END_REF] which might be related to the previously discussed energy cost of protein turnover. The genetic selection for higher feed intake and lean growth can thus be related to the higher FHP reported observed in modern and leaner breeds (Brown-Brandl et al., 2004;Renaudeau et al., 2006a;[START_REF] Barea | Energy utilization in pigs selected for high and low residual feed intake[END_REF]. Environmental conditions can also influence the FHP and RHP with lower values when pigs are exposed to high ambient temperature (Collin et al., 2001b;Quiniou et al., 2001a;Campos et al., 2014a) that is highly related to the reduction of the feeding level [START_REF] Labussière | Fasting heat production and metabolic body weight in growing animals[END_REF]. Immune challenge can also increase the RHP of pigs and it can be linked to the energetic cost of inflammatory and immune responses (Campos et al., 2014a).

Most of the observed diurnal changes in HP are related to the physical activity (AHP)

and TEF (Labussière et al., 2013;[START_REF] Pedersen | Influence of diurnal variation in animal activity and digestion on animal heat production[END_REF] which comprises about 10 to 20% and 20 to 25% of HP, respectively. In pigs, AHP is closely associated to the feeding activity [START_REF] Noblet | Energy cost of standing activity in sows[END_REF]Labussière et al., 2013;[START_REF] Pedersen | Influence of diurnal variation in animal activity and digestion on animal heat production[END_REF] as a result of the physical activity from standing up and walking to the feeder [START_REF] Van Milgen | Major determinants of fasting heat production and energetic cost of activity in growing pigs of different body weight and breed/castration combination[END_REF]. AHP can constitute up to 18-19% of HP when pigs have >10 meals/day compared to only 13-14% when they have ~7 meals/day (Quemeneur et al., 2020). AHP also increases with low ambient temperature from 17 to 23% in 22°C and 12°C, respectively (Quiniou et al., 2001b), related to the increased feeding activity and possibly shivering in cold conditions to maintain homeothermic body temperature. As the measure of AHP relies on signals of force sensors, AHP in high ambient temperature can increase despite the lower physical activity because of increased panting (Quiniou et al., 2001b). However, AHP proportion can also vary due to housing conditions which is slightly higher in individually housed pigs (~17% of HP) possibly due to less comfortable conditions compared to group housed pigs (~13% of HP) (Quiniou et al., 2001b).

Text box 3.

The heat exchange between the pig and its environment via the sensible (convection, radiation, and conduction) and the latent (evaporation) routes. Meanwhile, TEF, as suggested by its name, is highly influenced by the diet and the utilization of nutrients. As seen in Table 1, dietary nutrients have different metabolizable energy (ME) utilization efficiencies 3 . For example, starch and lipid utilization have relatively high ME efficiency at 0.842 and 0.883, respectively, but protein utilization has only 0.520 [START_REF] Van Milgen | Energetic efficiency of starch, protein and lipid utilization in growing pigs[END_REF] because of the high heat increment associated to the previously mentioned energetic cost of deamination process and of urea synthesis and excretion. Heat increment related to TEF can also be related to nutrient deposition and maintenance wherein lipid deposition (LD) is more energy efficient than protein deposition (PD) at 0.84 vs 0.62, respectively [START_REF] Noblet | Metabolic utilization of energy and maintenance requirements in growing pigs: effects of sex and genotype[END_REF] which is linked to the discussed energetically costly process of protein turnover [breakdown and (re)synthesis)].

Heat loss

Following the principles of thermodynamics, there is a constant heat exchange within the animal, and between the animal and its surroundings that is driven by the temperature or thermal gradient [START_REF] Wiegert | Thermodynamic considerations in animal nutrition[END_REF][START_REF] Curtis | Environmental management in animal agriculture[END_REF][START_REF] Deshazer | Chapter 1: Basic principles of the thermal environment and livestock energetics[END_REF][START_REF] Taylor | Considerations for the measurement of core, skin and mean body temperatures[END_REF][START_REF] Boldrini | A mathematical model for thermoregulation in endotherms including heat transport by blood flow and thermal feedback control mechanisms[END_REF]. Heat exchanges can be achieved by sensible routes (i.e. conduction, convection, and radiation) which highly relies on temperature gradient, and the latent route which is mainly done by water evaporation [START_REF] Curtis | Environmental management in animal agriculture[END_REF][START_REF] Deshazer | Chapter 1: Basic principles of the thermal environment and livestock energetics[END_REF][START_REF] Collier | Thermal biology of domestic animals[END_REF] (Text box 3). Although animals can still gain heat from the environment when environmental temperature exceeds core body temperature, this subsection focuses mainly on the heat losses (HL) of the animal to the environment (NRC, 1981;[START_REF] Curtis | Environmental management in animal agriculture[END_REF][START_REF] Mccafferty | Animal thermoregulation: a review of insulation, physiology and behaviour relevant to temperature control in buildings[END_REF].

As most of the heat produced by the animal comes from its core where most of the vital and visceral organs are located, heat transfer is 1) from the core to the skin (internal thermal gradient) and 2) then from the skin to the environment (external thermal gradient) [START_REF] Precht | Heat exchange with the environment[END_REF][START_REF] Curtis | Environmental management in animal agriculture[END_REF][START_REF] Speakman | Maximal heat dissipation capacity and hyperthermia risk: Neglected key factors in the ecology of endotherms[END_REF]) (Figure 3). Within the body, heat is transferred via conduction between the tissues, the skin, and the external insulation (e.g. fur or feathers), and via convection through the blood flow in the arterial-venous system as blood carries heat to the skin surface and brings "cooled" blood to the core [START_REF] Stolwijk | Temperature regulation in man -A theoretical study[END_REF][START_REF] Precht | Heat exchange with the environment[END_REF][START_REF] Charkoudian | Mechanisms and modifiers of reflex induced cutaneous vasodilation and vasoconstriction in humans[END_REF][START_REF] Witzman | Chapter 28. Regulation of body temperature[END_REF][START_REF] Mccafferty | Animal thermoregulation: a review of insulation, physiology and behaviour relevant to temperature control in buildings[END_REF]; Boldrini et al., 2018). Thus, heat transfer from core to skin depends on the conductivity of the tissues and the skin blood flow rate [START_REF] Stolwijk | Temperature regulation in man -A theoretical study[END_REF][START_REF] Huang | A two-node mechanistic thermophysiological model for pigs reared in hot climates -Part 1: Physiological responses and model development[END_REF].

F surface, heat can be dissipated to the environment by the sensible route (Text box 3). Heat exchanges can be achieved by conduction such as between the body surface to the floor when the pig is lying down, by convection which is highly influenced by the surrounding air temperature and velocity, or by radiation between the animal and the walls or ceilings, or even between animals [START_REF] Precht | Heat exchange with the environment[END_REF][START_REF] Curtis | Environmental management in animal agriculture[END_REF][START_REF] Deshazer | Chapter 1: Basic principles of the thermal environment and livestock energetics[END_REF].

Since conventional pigs are relatively hairless (or have very fine hair) [START_REF] Ingram | Evaporative Cooling in the Pig[END_REF][START_REF] Mota-Rojas | Efficacy and function of feathers, hair, and glabrous skin in the thermoregulation strategies of domestic animals[END_REF], the skin acts as the main surroundings and thus serves as the site for heat exchange. As already mentioned, sensible routes principally rely on the temperature gradient of the body surface and the environment thus the HL decreases when ambient temperature increase, driving pigs to rely more on the latent pathway to lose heat [START_REF] Bond | Heat and moisture loss from swine[END_REF][START_REF] Whittow | Chapter 3 -Ungulates[END_REF]National Research Council (NRC), 1981;[START_REF] Curtis | Environmental management in animal agriculture[END_REF][START_REF] Pedersen | A comparison of three balance methods for calculating ventilation rates in livestock buildings[END_REF]Huynh et al., 2005a;[START_REF] Collier | Thermal biology of domestic animals[END_REF] (Figure 4).

Animals can generally lose heat by the latent evaporative pathway through the skin by sweating and wallowing, and through the respiratory tract by panting [START_REF] Collier | Thermal biology of domestic animals[END_REF]. However, pigs not only have fewer sweat glands than other livestock species (30/cm 2 compared to 800 to 2000/cm 2 in cattle and 250/cm 2 in sheep) (Renaudeau et al., 2012) but these sweat glands are also non-functional [START_REF] Ingram | Stimulation of cutaneous glands in the pig[END_REF]. Thus, heat is lost through the evaporative route mainly by panting and wallowing in pigs [START_REF] Ingram | Evaporative Cooling in the Pig[END_REF][START_REF] Morrison | Adaptation of growing pigs to changes in environmental temperature[END_REF][START_REF] Huynh | Evaporative heat loss from group-housed growing pigs at high ambient temperatures[END_REF]. Whereas sensible HL is limited by temperature gradient, the latent route can be influenced by both the ambient temperature and the relative humidity (RH) since high RH slows down the evaporation rate (Huynh et al., 2005b). However, it has been suggested that HL efficiency in growing pigs by evaporation may not be limited until an ambient temperature above 30°C and at 90% RH [START_REF] Renaudeau | Effects of short-term exposure to high ambient temperature and relative humidity on thermoregulatory responses of European (Large White) and Caribbean (Creole) restrictively-fed growing pigs[END_REF]. Indeed, the effect of humidity on HL is more apparent under higher air temperatures (as shown by [START_REF] Fialho | Modelling the effect of heat stress on food intake, heat production and growth in pigs[END_REF]) (Figure 5) possibly due to the higher reliance on the evaporative HL pathway when the thermal gradient is decreased.

There are fewer studies on HL compared to HP [START_REF] Pedersen | Climatization of animal houses-heat and moisture production at animal and house level 4 th report of CIGR working group[END_REF]. Along with ambient temperature, skin and rectal temperatures are usually used to quantify internal and external thermal gradients (Renaudeau et al., 2012;[START_REF] Taylor | Considerations for the measurement of core, skin and mean body temperatures[END_REF] but there can be a [START_REF] Fialho | Modelling the effect of heat stress on food intake, heat production and growth in pigs[END_REF].

delay in the responses between the two temperatures (Huynh et al., 2005b). Furthermore, skin temperature varies depending on body part where it is measured and depending on the position of the pig [START_REF] Warriss | Estimating the body temperature of groups of pigs by thermal imaging[END_REF][START_REF] Mortola | Correlations between the circadian patterns of body temperature, metabolism and breathing in rats[END_REF][START_REF] Magnani | Difference of surface body temperature in piglets due to the backtest and environmental condition[END_REF][START_REF] Soerensen | Infrared skin temperature measurements for monitoring health in pigs: a review[END_REF][START_REF] Teixeira | Skin temperature of slaughter pigs with tail lesions[END_REF][START_REF] Stukelj | Stress-free measurement of body temperature of pigs by using thermal imaging -Useful fact or wishful thinking[END_REF]. Meanwhile, respiration rate is usually associated to a higher evaporative water loss [START_REF] Kamada | Effects of environmental temperature, humidity and air movement on heat loss particularly that of latent heat, from the pig[END_REF] but direct quantification of evaporative HL can be difficult. Mathematical models that aim to estimate the partition between sensible and latent HL according to the ambient temperature have been published by the CGIR4 group in different livestock animals [START_REF] Pedersen | Climatization of animal houses-heat and moisture production at animal and house level 4 th report of CIGR working group[END_REF].

Alternatively, latent HL can be measured directly in respiratory chambers (Text box 2). By using this method, sensible HL is calculated by difference between total HP and latent HL [START_REF] Kamada | Effects of environmental temperature, humidity and air movement on heat loss particularly that of latent heat, from the pig[END_REF]Huynh et al., 2005b;[START_REF] Huynh | Evaporative heat loss from group-housed growing pigs at high ambient temperatures[END_REF]Renaudeau et al., 2013).

Figure 6.

Model of thermoregulation as a negative feedback system for maintaining a homeothermic body temperature. If there is a deviation sensed in the body temperature, the system makes alterations to restore it toward its set point value. BAT: brown adipose tissue.

(Adapted from [START_REF] Sawka | Physiological responses to acute exercise-heat stress[END_REF][START_REF] Weller | Body temperature and its regulation[END_REF][START_REF] Modell | A physiologist's view of homeostasis[END_REF][START_REF] Fossion | A physicist's view of homeostasis: how time series of continuous monitoring reflect the function of physiological variables in regulatory mechanisms[END_REF][START_REF] Tan | Regulation of body temperature by the nervous system[END_REF].

Finding the equilibrium of heat production and losses

𝐻𝑒𝑎𝑡 𝑠𝑡𝑜𝑟𝑎𝑔𝑒 = 𝑚𝑒𝑡𝑎𝑏𝑜𝑙𝑖𝑐 𝐻𝑃 ± 𝑠𝑒𝑛𝑠𝑖𝑏𝑙𝑒 ℎ𝑒𝑎𝑡 𝑒𝑥𝑐ℎ𝑎𝑛𝑔𝑒5 -𝑒𝑣𝑎𝑝𝑜𝑟𝑎𝑡𝑖𝑣𝑒 ℎ𝑒𝑎𝑡 𝑙𝑜𝑠𝑠 Equation 1The concept of homeostasis is based on the ability of an animal (or any living organism)

to maintain a stable state of its internal environment through self-regulating processes [START_REF] Cannon | The Wisdom of the Body WW Norton & Company[END_REF]. As defined in Equation 1, core body temperature is indicative of the balance (or imbalance) of HP and HL and it is kept within narrow limits when the net body heat storage (or heat load) is near zero [START_REF] Weller | Body temperature and its regulation[END_REF][START_REF] Tansey | Recent advances in thermoregulation[END_REF]. In a homeostatic system, there is a regulated variable6 (i.e., core body temperature) that has to be maintained within a certain range (i.e., set point or threshold) through different controlled variables (i.e., thermoregulation responses) that can be altered when the feedback system senses deviation from the specified set point [START_REF] Kotas | Homeostasis, inflammation, and disease susceptibility[END_REF][START_REF] Modell | A physiologist's view of homeostasis[END_REF][START_REF] Fossion | A physicist's view of homeostasis: how time series of continuous monitoring reflect the function of physiological variables in regulatory mechanisms[END_REF]. Thermoregulation mechanisms are controlled by the hypothalamus [START_REF] Campbell | Body temperature and its regulation[END_REF] based on the information sent by the central and peripheral thermoreceptors. 7 When a deviation from the set point is sensed, physiological and behavioral thermal effectors are employed to maintain the core body temperature [START_REF] Hellon | Central thermoreceptors and thermoregulation[END_REF][START_REF] Morrison | Central neural pathways for thermoregulation[END_REF][START_REF] Tansey | Recent advances in thermoregulation[END_REF][START_REF] Tan | Regulation of body temperature by the nervous system[END_REF][START_REF] Morrison | Central mechanisms for thermoregulation[END_REF]) (Figure 6). There are four most important physiological effectors in mammals which are involuntary responses that either generate or dissipate heat: i. the shivering and non-shivering thermogenesis which are activated in cold conditions, ii. Cardiovascular adjustments via the skin blood flow wherein there is increased vasoconstriction to conserve heat in cold conditions and vasodilation to increase blood flow to the peripheral tissues in order to maximize HL in hot conditions, and iii.

An increased water evaporation to increase evaporative HL which can include sweating and salivary spreading in other mammals but, as discussed, is mainly via panting in pigs [START_REF] Ingram | Evaporative Cooling in the Pig[END_REF][START_REF] Charkoudian | Mechanisms and modifiers of reflex induced cutaneous vasodilation and vasoconstriction in humans[END_REF][START_REF] González-Alonso | Human thermoregulation and the cardiovascular system[END_REF][START_REF] Tan | Regulation of body temperature by the nervous system[END_REF]. General static view of the balance between heat production and heat losses Ambient temperature highly influences heat exchanges of the animal with its surroundings. The animal must have a higher body temperature than the environment to be able to lose heat, otherwise heat storage will constantly be positive and body temperature will continuously increase which can lead to hyperthermia (>42°C body temperature). On the other hand, when ambient temperature is too low, the thermal gradient between the body surface may be too high and the animal may lose more heat than it produces leading to hypothermia (<34°C body temperature) wherein metabolic processes are slowed down further impairing the ability to produce heat and thermoregulate [START_REF] Dickson | Temperature gradients in pigs during whole-body hyperthermia at 42 degrees C[END_REF][START_REF] Weller | Body temperature and its regulation[END_REF][START_REF] Taylor | Considerations for the measurement of core, skin and mean body temperatures[END_REF][START_REF] Collier | Thermal biology of domestic animals[END_REF].

Classical relationships between body temperature, energy intake, HP, and HL according to ambient temperature variation is shown in Figure 7. This diagram presents the standard view of how body temperature is maintained in different ambient temperatures. To briefly describe the figure, there is the concept of the thermoneutral zone (TNZ), generally defined as the range of ambient temperatures delimited by the lower and upper critical temperatures (LCT and UCT; denoted by points C and E, respectively) wherein a stable body temperature can be maintained with HP remaining relatively constant. Within the TNZ is the evaporative critical temperature (ECT; point D) that theoretically divides the TNZ into two parts. In the thermal comfort zone (points C-D), thermoregulation effort is minimal and pigs mainly rely on the sensible HL to maintain thermal balance. Meanwhile, between ECT and the UCT (points D-E), pigs are still able to maintain HP but have started to rely more on the latent HL pathways as thermal gradient between skin surface and environment decreases. Outside the TNZ, HP is increased below LCT (by increasing feed intake or by inducing shivering or nonshivering thermogenesis) and is decreased above UCT (by decreasing feed intake and metabolic rate) to maintain body temperature. Thus, the pigs start to expend energy outside the TNZ to ensure homeothermia. Meanwhile, below point A (lower extreme) and above point F (upper extreme), the pig can no longer do further thermoregulation which can lead to extreme thermal injury or death [START_REF] Ingram | The influence of deep body temperatures and skin temperatures on peripheral blood flow in the pig[END_REF][START_REF] Mount | Heat loss from animals and man[END_REF][START_REF] Ingram | Heat exchange between animal and environment BTman and animals in hot environments[END_REF][START_REF] Blaxter | Environmental factors and their influence on the nutrition of farm livestock[END_REF][START_REF] Curtis | Environmental management in animal agriculture[END_REF].

Critical temperatures can be influenced by many factors related to the animal (age, physiological stage), the management (feeding level, stocking density), and the environment (floor type, ventilation rate) (example in Table 2). For example, LCT can be from 9-16°C and [START_REF] Kruger | Australian pig housing seriessummer cooling[END_REF] and [START_REF] Lorschy | Definitions of ambient temperature requirements for pigs: A review[END_REF] UCT from 36-39°C in 90-kg growing pigs. However, in practice, critical temperatures are not fixed and they temperature is dynamic and can even be influenced by the duration of exposure. For example, even though 42°C is far above the UCT, pigs are able to recover from a repeated heat challenge of 42°C when exposed for 6 hours at a time but not when the challenge was constant for 24 hours [START_REF] Dickson | Temperature gradients in pigs during whole-body hyperthermia at 42 degrees C[END_REF]. Indeed, summarizing the relationship of heat exchanges and core body temperature as such in the graph is an oversimplification of the reality because it gives a static view of thermoregulation. Heat loss proportions reported and modelled by cited literature in the previous subsections are usually values calculated based on the total HP averaged on a long period in already acclimated animal but does not account for the HP variations that happens within a day or according to the duration of exposure to elevated temperature. Indeed, the continuous and varied metabolic heat produced by living organisms means that there is an ever-changing heat exchange with the environment and that thermal equilibrium (heat storage) is never constant [START_REF] Weller | Body temperature and its regulation[END_REF][START_REF] Mendoza | Thermoregulation[END_REF][START_REF] Taylor | Considerations for the measurement of core, skin and mean body temperatures[END_REF].

Maintaining thermal equilibrium is a dynamic process

In the following parts, the dynamics of thermoregulatory responses of pigs to high ambient temperature are discussed regarding their daily variations according to the duration of heat exposure and their within-day variations in simulated constant or cyclic heat challenges.

Variations of thermoregulatory responses during acclimation to heat exposure

Similar to other stress responses, thermoregulatory responses can be divided into the acute and the chronic phases. Pigs have a biphasic acclimation response to heat stress (HS) depending on the length of exposure and the ambient temperature [START_REF] Renaudeau | Effect of temperature on thermal acclimation in growing pigs estimated using a nonlinear function[END_REF]: the short term-term heat acclimation (STHA) and the long-term heat acclimation (LTHA). The STHA responses to a high ambient temperature involve the activation of the autonomic sympathetic system that diverts energy to processes that increases heat dissipation (e.g., increase cardiovascular activity with blood redirection to the skin and other peripheral tissues) [START_REF] Collier | Thermal biology of domestic animals[END_REF][START_REF] Mota-Rojas | Efficacy and function of feathers, hair, and glabrous skin in the thermoregulation strategies of domestic animals[END_REF]. These are highly accelerated responses that aims to alleviate the initial strain, even at the expense of homeostasis [START_REF] Horowitz | From molecular and cellular to integrative heat defense during exposure to chronic heat[END_REF]. Thus, STHA includes inefficient, rapid changes in the first 1-2 days of heat exposure to maintain thermal balance, characterized by a sudden drop in feed intake and a rapid increase in water intake, skin (cutaneous) temperature, respiratory frequency, and rectal temperature (Bernabucci et al., 2010;[START_REF] Renaudeau | Effect of temperature on thermal acclimation in growing pigs estimated using a nonlinear function[END_REF]. Meanwhile, the LTHA is defined by the gradual reduction in body temperature over time as the pigs acclimate to HS and it is characterized by increased thermal effector efficiency [START_REF] Horowitz | From molecular and cellular to integrative heat defense during exposure to chronic heat[END_REF][START_REF] Renaudeau | Effect of temperature on thermal acclimation in growing pigs estimated using a nonlinear function[END_REF]. Depending on the ambient temperature, skin temperature may remain high as long as pigs are in hot conditions as it is affected both by the internal (core) and ambient temperature [START_REF] Renaudeau | Effect of temperature on thermal acclimation in growing pigs estimated using a nonlinear function[END_REF]. Reduction in core body temperature may likely be attributed to the lower HP due to the reduced feeding and activity levels that are closely related to the lower TEF and AHP (lower meal frequency and shorter time spent in the feeder), and possibly a lower FHP from the reduced sizes of metabolically active organs in those subjected to chronic HS for longer periods [START_REF] Koong | Effects of plane of nutrition on organ size and fasting heat production in pigs[END_REF][START_REF] Brown-Brandl | Manual and thermal induced feed intake restriction on finishing barrows. I: Effects on growth, carcass composition, and feeding behavior[END_REF]Quiniou et al., 2000b;Collin et al., 2001c). These acclimation responses can also be related to different endocrine changes during HS since chronic responses are coordinated by the endocrine and homeorhetic system [START_REF] Collier | Thermal biology of domestic animals[END_REF].

Long-term exposure of the animal to a stressor results to endocrine-mediated chronic responses that can change tissue sensitivity to homeostatic signals resulting in a new physiological state with a modified set point [START_REF] Collier | Physiological limitations, nutrient partitioning[END_REF][START_REF] Baumgard | Effects of heat stress on postabsorptive metabolism and energetics[END_REF][START_REF] Monaghan | Stress and life history[END_REF]. The hypothalamic pituitary adrenal axis (HPA) axis plays a central role in the acclimation response of animals to thermal stress (Bernabucci et al., 2010) and is involved in growth hormone secretion which is reported to be reduced in HS pigs [START_REF] Cui | Proteomic changes of the porcine small intestine in response to chronic heat stress[END_REF]. Activation of the HPA axis is also implicated in the release of common stress biomarkers in swine such epinephrine, glucagon, and cortisol (Rhoads et al., 2013;[START_REF] Martínez-Miró | Causes, consequences and biomarkers of stress in swine: an update[END_REF]. Pigs exposed to high ambient temperature [START_REF] Alves | Environmental temperature and serum cortisol levels In growing-finishing pigs[END_REF][START_REF] Yu | Effect of heat stress on the porcine small intestine: A morphological and gene expression study[END_REF][START_REF] Hao | Nutritional and physiological responses of finishing pigs exposed to a permanent heat exposure during three weeks[END_REF][START_REF] Hung | Dietary nanochromium tripicolinate increases feed intake and decreases plasma cortisol in finisher gilts during summer[END_REF][START_REF] Cui | Proteomic changes of the porcine small intestine in response to chronic heat stress[END_REF] showed increased levels of blood cortisol which is a primary stress hormone involved in catabolism and increased glucose production from body protein and fat, increased cardiovascular activity (heart rate), and increased mitochondrial oxygen uptake [START_REF] Ulrich | Uptake of cortisol in vitro by rat liver mitochondria[END_REF][START_REF] Geverink | The effect of shot biopsy on behavior, salivary cortisol, and heart rate in slaughter pigs[END_REF][START_REF] Dallman | Regulation of the hypothalamo-pituitaryadrenal axis, chronic stress, and energy: the role of brain networks[END_REF][START_REF] Pradhan | Ghrelin: much more than a hunger hormone[END_REF][START_REF] Kuo | Regulation of glucose homeostasis by glucocorticoids[END_REF]. Increased inflammatory signaling was found in oxidative muscle from 4 to 12 hours after heat exposure but not any more after 24 hours wherein there was instead found an increased tumor necrosis factor alpha (TNF- [START_REF] Montilla | Heat stress causes oxidative stress but not inflammatory signaling in porcine skeletal muscle[END_REF][START_REF] Ganesan | Twelve hours of heat stress induces inflammatory signaling in porcine skeletal muscle[END_REF][START_REF] Ganesan | Acute heat stress activated inflammatory signaling in porcine oxidative skeletal muscle[END_REF], a known cytokine that promotes pro-inflammatory and pro-apoptotic (cell death) signaling cascades [START_REF] Kale | Chapter 8 -Role of gambogic acid in chemosensitization of cancer[END_REF]. Indeed, increased inflammatory responses such as cytokine production [START_REF] Kpodo | Effects of feed removal during acute heat stress on the cytokine response and short-term growth performance in finishing pigs[END_REF] and circulating endotoxins (Pearce et al., 2013b;[START_REF] Gabler | Diurnal heat stress reduces pig intestinal integrity and increases endotoxin translocation[END_REF] have been reported in pigs subjected to high ambient temperature.

Although not yet fully understood, animals under HS have altered physiological state which has been previously reviewed (Bernabucci et al., 2010;[START_REF] Baumgard | Effects of heat stress on postabsorptive metabolism and energetics[END_REF][START_REF] Ross | Physiological mechanisms through which heat stress compromises reproduction in pigs[END_REF]. For example, heat exposure has been widely reported to decrease circulating levels of thyroid hormones in pigs specifically the pro-hormone thyroxine (T4) and its active form triiodothyronine (T3) [START_REF] Marple | Physiological and endocrinological changes in response to terminal heat stress in swine[END_REF][START_REF] Kouba | Influence of a high ambient temperature on lipid metabolism in the growing pig[END_REF][START_REF] Collin | Effects of high temperature on body temperature and hormonal adjustments in piglets[END_REF][START_REF] Cui | Proteomic changes of the porcine small intestine in response to chronic heat stress[END_REF]Sanz Fernandez et al., 2015a) and this is associated with a decreased feed intake, basal metabolic rate, and HP [START_REF] Kim | Thyroid hormone as a determinant of energy expenditure and the basal metabolic rate[END_REF]. In terms of energy metabolism, there have been reports of decreased non-esterified fatty acids (NEFA) levels [START_REF] Kouba | Influence of a high ambient temperature on lipid metabolism in the growing pig[END_REF]Sanz Fernandez et al., 2015a;Sanz Fernandez et al., 2015b;[START_REF] Cottrell | The effect of heat stress on respiratory alkalosis and insulin sensitivity in cinnamon supplemented pigs[END_REF] in HS pigs that are associated with a reduced ability for lipid mobilization despite the decreased feed intake and the elevated stress hormones during HS [START_REF] Zhao | Heat stress decreases metabolic flexibility in skeletal muscle of growing pigs[END_REF]. Muscle or protein catabolism seems to be preferred during HS in line with the increased creatinine (Pearce et al., 2013a;[START_REF] Kim | Effects of recovery from short-term heat stress exposure on feed intake, plasma amino acid profiles, and metabolites in growing pigs[END_REF], increased muscle oxidation [START_REF] Fausnacht | Heat stress reduces metabolic rate while increasing respiratory exchange ratio in growing pigs[END_REF] and plasma Nt-methyl histidine, a marker of muscle catabolism, (Pearce et al., 2013a) during acute HS. In humans, exogenous glucose (i.e. from the diet) has been reported to be a less preferred energy source in hot conditions than muscle glycogen (Jentjens et al., 2002). The effect of HS on insulin has also been largely investigated especially in the past decade. Heat exposure has been reported to decrease insulin sensitivity [START_REF] Yuen | Influence of glucocorticoids and growth hormone on insulin sensitivity in humans[END_REF]Liu et al., 2017;[START_REF] Cottrell | The effect of heat stress on respiratory alkalosis and insulin sensitivity in cinnamon supplemented pigs[END_REF] or reduced glucose clearance rate (Sanz Fernandez et al., 2015a) with pigs exhibiting hyperinsulinemia demonstrated by increased basal and circulating insulin concentrations (Pearce et al., 2013a;Sanz Fernandez et al., 2015a;[START_REF] Mayorga | Effects of zinc amino acid complex on biomarkers of gut integrity and metabolism during and following heat stress or feed restriction in pigs1[END_REF][START_REF] Seibert | Effects of heat stress and insulin sensitizers on pig adipose tissue[END_REF]. Meanwhile, some studies also demonstrated an increased insulin sensitivity in HS pigs (Sanz Fernandez et al., 2015b;[START_REF] Campos | Postprandial insulin and nutrient concentrations in lipopolysaccharide-challenged growing pigs reared in thermoneutral and high ambient temperatures[END_REF] indicating that the effect of heat exposure on pig insulin levels is not yet fully elucidated. One theory on increased insulin secretion is that it may be a response to the increased endotoxins during HS [START_REF] Baumgard | Effects of heat stress on postabsorptive metabolism and energetics[END_REF]. The contradiction between the roles of increased cortisol levels (promoting gluconeogenesis and body depot mobilization) and increased insulin (promoting glycolysis and nutrient deposition) can also be hypothesized to be a factor in the reported insulin resistance. Nevertheless, insulin is a key . Body (intestinal) temperature of pigs with a high (400 g/meal) or low (320 g/meal) feed intake recorded at 5-min intervals. Pigs received their meals (0600, 1400, and 2200 h) and were exposed to a natural ambient temperature from around 29 to 36°C. (Adapted from Cervantes et al., 2018).

hormone in many metabolic processes and is theorized to play a key role in the adaptation of pigs and other mammals in coping with a heat challenge [START_REF] Baumgard | Effects of heat stress on postabsorptive metabolism and energetics[END_REF].

Nychthemeral variations of thermoregulatory responses

Pigs exposed to a diurnal or cyclic heat challenge shift their feeding and other heatproducing activities in the cooler parts of the day (Xin and DeShazer, 1991;Nienaber et al., 1996;Quiniou et al., 1996;[START_REF] Patience | Nutritional and physiological responses of growing pigs exposed to a diurnal pattern of heat stress[END_REF][START_REF] Cross | Feeding behavior of grow-finish swine and the impacts of heat stress[END_REF]. Wide temperature variation measured throughout the day (23.4 to 40.0°C) also results to largely fluctuating patterns of sensible and latent HP responses in pigs (Figure 8). In the mentioned study, rate of latent HP increases when ambient temperature increases or with increasing cumulated feed intake throughout the day. This further evidences what has been previously discussed on how pigs increase their reliance in latent HL when temperature gradient decreases whether from the increased ambient temperature of the environment or the higher TEF-related HP of the animal.

Fluctuating ambient temperature outside the TNZ can cause cyclic acute responses to the cyclic thermal challenge which can increase immune and inflammatory responses as indicated by increased leukocytes, cytokine responses, insulin resistance in pigs exposed to rapid changes in ambient temperature [START_REF] Nienaber | Cyclic temperature effects on growing-finishing swine[END_REF]Johnson et al., 2016a;Johnson et al., 2016b).

These studies demonstrate that the thermoregulation ability of pigs can be impaired with an exposure to a fluctuating ambient temperature due to the repeated activation of acute stress responses possibly preventing their ability to reach the LTHA state.

Dynamic variations of pig responses throughout the day

As described before, thermoregulatory responses and heat exchanges of the animal varies throughout the day according to the diurnal variation in ambient temperature. While most studies on body temperature and HP are set in thermoneutral conditions as cited above, only a few studies have looked at the dynamics of body temperature throughout the day when the pigs are exposed to a simulated or naturally cyclical high ambient temperature [START_REF] Ou | Potential of in vivo real-time gastric gas profiling: a pilot evaluation of heat-stress and modulating dietary cinnamon effect in an animal model[END_REF]Cervantes et al., 2018). As shown in Figure 9 intestinal temperature variations closely follow the pattern of ambient temperature and are influenced by the timing and the amount of feed intake. Indeed, there was a postprandial increase of body temperature each time the pigs ate which was higher in the afternoon and the evening meal during the warmer periods of the day (Cervantes et al., 2018). This higher extent in body temperature increase during the hotter parts of the day must mean HL pathways are more quickly saturated due to the lower sensible HL ability especially after a larger meal due to the TEF-related heat (recall Equation 1 on heat storage in the body).

Logically, postprandial kinetics are not only observed in body temperature but also in the plasma hormones and metabolites (Liu et al., 2017;[START_REF] Pluschke | Male grower pigs fed cereal soluble dietary fibres display biphasic glucose response and delayed glycaemic response after an oral glucose tolerance test[END_REF][START_REF] Campos | Postprandial insulin and nutrient concentrations in lipopolysaccharide-challenged growing pigs reared in thermoneutral and high ambient temperatures[END_REF][START_REF] Li | Dynamic changes of postprandial plasma metabolites after intake of corn-soybean meal or casein-starch diets in growing pigs[END_REF][START_REF] Cottrell | The effect of heat stress on respiratory alkalosis and insulin sensitivity in cinnamon supplemented pigs[END_REF][START_REF] Rodríguez-López | Determining insulin sensitivity from glucose tolerance tests in Iberian and landrace pigs[END_REF] among others) (Figure 10). Among the plasma parameters in the cited studies, glucose and insulin are the most widely reported proxies of the general state of energy metabolism. In general, blood glucose (along with other nutrients) rises after a meal and insulin is released by the pancreas in order to promote glucose uptake [START_REF] Röder | Pancreatic regulation of glucose homeostasis[END_REF]. There is also a decrease in NEFA levels after a meal which is related to the insulin-mediated suppression of adipocyte lipolysis [START_REF] Burdge | Effect of meal sequence on postprandial lipid, glucose and insulin responses in young men[END_REF]. Nevertheless, these suggest that changes in the dynamic pattern of HP, of body temperature, and of different hormones and metabolites are also highly related to the feeding activity and, possibly, the size and the timing of the meals.

Section summary

Maintaining a homoeothermic (set point) body temperature is a dynamic process that involves finding a balance between heat production and loss especially since heat is constantly involved. Exposure to a high ambient temperature decreases the thermal gradient between the body surface and the environment which decreases the efficiency of the animal to dissipate heat. When HL pathways are saturated and the pigs can no longer dissipate further heat to maintain body temperature, they enter into a state of HS which results to a cascade of thermoregulatory and physiological responses. The heat associated to physical activity and the TEF largely contributes to the instantaneous peaks of HP and body temperature during the day and is thus decreased during HS. However, pig responses to HS depends also on the characteristics of the heat challenge employed in different experimental conditions (e.g., duration, constant or cyclic, timing and size of meals), which probably have a huge impact on the thermoregulatory responses measured on the animal and on its performance.

Text box 4.

Brief description of the pair-feeding technique (Sources: [START_REF] Mitchell | The paired-feeding method in nutrition experiments and its application to the problem of cystine deficiencies in food proteins[END_REF][START_REF] Russell | Pair feeding-mediated changes in metabolism: stress response and pathophysiology in insulin-resistant, atherosclerosis-prone JCR:LA-cp rats[END_REF][START_REF] Ellacott | Assessment of feeding behavior in laboratory mice[END_REF] What is the pair-feeding method?

Pair-feeding is an experimental technique that aims to remove confounding effects among experimental groups that is due to the feeding level (same plane of nutrition). Since exposure to high ambient temperature decreases feed intake, pairfeeding in heat stress studies means restricting the feed intake of the thermoneutral control group to a level similar to those under heat stress.

In pig studies, feed allowance of pair-fed thermoneutral pigs are usually calculated based on the average daily feed intake of the heat stressed animals with the ration divided into two or three distributions per day. Caution must therefore be taken in comparing pair-fed /restricted-fed thermoneutral models to ad libitum-fed heat stressed animals because they are not subjected to the same types of stress. Imposed feed restriction can increase stress responses due to prolonged fasting or can disrupt the natural feeding behavior.

Nevertheless, pair-feeding has been a widely accepted technique especially in determining direct effects of heat stress in pigs, especially since there is a significant difference in feed intake between ad libitum-fed pigs raised in thermoneutral and in heat stressed conditions.

Failure to maintain thermal equilibrium during heat stress: consequences on pig performance Maintaining core body temperature in HS conditions generally reduce economically important traits such as reproduction and production performance in pigs and other livestock species (DeShazer et al., 2009;[START_REF] Ross | Physiological mechanisms through which heat stress compromises reproduction in pigs[END_REF][START_REF] Liu | Review: What have we learned about the effects of heat stress on the pig industry?[END_REF]. In growing pigs, the most described negative effect of HS is on feed intake reduction (Renaudeau et al., 2011;[START_REF] Da Fonseca De Oliveira | Impacts on performance of growing-finishing pigs under heat stress conditions: a metaanalysis[END_REF]. Other metabolic and physiological changes induced by heat exposure also contribute to the impairment of animal performance, body composition, and health [START_REF] Baumgard | Effects of heat stress on postabsorptive metabolism and energetics[END_REF][START_REF] Ross | Physiological consequences of heat stress in pigs[END_REF][START_REF] Ross | Physiological mechanisms through which heat stress compromises reproduction in pigs[END_REF]Gonzalez-Rivas et al., 2020;[START_REF] Liu | Review: What have we learned about the effects of heat stress on the pig industry?[END_REF]. This subsection tackles how pig responses to a heat challenge compromise pig performance directly (physiological changes due to heat exposure) and indirectly (induced by feed intake reduction). Some factors affecting severity of HS and possible interventions to mitigate HS effects are also discussed.

Direct effects of heat stress

Due to negative effect of HS on feed intake, most of the pig studies that evaluated direct effects of a heat challenge and that will be mentioned in this subsection attempt to remove the confounding HS effect of feeding level by using pair-feeding techniques (Text box 4).

Consequence of increased peripheral blood flow

As mentioned before, in high ambient temperatures, blood is redistributed towards organs implicated in HL dissipation such as the skin, ears, diaphragm, and lungs but away from the core and intestinal tissues (Collin et al., 2001a). This can cause a cascade of reactions (Figure 11) such as hypoxia due to the insufficient blood flow to these organs leading to increased production of reactive oxygen species and decreased cellular antioxidant activity, and thus to oxidative stress and to tissue damage [START_REF] Turrens | Mitochondrial formation of reactive oxygen species[END_REF][START_REF] Oliver | Hyperthermia induces injury to the intestinal mucosa in the mouse: evidence for an oxidative stress mechanism[END_REF][START_REF] Belhadj Slimen | Reactive oxygen species, heat stress and oxidative-induced mitochondrial damage. A review[END_REF][START_REF] Hao | Research progress on oxidative stress and its nutritional regulation strategies in pigs[END_REF]. There have already been many reports of impaired syndrome in pigs subjected to short-term HS (Pearce et al., 2013c;[START_REF] Liu | Selenium and vitamin E together improve intestinal epithelial barrier function and alleviate oxidative stress in heat-stressed pigs[END_REF][START_REF] Abuajamieh | The effect of recovery from heat stress on circulating bioenergetics and inflammatory biomarkers[END_REF][START_REF] Gabler | Diurnal heat stress reduces pig intestinal integrity and increases endotoxin translocation[END_REF]Kpodo et al., 2020b;[START_REF] Xia | Heat stress-induced mucosal barrier dysfunction is potentially associated with gut microbiota dysbiosis in pigs[END_REF]. Gastrointestinal tract mucosa represents the barrier between the body and the external environment allowing nutrient uptake and restricting translocation of pathogenic molecules and bacteria through the tight junction barrier [START_REF] Wiest | Bacterial translocation in the gut[END_REF][START_REF] Turner | Intestinal mucosal barrier function in health and disease[END_REF][START_REF] Vancamelbeke | The intestinal barrier: a fundamental role in health and disease[END_REF][START_REF] Assimakopoulos | The role of the gut barrier function in health and disease[END_REF][START_REF] Pluske | Gastrointestinal tract (gut) health in the young pig[END_REF]. Thus, impairment of gut integrity can compromise overall health and can affect other organs as it allows the passage of the bacteria and bacterial components such as endotoxins into intestinal tissue and then into the bloodstream [START_REF] Terciolo | Beneficial effects of Saccharomyces boulardii CNCM I-745 on clinical disorders associated with intestinal barrier disruption[END_REF][START_REF] Massier | Impaired intestinal barrier and tissue bacteria: Pathomechanisms for metabolic diseases[END_REF]. Not only does this impair gut health but oxidative stress also compromises immune system of animals exposed to HS [START_REF] Morrow-Tesch | Heat and social stress effects on pig immune measures[END_REF][START_REF] Chauhan | Impacts of heat stress on immune responses and oxidative stress in farm animals and nutritional strategies for amelioration[END_REF] which has metabolic and energetic consequences [START_REF] Gabler | 143 What are the nutritional and metabolic costs of immune system activation in pigs?[END_REF][START_REF] Huntley | 145 Immune system stimulation increases nursery pig maintenance energy requirements[END_REF] thereby decreasing efficiency of energy utilization.

Whether due to an increased endotoxin in the systemic circulation that impairs ovarian functions [START_REF] Mayorga | Biology of heat stress; the nexus between intestinal hyperpermeability and swine reproduction[END_REF] or to a decreased uterine and umbilical blood flow [START_REF] Dreiling | Maternal endocrine and fetal metabolic responses to heat stress[END_REF][START_REF] Limesand | Impact of thermal stress on placental function and fetal physiology[END_REF], heat exposure has been reported to reduce reproduction performance in pigs and in other mammalian species such as pregnancy loss (seasonal infertility) during early gestation, placental insufficiency during mid-gestation, and fetal growth retardation during late gestation [START_REF] Hansen | Effects of heat stress on mammalian reproduction[END_REF][START_REF] Ross | Physiological mechanisms through which heat stress compromises reproduction in pigs[END_REF][START_REF] Mayorga | Biology of heat stress; the nexus between intestinal hyperpermeability and swine reproduction[END_REF][START_REF] Liu | Review: What have we learned about the effects of heat stress on the pig industry?[END_REF]. As reviewed by [START_REF] Hansen | Effects of heat stress on mammalian reproduction[END_REF], maternal hyperthermia has also been\ reported to impair oocyte development and maturations, early embryonic development, cardiovascular and brain functions, and other physical and physiological defects. This is in line with reported endometrial integrity deterioration and uterine tissue damage [START_REF] Lian | Heat stress impairs maternal endometrial integrity and results in embryo implantation failure by regulating transport-related gene expression in Tongcheng pigs[END_REF], placental nutrient transport capacity impairment [START_REF] Zhao | Maternal heat stress alters expression of genes associated with nutrient transport activity and metabolism in female placentae from mid-gestating pigs[END_REF], and placental insufficiency (Zhao et al., 2020) when sows are exposed to HS. Heat exposure can thus negatively affect not just the pregnant sow and but also the progeny.

Consequence of elevated evaporative heat loss

As the ambient temperature increases, sensible HL pathways becomes limited leading to panting. The related rise in respiratory frequency is accompanied by a decreased tidal volume that increases ventilation of the upper respiratory tract and preserves alveolar ventilation [START_REF] Robertshaw | Mechanisms for the control of respiratory evaporative heat loss in panting animals[END_REF][START_REF] Tsuji | Characteristics of hyperthermiainduced hyperventilation in humans[END_REF]. However, in cases of extreme heat and body temperature continues to rise, there is a shift to a slower deeper panting phase characterized by an increase of alveolar ventilation rate [START_REF] Hales | Respiratory function during thermal tachypnoea in sheep[END_REF]Renaudeau et al., 2012). As reported in HS pigs, a significant increase in respiratory rate can result to hypocapnia (low blood CO2) and respiratory alkalosis in the blood (Liu et al., 2017;Liu et al., 2018a;[START_REF] Le | PSIV-8 Effect of selenium and superoxide dismutase supplementation on heat stressed pigs[END_REF]Cottrell et al., 2020). The increased evaporative HL can also lead to dehydration and electrolyte imbalance ---). (Adapted from Le Bellego et al., 2002b).

with negative subsequent effects on growth performance [START_REF] Patience | Effect of dietary electrolyte balance on growth and acid-base status in swine[END_REF][START_REF] Haydon | Effect of dietary electrolyte balance on performance and blood parameters of growing-finishing swine fed in high ambient temperatures[END_REF][START_REF] Ahmad | Dietary electrolyte balance: implications in heat stressed broilers[END_REF][START_REF] Brêtas | Effect of dietary electrolyte balance on the performance of swine maintained in conditions of thermal discomfort[END_REF][START_REF] Ortega | The effect of heat stress and vitamin and micro-mineral supplementation on some mineral digestibility and electrolyte balance of pigs[END_REF].

Consequences of the metabolic and physiological changes

In thermoneutral conditions, pigs are inclined to prioritize and use higher proportion of ME intake for protein instead of lipid deposition even in restricted-fed pigs until maximum PD (PDmax)8 has been reached (Campbell et al., 1985;[START_REF] Bikker | Performance and body composition of finishing gilts (45 to 85 kilograms) as affected by energy intake and nutrition in earlier life: II. Protein and lipid accretion in body components[END_REF]Quiniou et al., 1996;[START_REF] Moughan | A genetic upper limit to whole-body protein deposition in a strain of growing pigs[END_REF]. However, heat exposure would limit PDmax in growing pigs (Le Bellego et al., 2002b). As shown in Figure 12, this effect seems to be unrelated to the feeding level and could be related to higher energetic cost of PD or may also be due to the increased insulin responses as discussed in a previous section of the literature review. This modified energy partitioning for PD and LD upon heat exposure is also observed in HS pigs when compared to pair-fed TN group (Pearce et al., 2013a) with fat deposition generally occurring in internal compartments [START_REF] Christon | The effect of tropical ambient temperature on growth and metabolism in pigs[END_REF][START_REF] Kouba | Influence of a high ambient temperature on lipid metabolism in the growing pig[END_REF][START_REF] Wu | Effects of high ambient temperature on lipid metabolism in finishing pigs[END_REF]. This may be related to effect of high ambient temperature on the decreased activity of stearoyl-CoA-desaturase 9 in the fat and muscle depots but not in the liver [START_REF] Kouba | Influence of a high ambient temperature on stearoyl-CoA-desaturase activity in the growing pig[END_REF] alongside the increased triglycerides transported by VLDL 10 (Kouba et al., 2001). The increased internal rather than external fat deposition has been hypothesized to be an acclimation response to reduce body insulation (thus increasing heat dissipation capacity) during HS (Le Dividich et al., 1998). As insulin inhibits hydrolysis of triglycerides in adipose cells into glycerol and free fatty acids [START_REF] Laws | Differences in insulin suppression of free fatty acid levels by gender and glucose tolerance status[END_REF], the direct effect of HS on fat deposition might be connected to the reported altered insulin response during HS. On the other hand, presence of NEFA also inhibits insulinstimulated glucose uptake and glycolysis [START_REF] Boden | Mechanisms of fatty acidinduced inhibition of glucose uptake[END_REF][START_REF] Roden | How free fatty acids inhibit glucose utilization in human skeletal muscle[END_REF] causing insulin resistance which is also correlated with visceral adiposity in HS animals [START_REF] Frayn | Visceral fat and insulin resistancecausative or correlative?[END_REF][START_REF] Bergman | Why visceral fat is bad: mechanisms of the metabolic syndrome[END_REF][START_REF] Usui | Visceral fat is a strong predictor of insulin resistance regardless of cardiorespiratory fitness in non-diabetic people[END_REF]. However, increased carcass fatness could also be an indirect consequence of a reduced net lean deposition rather than an increased fat deposition as indicated by previously discussed increased markers of muscle catabolism such as creatinine, leucine oxidation, and plasma Nt-methyl histidine (Pearce et al., 2013a;[START_REF] Zhao | Heat stress decreases metabolic flexibility in skeletal muscle of growing pigs[END_REF][START_REF] Kim | Effects of recovery from short-term heat stress exposure on feed intake, plasma amino acid profiles, and metabolites in growing pigs[END_REF].

Firmicutes (+) Proteobacteria (+) Bacteroidetes (-) Cyanobacteria (-) Lactobacillus (+),Bacteroides (-), Dorea (-), Faecalibacterium (-),
Oscillospira (-) [START_REF] Shi | Impact of gut microbiota structure in heat-stressed broilers[END_REF] 3 weeks Cecum 31-33°C vs.

21-23°C 2 weeks

Firmicutes (+) Bacteroidetes (-)

Anaerofustis (+), Pseudonocardia (+), Rikenella (+), Tyzzerella (+), Parabacteroides (-), Romboutsia (-), Saccharimonas (-),Weissella (-) (Liu et al., 2020) 4 weeks Ileum 30-33°C vs.

20-22°C 2 weeks

Firmicutes (+) Bacteroidetes (-) Proteobacteria (-) ), Streptococcus (-) [START_REF] Wang | Effects of high ambient temperature on the community structure and composition of ileal microbiome of broilers[END_REF] Laying hen

Alistipes (+), Azospirillum (+), Clostridium XIVb (+), Faecalibacterium (+), Oscillibacter (+), Rothia (+), Streptophyta (+), Coprococcus (-

weeks Cecum

24-30°C vs. 22°C 10 weeks

Bacteroidetes (+) Euryarchaeota (-)

Akkermansia (+), Desulfovibrio (+), Faecalibacterium (-) [START_REF] Wang | Mild heat stress changes the microbiota diversity in the respiratory tract and the cecum of layer-type pullets[END_REF] 28 weeks cecum 29-35°C vs.

20-22°C 4 weeks

No differences

Anaerosporobacter (+), Barnesiella (+), Clostridium_sensu_stricto_1 (+), Escherichia_Shigella (+), Gallibacterium (+), Odoribacter (+), Rikenellaceae_RC9_gut_group(+)
, Sphaerochaeta (+), Bacteroides (-), Lactobacillus (-), Prevotellaceae_Ga6A1_group (-), Ruminococcaceae_UCG_005 (-) [START_REF] Xing | Changes in the cecal microbiota of laying hens during heat stress is mainly associated with reduced feed intake[END_REF] 45 weeks feces 25-34°C vs. 21-28°C

6.6 weeks

Bacteroidetes (+) Firmicutes (-) Fusobacteria (-) Proteobacteria(-) norank_p_Candidatus_ Saccharimonas (+), Cloacibacillus (+), Enterorhabdus (+), Flavihumibacter (+), Prolixibacter (+), Sunxiuqinia (+), norank_f_Synergistaceae (+), Synergistes (+), Dorea (-), Exiguobacterium (-), norank_f_Lachnospiraceae (-),
Marvinbryantia (-) [START_REF] Zhu | Heat stress mediates changes in fecal microbiome and functional pathways of laying hens[END_REF] Adapted from [START_REF] He | Potential use of gut microbiota composition as a biomarker of heat stress in monogastric species: a review[END_REF] Indirect effects of heat stress (mediated by a reduction in feed intake)

Average daily feed intake (ADFI) is the main determining factor of growth performance in pigs since tissue (lean and fat) deposition and subsequent maintenance needs dietary energy and nutrients [START_REF] Nyachoti | Voluntary feed intake in growing-finishing pigs: A review of the main determining factors and potential approaches for accurate predictions[END_REF][START_REF] Patience | A review of feed efficiency in swine: biology and application[END_REF]. The negative effect of HS on feed intake is well-documented and usually associated with the reduction in TEFrelated HP (Collin et al., 2001b;Quiniou et al., 2001a;Renaudeau et al., 2013;Campos et al., 2014a;Labussière et al., 2021). Although there is little to no effect on feed efficiency except at very high ambient temperature (above 32°C), the main consequence is a reduction in growth [as reported in both meta-analyses by Renaudeau et al. (2011) and da Fonseca de Oliveira et al. ( 2019)]. For example in 50-kg BW pigs, feed conversion ratio (FCR) remains at an average of 2.7 kg/kg from 20 to 30°C (Renaudeau et al., 2011), but ADFI and average daily BW gain (ADG) are reduced by -32 and -1 , respectively (Renaudeau et al., 2011). The reduction in ADFI and the consequent reduction in ADG results in longer time to reach the market weight [START_REF] Cruzen | Carcass composition of market weight pigs subjected to heat stress in utero and during finishing[END_REF]. The decreased slaughter BW of pigs exposed to high ambient temperature might also be related to a reduction in the absolute weight of the visceral organs [START_REF] Collin | Effects of high temperature on body temperature and hormonal adjustments in piglets[END_REF] which can mean a positive effect on the dressing BW. Indeed, when expressed as a percentage of empty BW, carcass proportion increased by 0.9 points upon heat exposure while head, total viscera, and liver proportion (%) of empty BW decreased by 3.9, 5.6, and 7.3 points, respectively (Johnson et al., 2015b).

Changes in the gut microbiota

Gut microbiota is particularly important in monogastric species when considering feed utilization especially its influence on dietary fiber digestion [START_REF] Sciellour | Effect of dietary fiber content on nutrient digestibility and fecal microbiota composition in growing-finishing pigs[END_REF][START_REF] Niu | Characteristics of gut microbiota in sows and their relationship with apparent nutrient digestibility[END_REF]. However, the past decade has provided a vast amount of pig gut microbiota research on health and production (as reviewed by [START_REF] Gresse | Gut microbiota dysbiosis in postweaning piglets: understanding the keys to health[END_REF][START_REF] Nowland | Development and function of the intestinal microbiome and potential implications for pig production[END_REF]and Duarte and Kim, 2022 among others). While it is still unclear, the effect of HS on pig microbiota may be related to the reduced feeding level since feed intake restriction in itself can modify microbiota composition but comparing HS to pair-fed (PF) TN controls also suggest a direct effect of heat exposure [START_REF] Xia | Heat stress-induced mucosal barrier dysfunction is potentially associated with gut microbiota dysbiosis in pigs[END_REF][START_REF] Xiong | Alterations in intestinal microbiota composition coincide with impaired intestinal morphology and dysfunctional ileal immune response in growing-finishing pigs under constant chronic heat stress[END_REF]. Nevertheless, some studies looking at HS effects in monogastric animals are provided in Table 3 (reviews by [START_REF] He | Potential use of gut microbiota composition as a biomarker of heat stress in monogastric species: a review[END_REF]and Wen et al., 2021). In growing pigs, heat exposure can result to increase in Proteobacteria relative to

Firmicutes and Bacteroidetes [START_REF] Xiong | Effects of acute heat stress on intestinal microbiota in grow-finishing pigs, and associations with feed intake and serum profile[END_REF]. Decrease in abundance of genera

Lactobacillus and Clostridium has also been reported under HS conditions and was theorized to an increase in pathogenic bacteria colonization as it coincided with an increase in potentially pathogenic Prevotellaceae [START_REF] Sciellour | Effect of chronic and acute heat challenges on fecal microbiota composition, production, and thermoregulation traits in growing pigs1,2[END_REF]. Meanwhile, heat exposure increased bacterial genera Chlamydia, Lactobacillus, Succinivibrio, Bifidobacterium, and

Lachnoclostridium which has been correlated with oxidative stress, intestinal barrier dysfunction, and inflammation and decreased Lactobacillus and Bifidobacterium which are suggested to be part of the initial responses in gut dysbiosis and inflammation [START_REF] Xia | Heat stress-induced mucosal barrier dysfunction is potentially associated with gut microbiota dysbiosis in pigs[END_REF].

Meanwhile, some species such as Lactobacillus reuteri, Clostridium chartatabidum, and

Anaerovibrio lipolyticus are suggested to be important during heat exposure as they are positively related to an improved feed intake and energy efficiency (Labussière et al., 2021).

Factors affecting severity of heat stress in growing pigs

Severity of HS effect on pigs depends on many factors relating both to the animal and the surrounding environment (Figure 13). While the goal of the literature review is not to enumerate all these factors, this subsection discusses some animal and environment related factors.

Animal related factors

Body weight

The severity of feed intake reduction increases not only with ambient temperature but also with BW (Quiniou et al., 2000a;[START_REF] Zhang | Integrative analysis of energy partition patterns and plasma metabolomics profiles of modern growing pigs raised at different ambient temperatures[END_REF]. As shown in Figure 14, the curvilinear effect of elevated temperature on ADFI is dependent on the BW of the pigs wherein ADFI was reported to decline between 20 and 30°C with an average of -32 and -1 °C 1 for 50 kg and 100-kg pigs, respectively (Renaudeau et al., 2011). The non-linear model proposed by the latter paper clearly shows that the critical temperature (inflection point) in which ADFI and thus ADG is lower in heavier pigs. The higher susceptibility of heavy animals is usually explained by lower HL capacity due lower surface area to mass ratio and the thicker subcutaneous fat (thicker insulation) and to a higher metabolic rate (Bruce and Clark, 1979).

Growth potential and higher productivity

It is generally accepted that selection for higher productivity increases environmental sensitivity [START_REF] Kolmodin | Selection in the presence of a genotype by environment interaction: response in environmental sensitivity[END_REF][START_REF] Knap | Genotype by environment interaction for litter size in pigs as quantified by reaction norms analysis[END_REF] with modern pig breeds or pigs with higher growth potential being more susceptible to the negative effects of HS (Renaudeau et al., 2006b;[START_REF] Rauw | Effects of diet and genetics on growth performance of pigs in response to repeated exposure to heat stress[END_REF][START_REF] Gourdine | The genetics of thermoregulation in pigs: A review[END_REF]. In comparing three pig lines, i.e., commercial, high and low residual feed intake (RFI) 11 , pigs with lower growth potential (low and high RFI)

1 1 but maintained their growth performance better upon exposure to high ambient temperature (Figure 15). Similar results were reported in an earlier study comparing conventional Large

White breed with a local Carribean breed (Creole) wherein the local breed with a lower lean deposition had lower reduction in ADFI and ADG (-5 and -3%, respectively) than the conventional breed (-12 and -9%, respectively) when exposed to a naturally high ambient temperature (Renaudeau et al., 2006b). Indeed, increased lean deposition in modern pig breeds translates to higher energy intake and HP rendering them more susceptible to HS [START_REF] Brown-Brandl | Thermoregulatory profile of a newer genetic line of pigs[END_REF].

Sex

With the previously discussed higher protein deposition, growth rate, and HP of entire male pigs [START_REF] Moughan | Description and validation of a model simulating growth in the pig (20-90 kg liveweight). New Zeal[END_REF][START_REF] Campbell | Genotype and sex effects on the relationship between energy intake and protein deposition in growing pigs[END_REF]Labussière et al., 2013), they are assumed to be more likely negatively affected by HS than castrated males or female pigs.

Sexual dimorphism on the response of pigs to HS has not been fully explored but barrows were reported to have lower feed efficiency in HS conditions than gilts (Renaudeau et al., 2006b).

Meanwhile, studies from one laboratory have suggested that there might be an underlying effect of sex in the heat-induced oxidative stress status of pigs. Skeletal muscles of barrows were unaffected [START_REF] Rudolph | The effect of Mitoquinol (MitoQ) on heat stressed skeletal muscle from pigs, and a potential confounding effect of biological sex[END_REF] while those of gilts showed increased oxidative stress upon HS [START_REF] Montilla | Heat stress causes oxidative stress but not inflammatory signaling in porcine skeletal muscle[END_REF] although there might be the confounding effect of age with former study having used ~60 kg barrows and the latter ~35 kg gilts. However, these have yet to be demonstrated in the animal level and in pigs of similar ages.

Table 4. Potential effects of in utero heat stress on the swine industry.

Production stage Biological effect Potential swine production effect References

Gestation and farrowing

Reduced piglet birth weight

Reduced piglet survivability and weaning weight [START_REF] Lucy | Litter characteristics and thermoregulatory behavior of first parity sows exposed to a controlled heat stress (HS) during gestation[END_REF] Decreased boar fertility Greater number of services per sow and reduced reproductive efficiency [START_REF] Lugar | In utero heat stress causes reduced testicular area at puberty, reduced total sperm production, and increased sperm abnormalities in boars[END_REF] Increased piglet cortisol Greater risk of mortality at farrowing due to increased stress response [START_REF] Machado-Neto | Immunoglobulins in Piglets from Sows Heat-Stressed Prepartum[END_REF] Altered lactation performance Impact on piglet growth from farrowing to weaning [START_REF] Safranski | Reproductive performance of gilts having developed in heat stressed dams[END_REF]Wiegert et al., 2015) Postweaning (nursery)

Greater stress response

Greater likelihood of morbidity and mortality following weaning [START_REF] Chapel | Determining the effects of early gestation in utero heat stress on postnatal fasting heat production and circulating biomarkers associated with metabolism in growing pigs[END_REF] Altered immune function Increased disease susceptibility [START_REF] Machado-Neto | Immunoglobulins in Piglets from Sows Heat-Stressed Prepartum[END_REF]

Growingfinishing

Increased maintenance costs

Reduced energy available for growth and lean meat production [START_REF] Chapel | Determining the effects of early gestation in utero heat stress on postnatal fasting heat production and circulating biomarkers associated with metabolism in growing pigs[END_REF]

Altered body composition

Increase adiposity and reduced lean accretion reduces meat yield and quality (Boddicker et al., 2014;Johnson et al., 2015a) Reduced thermotolerance

Greater heat stress susceptibility resulting in increased morbidity and mortality [START_REF] Johnson | Effects of mammalian in utero heat stress on adolescent body temperature[END_REF]Johnson et al., 2015b)

Decreased feed efficiency

Decreased profitability due to an increase in feed costs (Johnson et al., 2015a) Altered postabsorptive metabolism Impact on nutrient partitioning and carcass quality (Boddicker et al., 2014)

Market

Altered body composition

Increase adiposity and reduced lean accretion reduces meat yield and quality (Boddicker et al., 2014;Johnson et al., 2015a) Increased muscle fiber size Tougher meat products [START_REF] Wilmoth | Effects of in utero heat stress on muscle development of barrows[END_REF] Reduced live weight Less meat produced per pig harvested [START_REF] Wiegert | Effects of in utero heat stress on subsequent lactational performance of gilts and transgenerational effects on offspring[END_REF] Adapted from [START_REF] Johnson | Physiology symposium: Postnatal consequences of in utero heat stress in pigs[END_REF] Previous heat stress experiences

Another aspect that affects severity of HS effects does not come from the immediate previous exposure to a HS challenge. In pigs, a previous heat exposure resulted to an attenuated increase in body temperature upon a new heat challenge [START_REF] Rauw | Effects of diet and genetics on growth performance of pigs in response to repeated exposure to heat stress[END_REF][START_REF] Renaudeau | Impact of single or repeated short-term heat challenges mimicking summer heat waves on thermoregulatory responses and performances in finishing pigs[END_REF] (Figure 16). Pigs that were already subjected to a heat challenge twice (HHH) decreased ADFI and ADG when subjected to another HS challenge in Period 3 but to a lesser extent than pigs subjected to HS for the first time (TTH) [START_REF] Renaudeau | Impact of single or repeated short-term heat challenges mimicking summer heat waves on thermoregulatory responses and performances in finishing pigs[END_REF]. Similarly, in pigs subjected three times to an ambient temperature of 32°C, the drop in feed intake and BW gain was larger in the first and -529 1 , respectively) than in the second and -294 1) and third and -234 1 )

heat challenge [START_REF] Rauw | Effects of diet and genetics on growth performance of pigs in response to repeated exposure to heat stress[END_REF]. Meanwhile, a previous exposure to a heat challenge seems to limit the [START_REF] Mayorga | Effects of zinc amino acid complex on biomarkers of gut integrity and metabolism during and following heat stress or feed restriction in pigs1[END_REF][START_REF] Renaudeau | Impact of single or repeated short-term heat challenges mimicking summer heat waves on thermoregulatory responses and performances in finishing pigs[END_REF] that may be associated to gut integrity impairment [START_REF] Mayorga | Effects of zinc amino acid complex on biomarkers of gut integrity and metabolism during and following heat stress or feed restriction in pigs1[END_REF].

Recent studies have also shown that the prenatal thermal environment can influence pig responses to a postnatal heat challenge. Studies in other mammalian species have reported that the HS during pregnancy can affect their progeny [START_REF] Shiota | Effects of prenatal heat stress on postnatal growth, behavior and learning capacity in mice[END_REF][START_REF] Edwards | Effects of heat on embryos and foetuses[END_REF][START_REF] Hansen | Effects of heat stress on mammalian reproduction[END_REF]. In farm animals, prenatal epigenetic adaptation and thermal manipulation has been well-studied in poultry species to improve the -hatch thermal challenge [START_REF] Yahav | Thermotolerance acquisition in broiler chickens by temperature conditioning early in life-the effect of timing and ambient temperature[END_REF][START_REF] Tzschentke | Early development of neuronal hypothalamic thermosensitivity in birds: influence of epigenetic temperature adaptation[END_REF][START_REF] Moraes | Effect of thermal conditioning during embryonic development on aspects of physiological responses of broilers to heat stress[END_REF][START_REF] Yahav | Thermal manipulations during broiler chick embryogenesis: effects of timing and temperature[END_REF][START_REF] Tzschentke | Attainment of thermoregulation as affected by environmental factors[END_REF], or to influence carcass characteristics [START_REF] Collin | Effects of thermal manipulation during early and late embryogenesis on thermotolerance and breast muscle characteristics in broiler chickens[END_REF][START_REF] Loyau | Thermal manipulation of the embryo modifies the physiology and body composition of broiler chickens reared in floor pens without affecting breast meat processing quality1[END_REF][START_REF] Loyau | Thermal manipulation of the chicken embryo triggers differential gene expression in response to a later heat challenge[END_REF]. In ruminants, maternal HS has been found to lower stress response of newborn calves and negatively affect the fetal development (Guo et al., 2016), and to reduce feed intake and thus growth of calves coming from cows subjected to HS during late gestation [START_REF] Monteiro | Effect of maternal heat stress during the dry period on growth and metabolism of calves[END_REF].

As previously discussed, direct effects of heat exposure in pregnant sows can include impaired endometrial integrity and reduced uterine blood flow which results to a cascade of negative effects to the offspring and thus to the overall pig production efficiency (as reviewed by [START_REF] Ross | Physiological mechanisms through which heat stress compromises reproduction in pigs[END_REF][START_REF] Johnson | Physiology symposium: Postnatal consequences of in utero heat stress in pigs[END_REF]and Liu et al., 2021) (Table 4). Some studies have reported that pigs born from HS pregnant sows are reported to be less heat tolerant due to increased basal body temperature [START_REF] Johnson | Effects of mammalian in utero heat stress on adolescent body temperature[END_REF]Johnson et al., 2015c) and increased cortisol response of pigs when subjected to a postnatal stressor [START_REF] Chapel | Determining the effects of early gestation in utero heat stress on postnatal fasting heat production and circulating biomarkers associated with metabolism in growing pigs[END_REF][START_REF] Johnson | Early life thermal stress: Impact on future thermotolerance, stress response, behavior, and intestinal morphology in piglets exposed to a heat stress challenge during simulated transport[END_REF]. Carcass fatness is also associated with pigs that were exposed to a prenatal HS environment and has been associated to seasonal fatness of pigs born during the summer season [START_REF] Trezona | The causes of seasonal variation in backfat thickness of pigs in Western Australia[END_REF][START_REF] Rodríguez-Sánchez | The effect of seasonality of the growing-finishing period on carcass, meat and fat characteristics of heavy barrows and gilts[END_REF]Choi et al., 2019a;Liu et al., 2020a). However, results differ across studies with increased fatness in utero HS pigs observed in some studies (Boddicker et al., 2014;Johnson et al., 2015b) but not in others [START_REF] Cruzen | Carcass composition of market weight pigs subjected to heat stress in utero and during finishing[END_REF]Johnson et al., 2015a) which might be related to many factors such as the stage and length of the heat challenge during gestation and the growth stage of the progeny.

The reported decrease in placental sufficiency and decrease in the number of skeletal muscle fibers of the offspring of sows subjected to mid-gestation HS [START_REF] Zhao | Controlled elevated temperatures during early-mid gestation cause placental insufficiency and implications for fetal growth in pregnant pigs[END_REF]) also lend support to the altered body composition of prenatal HS pigs. Nevertheless, most of the pig studies related to prenatal HS that have been published considered only primiparous sows (Boddicker et al., 2014;[START_REF] Cruzen | Carcass composition of market weight pigs subjected to heat stress in utero and during finishing[END_REF]Johnson et al., 2015c;Johnson et al., 2015a;Johnson et al., 2015b;[START_REF] Chapel | Determining the effects of early gestation in utero heat stress on postnatal fasting heat production and circulating biomarkers associated with metabolism in growing pigs[END_REF][START_REF] Zhao | Controlled elevated temperatures during early-mid gestation cause placental insufficiency and implications for fetal growth in pregnant pigs[END_REF] thus results cannot be easily extrapolated to multiparous sows that might be more susceptible to HS due to the higher BW.

Environmental related factors

Relative humidity

As previously discussed, high air humidity slows down evaporation and can exacerbate HS effects especially in high ambient conditions. For example, increase in respiratory rate and subsequent increase in rectal temperature happens at a lower temperature (inflection point) in 60-kg group-housed pigs at 80% RH (21.3 and 24.6°C, respectively) than at 65% RH (22.6 and 27.1°C, respectively) or at 50% RH (23.4 and 26.6°C, respectively) (Huynh et al., 2005b) (Figure 17). Indeed, it has been suggested that an 18% increase in RH at 30°C is similar to a 1°C increase in air temperature [START_REF] Curtis | Environmental management in animal agriculture[END_REF]. This had evidently consequences on feed intake (decrease of -81, -99, and -1 per 1°C above inflection point at 50, 65, and 80% RH, respectively) and thus on growth (ADG of 721, 644, and 630 g d 1 at 50, 65, and 80% RH, respectively) (Huynh et al., 2005b). Meanwhile, in individually housed pigs, decreased HL efficiency by panting only started at 90% RH and at ambient temperature above 30°C [START_REF] Renaudeau | Effects of short-term exposure to high ambient temperature and relative humidity on thermoregulatory responses of European (Large White) and Caribbean (Creole) restrictively-fed growing pigs[END_REF].

Housing conditions (stocking density, floor type)

A higher stocking density can increase HS susceptibility of pigs since heat is conserved better with surrounding warm bodies. This can be one of the many factors explaining the discrepancy in the previously mentioned RH effects wherein respiratory rate increased at 80% Reduce heat increment Compensate for feed intake reduction [START_REF] Renaudeau | Effects of exposure to high ambient temperature and dietary protein level on performance of multiparous lactating sows[END_REF]Le Bellego et al., 2002a;Wang et al., 2022) Feed withdrawal after acute HS More rapid decrease in body temperature due to no further TEFrelated heat production (Kpodo et al., 2020)

Increase heat losses

Floor cooling Increase temperature gradient thereby increasing conductive heat loss [START_REF] Silva | Effect of floor cooling and dietary amino acids content on performance and behaviour of lactating primiparous sows during summer[END_REF]Cabezón et al., 2017a) Wetting of the skin or water provision to increase wallowing (fogging, misting, etc.)

Increase evaporative heat loss through the skin [START_REF] Black | Lactation in the sow during heat stress[END_REF][START_REF] Huynh | Evaporative heat loss from group-housed growing pigs at high ambient temperatures[END_REF][START_REF] Justino | The impact of evaporative cooling on the thermoregulation and sensible heat loss of sows during farrowing[END_REF][START_REF] Godyń | Impact of fogging system on thermal comfort of lactating sows[END_REF] Provision of chilled drinking water

Increase heat loss (by warming water to body temperature level) [START_REF] Jeon | Effects of chilled drinking water on the performance of lactating sows and their litters during high ambient temperatures under farm conditions[END_REF] Increasing air speed Increase convective heat loss [START_REF] Bjerg | Modeling skin temperature to assess the effect of air velocity to mitigate heat stress among growing pigs[END_REF] Night feeding Increase feeding during the cooler parts of the day (higher temperature gradient) (Choi et al., 2019)

Attenuate direct effects of heat stress

Antioxidant supplementation (Vit. E, Selenium, Mitoquinol)

Reduce oxidative stress Improve intestinal integrity [START_REF] Liu | Selenium and vitamin E together improve intestinal epithelial barrier function and alleviate oxidative stress in heat-stressed pigs[END_REF]Liu et al., 2018a;Liu et al., 2018b;[START_REF] Le | PSIV-8 Effect of selenium and superoxide dismutase supplementation on heat stressed pigs[END_REF][START_REF] Rudolph | The effect of Mitoquinol (MitoQ) on heat stressed skeletal muscle from pigs, and a potential confounding effect of biological sex[END_REF] Pre-and probiotic supplementation

Gut microbiota modulation Improve intestinal integrity Improve immune functions

Reduce pro-inflammatory responses (Review by [START_REF] He | Potential use of gut microbiota composition as a biomarker of heat stress in monogastric species: a review[END_REF] Betaine supplementation Energy-sparing (as osmolyte) to compensate for feed intake reduction Decrease intestinal permeability [START_REF] Schrama | Dietary betaine supplementation affects energy metabolism of pigs[END_REF]Cabezón et al., 2017b;[START_REF] Le | Betaine and isoquinoline alkaloids protect against heat stress and colonic permeability in growing pigs[END_REF] Electrolyte provision Maintain electrolyte balance [START_REF] Ortega | The effect of heat stress and vitamin and micro-mineral supplementation on some mineral digestibility and electrolyte balance of pigs[END_REF] Plant extracts Decrease intestinal permeability [START_REF] Le | Betaine and isoquinoline alkaloids protect against heat stress and colonic permeability in growing pigs[END_REF] Chromium supplementation Improve insulin sensitivity (Liu et al., 2017) [START_REF] Hung | Dietary nanochromium tripicolinate increases feed intake and decreases plasma cortisol in finisher gilts during summer[END_REF] Cinnamon supplementation Improve insulin sensitivity [START_REF] Cottrell | The effect of heat stress on respiratory alkalosis and insulin sensitivity in cinnamon supplemented pigs[END_REF] Thiazolidinedione or lipoic acid supplementation

Improve insulin sensitivity (theorized; not yet studied) (Rhoads et al., 2013) RH at 21°C with group-housed pigs (Huynh et al., 2005b) compared to 90%RH at >30°C in individually housed pigs [START_REF] Renaudeau | Effects of short-term exposure to high ambient temperature and relative humidity on thermoregulatory responses of European (Large White) and Caribbean (Creole) restrictively-fed growing pigs[END_REF]. An environment that allows pigs to modify temperature gradient, e.g. decreased huddling in case of group housing or increased contact surface that can increase sensible HL (such as lying on the lateral instead of sternal side) (Huynh et al., 2005a) can attenuate severity of HS effects. Studies also suggest that floor type influences pig response to HS with pigs increasing their preference to lie in slatted floors than in solid floors in high ambient temperatures (Huynh et al., 2005c).

Possible interventions to mitigate heat stress effects

Many nutritional and management solutions have already been proposed and studied to mitigate HS effects in pigs and in other livestock species; some of which are presented in Table 5. Some of these interventions target the improvement of body heat balance (increase temperature gradient) either by reducing HP (e.g., decreasing heat increment) or increasing capacity for HL of the animal. Meanwhile, some aim to attenuate direct effects of HS in terms of reducing oxidative stress, improving intestinal integrity, increasing insulin sensitivity, and improving immune functions among others. While the ideal is to improve facilities and improve climate control of the building, depending on the energy cost it can be much more expensive than to incorporate nutritional and feeding management techniques to current intensive production systems [START_REF] Mayorga | Heat stress adaptations in pigs[END_REF]. Nutritional interventions to mitigate HS effects in livestock animals have indeed been reviewed extensively (Cottrell et al., 2015) with other reviews focusing on the importance of improving insulin sensitivity (Rhoads et al., 2013) or improving gut health and integrity [START_REF] He | Potential use of gut microbiota composition as a biomarker of heat stress in monogastric species: a review[END_REF][START_REF] Wen | Microbiota-gut-brain axis and nutritional strategy under heat stress[END_REF]. Based on current literature, there are no studies to our knowledge on modifying feeding frequency to improve performance during a heat challenge. In TN conditions, modifying feeding behavior via feeding frequency has been shown to improve growth performance and alter metabolic responses [START_REF] Schneider | Effects of restricted feed intake on finishing pigs weighing between 68 and 114 kilograms fed twice or 6 times daily[END_REF][START_REF] R1703 | Meal frequency changes the basal and time-course profiles of plasma nutrient concentrations and affects feed efficiency in young growing pigs1[END_REF]. With the large contribution of TEF and AHP to the instantaneous peaks of HP, increasing feeding frequency might be worth studying with the idea that splitting HP into smaller increments via provision of feed into smaller portions might help avoid overwhelming HL pathways. supplementation is indeed a promising nutritional intervention during HS. Live yeast supplementation has already been reported to mitigate HS effects on feed intake in growing and finishing pigs, lactating sows, young calves, and dairy cows [START_REF] Brewster | Saccharomyces cerevisiae boulardii improves performance of pigs fed low and high energy diets in summer[END_REF]Lee et al., 2019;Perdomo et al., 2020;[START_REF] Domingos | Saccharomyces Cerevisiae var. Boulardii CNCM I-1079 during late gestation and lactation improves voluntary feed intake, milk production and litter performance of mixed-parity sows in a tropical humid climate[END_REF]Labussière et al., 2021). This translated to improved feed intake and energy efficiency in finishing pigs (Labussière et al., 2021), and milk production and piglet weight in lactating sows [START_REF] Lee | Effect of saccharomyces boulardii supplementation on performance and physiological traits of holstein calves under heat stress conditions[END_REF][START_REF] Domingos | Saccharomyces Cerevisiae var. Boulardii CNCM I-1079 during late gestation and lactation improves voluntary feed intake, milk production and litter performance of mixed-parity sows in a tropical humid climate[END_REF][START_REF] Rocha | Effects of Saccharomyces cerevisiae var. boulardii CNCM I-1079 on performance, colostrum and milk composition, and litter performance of mixed-parity sows in a tropical humid climate[END_REF] subjected to high ambient temperature.

Potential mechanisms of live yeast in livestock animals during heat stress

Live yeasts belong to the category of probiotics which are live microorganisms beneficial to the hosts when ingested in appropriate and regular quantities (Chaucheyras-Durand and Durand, 2010). Probiotic activity is strain-dependent; thus this subsection mostly focuses on Saccharamyces cerevisiae var. boulardii or S. boulardii and the term live yeast (LY) will refer to this strain unless stated otherwise. This particular yeast is considered to be the probiotic strain because of its known thermotolerance with optimum temperature at 37°C, its ability to survive gastrotintestinal (GIT) transit (more resistant than S. cerevisiae in a gastric environment), and its inhibition of microbial pathogenic growth [START_REF] Czerucka | Review article: yeast as probiotics -Saccharomyces boulardii[END_REF]. The Mechanism of S. boulardii during HS is still unclear but supplemented HS animals showed altered thermoregulatory responses (e.g. decreased rectal or skin temperature), altered meal behavior (increased meal frequency or shorter inter-meal intervals), decreased inflammatory responses, and modulated gut microbiota with an increase in Lactococcus lactis that was correlated to an improved feed intake during HS [START_REF] Lee | Effect of saccharomyces boulardii supplementation on performance and physiological traits of holstein calves under heat stress conditions[END_REF]Perdomo et al., 2020;Labussière et al., 2021;[START_REF] Mayorga | Effects of dietary live yeast supplementation on growth performance and biomarkers of metabolism and inflammation in heat-stressed and nutrient-restricted pigs[END_REF]. Other potential mechanisms of S. boulardii may be related to decreased bacterial adhesion in intestinal cells, reduced pro-inflammatory responses, and increased immune responses of pigs [START_REF] Lessard | Administration of Pediococcus acidilactici or Saccharomyces cerevisiae boulardii modulates development of porcine mucosal immunity and reduces intestinal bacterial translocation after Escherichia coli challenge[END_REF][START_REF] Collier | Oral administration of Saccharomyces cerevisiae boulardii reduces mortality associated with immune and cortisol responses to Escherichia coli endotoxin in pigs1[END_REF][START_REF] Daudelin | Administration of probiotics influences F4 (K88)-positive enterotoxigenic Escherichia coli attachment and intestinal cytokine expression in weaned pigs[END_REF][START_REF] Oswald | Mycoplasma vaccination responses in immunodepressed weanling pigs supplemented with S. cerevisiae boulardii[END_REF][START_REF] Gresse | Saccharomyces Cerevisiae var Boulardii CNCM I-1079 reduces expression of genes involved in inflammatory response in porcine cells challenged by enterotoxigenic E. Coli and influences bacterial communities in an in vitro model of the weaning piglet colon[END_REF] which may attenuate discussed compromised immune system of HS animals. Intestinal and mucosal integrity were also improved by LY supplementation in healthy and immunocompromised pigs [START_REF] Bontempo | Live yeast dietary supplementation acts upon intestinal morpho-functional aspects and growth in weanling piglets[END_REF][START_REF] Collier | Oral administration of Saccharomyces cerevisiae boulardii reduces mortality associated with immune and cortisol responses to Escherichia coli endotoxin in pigs1[END_REF][START_REF] Oswald | Mycoplasma vaccination responses in immunodepressed weanling pigs supplemented with S. cerevisiae boulardii[END_REF][START_REF] Trevisi | Effect of feed supplementation with live yeast on the intestinal transcriptome profile of weaning pigs orally challenged with Escherichia coli F4[END_REF] which might help with the discussed leaky gut symptoms observed in hot conditions. The beneficial effect in the GIT of S. boulardii that makes it an established therapeutic treatment for gut symbiosis, diarrhea, and constipation in humans [START_REF] Kelesidis | Efficacy and safety of the probiotic Saccharomyces boulardii for the prevention and therapy of gastrointestinal disorders[END_REF] might also apply to pigs especially since HS pigs seem to show symptoms like in humans with inflammatory bowel disease and diabetes [START_REF] Jurjus | Inflammatory bowel disease, colorectal cancer and type 2 diabetes mellitus: The links[END_REF][START_REF] Hu | A comprehensive analysis of the colonic flora diversity, short chain fatty acid metabolism, transcripts, and biochemical indexes in heat-stressed pigs[END_REF]. Reported attenuation of diabetes-related complications by S. boulardii such as hyperglycemia (insulin resistance) and leaky gut by gut modulation, decreased inflammatory and oxidative responses in the gut and liver, and an overall improved cardiovascular health (Brandão et al., 2018;Albuquerque et al., 2019;Cunha et al., 2020) thus might also be potential mechanisms during HS.

Section summary

Physiological and metabolic responses of growing pigs to maintain their body temperature in high ambient temperature results to reduced growth performance which is mainly due to the reduction in feed intake. However, heat exposure has also many less known direct effects on the metabolic and physiological responses of pigs that are usually evaluated using pair-feeding models in which ad libitum-fed HS pigs are usually compared with restricted-fed TN controls with strict feeding schedule. While severity of HS effects depends ough this remains to be elucidated. Possible nutritional and management interventions have also been discussed but none seem to consider the importance of feeding behavior (timing, size, or frequency of meals) in the saturation of HL pathways during the course of the day. Thus, LY supplementation seems to be a potential nutritional intervention during HS especially with its reported effects on feeding behavior modification and energy efficiency.

Conclusion of the literature review

This literature review demonstrates that regardless of environmental conditions, pigs will strive to maintain a constant homeothermic body temperature conducive for biological process because failure to do so can cause heat injury and, in extreme cases, death. During a thermal challenge, animals maintain a thermal balance via biochemical, physiological, and/or behavioral adjustments collectively known as thermoregulation. Exposure to high ambient temperature limits their HL ability thereby disrupting the thermal equilibrium. When their heat load surpasses their capacity for heat dissipation and they can no longer maintain body temperature, they enter a state of HS. Decreased growth during a heat challenge is mainly related to the reduced feed intake since it is the main adaptation response to limit heat production. However, in the recent years, more aspects of HS in pigs have been coming into light that have been previously unexplored.

Recent studies have shown that it is not just the immediate surroundings but also the prenatal thermal environment that affects pig performance in terms of their core body temperature, body composition, and their responses to a postnatal heat challenge. Results, however, are inconsistent perhaps due to differences in duration and stages of prenatal heat challenge across studies. Increasing number of studies have also been looking on the direct effects of heat exposure unrelated to feed intake such as increased adiposity and decreased metabolic flexibility that are demonstrated using pair-feeding models. However, it is difficult to assess which effects are due to heat exposure and which are confounded with the feeding pattern since TN pair-fed models with strict feeding schedule are usually compared with ad libitum-fed HS pigs.

Another aspect of HS in pigs often overlooked is its impact on feeding behavior even though both feeding level and behavior largely contribute to the instantaneous peaks of HP due to feeding-related AHP and TEF. These variations in HP and body temperature must also mean that maintaining thermal balance is a dynamic process throughout the day. Yet, most nutritional strategies to mitigate HS effects focus mainly on reducing the global heat load (e.g. improving energy utilization or reducing heat increment) and there are only a few studies on pigs investigating feeding management strategies (e.g. time or frequency of feeding). One nutritional intervention, however, has been recently demonstrated to modify feeding behavior in pigs and in dairy cows. Still, it is unclear whether the positive effects of LY (Saccharomyces cerevisiae var boulardii CNCM I-1079) supplementation on feed intake and energy efficiency of HS pigs were due to an altered feeding behavior (higher meal frequency) and/or energy metabolism.

There are certainly many more elements about HS which need to be understood. The main goal of this thesis project is to contribute in bridging knowledge gaps in some emerging aspects of HS in pigs in four parts:

1.

To investigate effects of a chronic prenatal heat challenge on the growth, body composition, and physiological responses of pigs in postnatal TN or HS conditions, 2.

To evaluate and to confirm indirect (caused by a reduced feed intake) and direct HS effects (regardless of feed intake and pattern) on pig performance, 3.

To determine whether the positive effects of LY supplementation in HS pigs is due to changes in feeding behavior or energy metabolism, and if these effects can be replicated by imposing an increased meal frequency, and 4.

To further understand the dynamic process of how body heat balance is maintained by looking at variations in thermoregulatory and physiological responses during the day of pigs subjected to high ambient temperature.

General experimental strategy

Figure 19. Summary of the experimental designs of the trials performed in the thesis project. TN: thermoneutral; HS: heat stressed; Diet N: control (0 g live yeast/ton); Diet Y: supplemented with live yeast (100 g live yeast/ton); PF2: pair-fed with 2 feed distributions; PF8: pair-fed with 8 feeding distributions; FW: feeding window. This experiment was also designed to distinguish direct and indirect HS effects using a pairfeeding approach based on both feed restriction and change in meal frequency distribution.

General experimental strategy

Exp. 3 was conducted in the respiratory chambers in order to determine whether or not LY effect was due to the altered feeding behavior or to modified energy metabolism that can explain improved energy efficiency (Labussière et al., 2021). In contrast to Exp. 2, feeding restriction in Exp. 3 was only limited to the time and duration of feeder access but not the amount of intake. Lastly, Exp. 4 was designed to have a link between Exp. 2 (all females) and Exp. 3 (all entire males) and with the idea that heat tolerance and response to LY supplementation adaptation may differ between sexes [START_REF] Tower | Sex differences in the response to oxidative and proteolytic stress[END_REF][START_REF] Rudolph | The effect of Mitoquinol (MitoQ) on heat stressed skeletal muscle from pigs, and a potential confounding effect of biological sex[END_REF].

Animals

The sows used in Exp. 1 were all Large White × Landrace from the UE3P experimental herd. Most pig studies related to prenatal HS considered only primiparous sows (Boddicker et Johnson et al., 2015c;[START_REF] Chapel | Determining the effects of early gestation in utero heat stress on postnatal fasting heat production and circulating biomarkers associated with metabolism in growing pigs[END_REF][START_REF] Zhao | Controlled elevated temperatures during early-mid gestation cause placental insufficiency and implications for fetal growth in pregnant pigs[END_REF], but parity can affect sow response to high ambient temperature [START_REF] Spencer | Early weaning to reduce tissue mobilization in lactating sows and milk supplementation to enhance pig weaning weight during extreme heat stress[END_REF][START_REF] Iida | Timing and temperature thresholds of heat stress effects on fertility performance of different parity sows in Spanish herds[END_REF]Renaudeau et al., 2021). Thus, mixed parity sisters (or at least half-sister sows) were used in the experiment to consider effects that might be related to age or parity and to remove the confounding effect of the litter origin of the sows. In addition, within each parity and experimental groups, females were inseminated using the same Pietrain sire origin (4 in total). In all experiments, pigs were blocked according to litter origin (full-or half-siblings) and according to the BW in order to have a uniform origin and average initial BW among the experimental groups.

For Exp. 1, the plan was to be able to compare also the offspring microbiota composition to that of their sow origin thus it was decided to use only female offspring (a total of 48 pigs) in the experiment since sex can have an effect on microbiota profile [START_REF] Zhang | Sex-dependent changes in the microbiota profile, serum metabolism, and hormone levels of growing pigs after dietary supplementation with Lactobacillus[END_REF]. In Exp. 2, according to the numerous variation factors to be evaluated (ambient temperature, feeding management, and LY supplementation) it was not possible to test a sex effect. Thus, only female growing pigs were used (a total of 128 pigs). Due to building constraints, only 32 individual pens (2 rooms) were available at a given time and thus the experiment was conducted in 4 successive replicates (32 pigs per replicate). The plan for the experiment in the respiratory chambers (Exp. 3) was already to use entire male pigs to provide a link with the previous study which was the basis of the experimental design on LY and feeding frequency (Labussière et al., 2021). Since only two pigs can be measured at the same time and because pigs from the experimental herd were only available every 3 weeks, Exp.3 was conducted in 9 replicates with 4 pigs coming from the same litter per replicate for a total of 36 entire male growing pigs. Finally, both females and entire males (a total of 64 pigs; 32 each sex) blocked according to their litter origin were used in Exp. 4 to connect the experiments with LY (Exp. 2 and Exp. 3) in terms of similarities and differences of HS responses based on the sex of the animal. Exp. 4 was conducted in two successive replicates [16 female (F) pigs and 16 entire male (EM) pigs per replicate] because to similar building constraints as described for Exp. 2).

Housing

All experimental rooms used in the study were climate controlled and the ambient temperature of the rooms are presented in Figure 20. Experiments 1, 2, and 4 were conducted in the same experimental facilities in which two rooms were used. For Exp. 1 and 2, one room was maintained in TN conditions (18 to 24°C for Exp. 1 or 22°C for Exp. 2) for the whole experimental period, while the other room was in TN conditions during the adaptation and prechallenge period and was afterwards maintained in HS conditions (28 to 34°C for Exp. 1 or 32°C for Exp. 2). These same two rooms were used in Exp. 4 but with both rooms maintained at 22°C for 7 d and then at 32°C for 3 d during the adaptation period before the 27 d experimental period at 32°C. For the transition to the heat challenge period, ambient temperature in Exp. 1 gradually changed at a rate of 1°C/h from 0600 to 1800 hours and was thereafter maintained in cyclic HS conditions of 28 to 34 °C while ambient temperature in Exp.

2 and 4 was gradually changed at a rate of 2°C/h from 1200 to 1800 hours and was thereafter maintained in constant HS conditions of 32°C. Relative humidity was not controlled but was measured for the aforementioned experiments. For Exp. 3, ambient temperature was maintained during the experimental period at TN conditions of 22°C during the first 5 d of measurement and was increased for the last 5 d to 28°C with the transition on d 0 (fixed at 25°C at 0800h and an increase of 1°C/h from 0800h to 1100h) regardless of the experimental group. Relative humidity was kept constant at 65%. In Exp. 1, each room had 8 slatted pens (4 m 2 ) designed for housing three pigs each and equipped with a feeder and nipple drinker. In Exp. 2 and 4, the same experimental rooms were used but were partitioned into 16 slatted pens (2 m 2 ) per room in which pigs were individually housed. These two rooms had different climate control systems in which one room has a full air-conditioning system while the other only has a heating system (thus less homogeneity in the room). In Exp. 2, a makeshift automatic feed distributor system was placed in the pens that were assigned for pair-fed groups with 8 feeding distributions (Figure 21). In experiment 3, pigs were individually housed in metabolic crates equipped with an automatic feeder (with a feeding trap) and a bowl drinker during the adaptation period. They were then moved to another metabolic crate (3.3 m 2 ) in a 12-m 3 open circuit respiration chamber (as described by Vermorel et al., 1973) for the duration of the experimental period (Figure 22). Automatic feeder and bowl drinker was also provided in inside the respiratory chamber similar to those used during the adaptation period.

Heat challenge

As discussed in the literature review, responses elicited can differ depending on the type (constant or cyclic ambient temperature) and duration (acute or chronic) of the heat challenge. In Exp. 1, the sows and offspring growing pigs were subjected to the same cyclic and chronic thermal challenge to simulate the natural diurnal variations in ambient temperature during summer. Meanwhile, a constant ambient temperature was chosen for the experiments with LY supplementation (Exp. 2, 3, and 4) due to its hypothesized effect on feeding behavior and thus to avoid possible confounding effect of a cyclic heat challenge on feeding behavior, e.g. increased feeding activity in cooler parts of the day (Xin and DeShazer, 1991;Quiniou et al., 2000c;[START_REF] Patience | Nutritional and physiological responses of growing pigs exposed to a diurnal pattern of heat stress[END_REF]. The chosen ambient temperature considered to be the heat challenge for Exp.3 was also different. Due to the climate-controlled, relatively small, and fully insulated compartment of the respiratory chamber (Exp. 3) compared to the climate-controlled, mechanically ventilated rooms in Exp. 2 and 4, the air was at the same temperature as all the objects and equipment inside the chamber which could be why pigs already have trouble coping with an ambient temperature of 28°C in this housing condition as described in a previous experiment (Labussière et al., 2021). Thus, an ambient temperature of 28°C was deemed sufficient to elicit HS responses in pigs housed in respiratory chambers compared to 32°C in the climate-controlled rooms used in the other experiments.

Diet and feeding management

Experimental diets

The experimental diets are presented in Table 7 and were all provided in pelleted form.

Commercial diet formulations were used for Exp. 1 (growing diets) and for Exp. 2 and 4 (finishing diets), while Exp. 3 diets were especially formulated for the experiment and was produced in the UE3P experimental feed mill. For the supplemented diets, LY was incorporated at 100 g/ton of feed (1×10 6 CFU Saccharomyces cerevisiae var. boulardii CNCM I-1079/g of feed) in the feed before pelleting in the experimental feed mill for Exp. 2 and 4, and within the formula for Exp. 3. In order to ensure survival of the live yeast in feed, pelleting temperature did not exceed 65°C. Live yeast count in the feed was also conducted before each experiment by an external laboratory before feeding to the animals to ensure that the feed actually contained the minimum CFU count. In a previous experiment (Labussière et al., 2021), improved feed intake and retained energy with live yeast supplementation was mainly associated to increased deposition of fat and only tended to increase that of protein. Thus, the experimental finishing diets for Exp. 3 were formulated to be limiting in protein (0.65 vs. 0.84 Table 7. Basal composition of the experimental growing and finishing diets. SID Lys:NE of the diets in the other experiments) in order to more clearly evaluate differences in protein deposition among the experimental groups.

Feeding management

All pigs were adapted to their assigned experimental diets and feeding distribution schedule before the beginning of measurements during the experimental period (5,7,[START_REF] Quiniou | Modelling heat production and energy balance in group-housed growing pigs exposed to low or high ambient temperatures[END_REF], and 10 d of adaptation for Exp. 1, 2, 3, and 4 respectively). Feeding management was kept simple for Exp. 1 and 4 with all pigs having free access to feed during the whole day (ad libitum feeding), but with 2 manual feed distributions per day (one at 0900h and one at 15h00) in order to avoid spillage and soiling of the feed.

Three feeding managements were employed in Exp. 2 depending on the experimental groups: ad libitum feeding, and pair-feeding with 2 (PF2) or 8 (PF8) feeding provisions. Feed was distributed twice at 0900h and at 1500h for the ad libitum-fed and PF2 groups, while it was distributed at every 90-min interval from 0900h to 1930h in the PF8 groups. Despite its limitations, pair-feeding is a widely accepted technique to serve as a control for feed intake [START_REF] Russell | Pair feeding-mediated changes in metabolism: stress response and pathophysiology in insulin-resistant, atherosclerosis-prone JCR:LA-cp rats[END_REF][START_REF] Curfs | Nutrient requirements, experimental design, and feeding schedules in animal experimentation[END_REF] and the different number of feed provisions were added to allow evaluation of the effects of feeding frequency. In this experiment, pair-feeding was -d average feed intake per kg BW of either the non-supplemented ad libitum or live yeast supplemented ad libitum group in the HS room to smooth the inter-day variations. The calculated daily ration of the PF groups was then divided into two or eight feed provisions depending on their experimental group. On d 1 and 2 of the heat challenge, the HS ad libitum groups were -challenge ADFI to remove the risk of inaccurate pair-feeding on the first 2 days) were based on the results obtained in Exp. 1.

Many comparisons were considered in designing Exp. 2 as follows:

1. Ad libitum groups were used to compare the effect of HS in non-supplemented (Trt 1 vs. Although time and amount of feed provision were regulated in Exp. 2, it must be noted that due to the limitations of the facilities, pigs still had access to the feeding trough any time of the day regardless of if there was feed in trough or not.

Exp. 3 was also designed to evaluate LY and feeding frequency in hot conditions. A total of : 3 feeding management practices called feeding windows (FW) was applied: unlimited access (Unli; 0900h to 1545h and 1600h to 0700h), 2FW of 1 h each (from 0900h to 1000h and from 1500h to 1600h), and 8FW of 15 min each (at every 90-min interval from 0900h to 1930h), in which both the 2FW and 8FW groups were still allowed a total of 2 h of feed access per day. In contrast to Exp. 2, only time and duration of feeding were restricted in Exp. 3 but not the amount of feed available in the feeder. This was made possible due to the automated feeding traps designed to provide or restrict access to the feeder. Time of feed distribution were at 0855h and 1455h for the Unli and 2FW groups, and 5 min before each feeding window for the 8FW groups.

Measurements

Summary of the measurements taken in the four experiments are shown in Table 8.

Description of the methodology for the measurements and calculations done in Exp. 1, 2, and 3 are described in more details in Publications 1, 2, and 3 respectively. For Exp. 4, growth and feed intake measurements are similar to the method applied as described in Publication 2.

Growth and body composition

Individual pig BW was measured manually at the beginning and at the end of adaptation period and then every 2 weeks for Exp. 1 and every week for Exp. 2 during the experimental were manually weighed before entering and after exiting the respiration chambers but during the experimental period, BW was automatically transmitted and registered each time the pig stepped in front of the feeding through a platform equipped with a set of calibrated load sensors.

The general practice in an experiment in the respiratory chamber is to take the pig out of the respiratory chamber to weigh them manually in between periods. However, it was necessary to have enough days wherein the pigs were adapted in the chambers with the least disturbance as possible to the animal since time was limited (only 5 d each for periods 1 and 2). Furthermore, moving the pigs out to be weighed can be a stressful event which can likely affect adaptation to HS, thus the automatic body weighing system was put in place to still be able to measure the BW without the pigs exiting the chambers.

In Exp. 1 and 2, a comprehensive slaughter performance measurement was conducted in the experimental abattoir of INRAE-UE3P which included measurement of different body components, while in Exp. 4, only slaughter body weight and lean meat percentage were measured in a commercial slaughterhouse. In Exp. 3, protein and fat deposition were calculated based on the nitrogen and energy balance measurements (Figure 23). For digestibility measurements, the indigestible marker method was chosen over the total collection method to have the least disturbance to the animal during the transition in between periods.

Behavior (feeding behavior and physical activity)

Feed intake was measured in all the four experiments, feeding behavior in Exp. 2 and 3, and drinking behavior in Exp. 3. For feed intake measurement, in brief, feed allowance and refusals were weighed per day. The daily feed intake was measured as the difference between offered and refused feed and, depending on the experiment, was then averaged per day and per period. For feeding behavior measurement in Exp. 2, an infrared barrier system was installed in each of the individual feeding trough. Feeder presence was thus measured each time the infrared barrier was interrupted by the pig when eating. For Exp. 3, the feeding trough and the water container (outside the chamber) were equipped with weight sensors whose data was continuously transmitted and registered. Thus, feeding and drinking behavior were detected based on weight differences. For the measurement of physical activity in Exp. 3, the metabolic crate was mounted on force sensors producing an electrical signal proportional to the physical activity of the pig which was also continuously transmitted and registered.

Thermoregulation responses

In order to evaluate the thermal status of the pigs, rectal and skin temperature in both

Exp. 1 and 2, and respiratory frequency (in Exp. 2 only) were measured on certain days of periods 1 and 2. Additional rectal and skin temperature, and respiratory frequency measurements were conducted in Exp. 2 at 0, 2, 4, and 6 h after the initial increase of ambient temperature during the transition period in order to evaluate acute responses in Exp. 2. In addition, a temperature probe that was implanted within the brachiocephalic muscle also allowed continuous measurements of internal body temperature in Exp. Blood sampling and subsequent analysis of plasma hormones and metabolites were also conducted in order to evaluate physiological, metabolic, and oxidative status of the animals in different ambient temperatures in Exp. 1, 2, and 3 (Table 9). Blood sampling was done at

Results

This section consists of four chapters corresponding to each of the four experiments: Table 1. Effect of the prenatal and postnatal (growing) climatic environment on the growth performance of growing pigs. Literature cited

Chapter 2. Effect of heat stress and feeding management on growth performance and physiological responses of finishing pigs

The summary of the growth performance of the eight experimental groups of Experiment 2 is presented in Annex table 1.This chapter presents the results of Experiment 2 focusing only on the non-supplemented groups (4 out of 8 experimental groups: TN-AL, TN-PF8, HS-AL, and HS-PF8). This decision was taken because LY supplementation did not have any significant effect on the growth parameters measured and due to the unprecedented effect of the experimental design on the feeding behavior especially of the ad libitum groups (Annex figure 2). Since each of the two rooms according to the temperature treatment housed all the three feeding managements together (ad libitum, PF2, and PF8), there was a stimulation of feeding activity in the other groups (especially apparent in the ad libitum groups) each time there was a feed distribution for the PF8 pigs.

The results of Experiment 2 dealing with the non-supplemented groups were initially presented in the 71 st Annual Meeting of the European Federation of Animal Science (EAAP) in 2020 as shown in Annex 2 and were afterwards published in the Journal of Animal Science in 2020 as presented in the following pages.

Materials and Methods

Table 1. Composition of the experimental diet Table 2. Effect of HS and feeding management on growth performance and feeding behavior of finishing pigs.

Figure 1. Graphical illustration of how the area under the curve was calculated for a specific meal using the Tmuscle (°C) of the PF8 pigs and the time passed (min) after each meal provision.

Table 3. Effect of HS and feeding management on slaughter traits of finishing pigs. Introduction Climatic environment is a major problem in pig production in many countries especially with global warming and more frequent heat waves. Pigs exposed to high ambient temperature enter a state of heat stress (HS) when their heat load exceeds their ability to dissipate heat and consequently they are no longer able to maintain a constant core body temperature (1) . Genetic selection for higher feed intake and faster lean growth resulted to higher metabolic HP in modern pigs (2) making them more susceptible to heat stress especially entire male pigs due to higher basal heat production (HP) (3) and higher protein deposition associated with higher heat increment (4) . The reduced growth during HS is mainly due to the reduced feed intake as an adaptation to reduce HP related to the thermic effect of feed (TEF) (5) . Several nutritional strategies to decrease total heat load (e.g. to improve energy utilization and to reduce heat increment) have thus been studied to mitigate HS effects in livestock animals (6,7) . Nevertheless, most of these solutions aim at reducing the daily HP produced rather than considering that instantaneous body temperature results from the dynamic equilibrium between heat production and dissipation.

Indeed, HP and body temperature are not constant during the day but are closely related to variations in feeding behavior and in physical activity (3,8) in addition to a biological diurnal pattern. It is possible that rather than reducing the daily heat load, strategies to avoid saturating heat loss pathways (to limit instantaneous HP peaks and/or to improved heat losses) could help the pigs in coping better with HS. Recent studies have shown that live yeast (LY) supplementation improved HS response of pigs (9) and of dairy cows (10) possibly mediated by a modified feeding behavior changes (higher number of meals or shorter inter-meal intervals)

and by an improved energy efficiency. One theory is that this increased meal frequency might have helped by splitting the heat increment into several events of smaller amplitude rather than a small number of events of a larger amplitude. Meanwhile, reported changes in gut microbiota composition (9), and improvement in immune responses and intestinal integrity of yeastsupplemented animals (11,12) could also be linked to an improved energy efficiency.

The present study thus aimed to evaluate whether or not the improved HS response due to LY supplementation in pigs is linked to a modification of feeding behavior and/or of energy metabolism, and to determine if increasing the meal frequency can replicate the observed positive effects of LY supplementation in HS pigs.

Materials and methods

The experiment was conducted in accordance with the French legislation on animal experimentation and was approved by a Committee for Consideration of Ethics in Animal Experimentation (Authorization: APAFiS #18973-2019020622003043).

Animals and Treatments

The study was designed to investigate the effect of LY supplementation and of increasing meal frequency on finishing pigs exposed to hot conditions in a 2×3 experimental design with 2 levels of Levucell SB TITAN supplementation: 0 (diet N) vs. 100 g/ton of feed (diet Y; 1×10 6 CFU Saccharomyces cerevisiae var. boulardii CNCM I-1079/g of feed), and 3

feeding management practices called feeding windows (FW): unlimited access (Unli; 0900h to 1545h and 1600h to 0700h), 2FW of 1 h each (from 0900h to 1000h and from 1500h to 1600h), and 8FW of 15 min each (at every 90-min interval from 0900h to 1930h). The experimental diets (9.60 MJ/kg; 0.65 SID Lys:NE; shown in Table 1) were formulated to be slightly limiting in protein in order to evaluate differences in protein deposition among the groups. Titanium dioxide (TiO2) was added in the diets (3 g/kg feed) as indigestible marker.

Feed was automatically distributed twice for groups with unlimited and 2 FW (at 0855h and at 1455h) and eight times for groups with 8 FW (5 min before each feeding window). There were thus 6 experimental groups (N-Unli, Y-Unli, N-2FW, Y-2FW, N-8FW, and Y-8FW) in which feeding behavior, energy metabolism, and thermoregulation responses were measured.

In order to have 6 pigs per experimental group, 36 individually housed and fed entire male finishing pigs [Pietrain × (Large White × Landrace); 62.2 ± 1.0 kg initial live BW] were used in the experiment using two similar respiration chambers. Since only two pigs can be measured at the same time and because pigs from the experimental herd were only available every 3 weeks, the experiment was conducted in 9 replicates with 4 pigs coming from the same litter per replicate. There were 2 blocks in each replicate spaced 10 d apart and group allotment was decided to have a balanced comparison between experimental groups. For each block, two pigs were individually housed in metabolic crates and were fed the corresponding experimental diet (Table 1) during a 17-d adaptation period (Fig. 1). Thereafter, pigs were moved in a 12-m 3 open circuit respiration chamber (as described by Vermorel et al. 13 ) for 10 d of measurements allow for successive blood samplings and a temperature probe (Anipill, Body Cap, Hérouville Saint-Clair, France) was implanted 2 to 3 cm deep into the brachiocephalic muscle of the neck of the pig (method described by Renaudeau 15 ). The surgery and implantation were then followed by a recovery period during which the animals were also adapted to their diet. For the first 10 d of adaptation, the pigs were adapted to the feed and feeding distribution schedule with unlimited feed access for the first 10 d and then the feeding windows were imposed in the last 7 d of adaptation. All throughout the experimental period, relative humidity (RH) was kept constant at 65% (actual 67.1 ± 2.7%), light was turned on from 0730h to 1930h, feeder was blocked from 0700h to 0900h for animal care and samplings, and water was provided ad libitum.

Measurements and samplings

The pigs were manually weighed twice at 0830h: before entering and after exiting the respiration chambers. Live BW data were also transmitted and registered each time the pig stepped in front of the feeding through a platform equipped with a set of load sensors calibrated to provide the live BW. Feed allowance was weighed daily and refusals were weighed per period. For each of the four batches of feed fabrication, diets were sampled during feed preparation and pooled samples were taken over the experimental period. Feed and water intake (time, duration, quantity) were continuously recorded through weight sensors placed below the feeding trough and the water storage tank (outside the chamber), respectively. Total fecal collection was done at the end of the 10-d measurement in the chamber and spot fecal sampling were done twice during each period between 0800h to 0900h (d -3 and d-1 for P1 and d 1 and d 4 for P2) in order to have separate digestibility coefficients per period. The spot fecal samples were pooled per period. Urine was collected in a bucket containing 250 ml of 10% H2SO4 to prevent from microbial fermentations resulting in ammonia losses. Urine was collected, weighed, and sampled (2% of weight) daily, and the representative sample cumulated per period was stored at -20°C awaiting analyses. Nitrogen losses in the chamber recovered in the condensed water and outgoing air of the chamber were measured per period (methods as described by Noblet et al. 16 ).

Gas concentrations of O2, CO2, and CH4 of outgoing air were continuously measured with a paramagnetic differential analyzer (Oxymat 6, Siemens AG, Munich, Germany) for O2

and with two infrared analyzers (Ultramat 6, Siemens AG, Munich, Germany) for the CO2 and CH4 concentration. Gas extraction rate of the air of the chamber was measured with a mass gas meter (Teledyne Brown Engineering, Hampton, VA) and corrected for humidity content. The cage in the respiration chamber was mounted on force sensors (9104A, Kistler, Winterthur, Switzerland) producing an electrical signal proportional to the physical activity of the pig (17) .

Gas concentrations, the signals of the force sensors, the weight of the trough and water tank, gas flow rate, RH, ambient temperature, and atmospheric pressure in the respiration chamber were measured 60 times per second, averaged over 10-s intervals, and recorded for further calculations (as described by van Milgen et al. 18 ). The recovery of known amounts of CO2 and N2 was measured before and after the experiment; it averaged 99.7 and 99.9 % for CO2, and 100.2 and 99.5 % for N2 for respiration chambers 1 and 2, respectively. Temperature of the drinking water was also continuously recorded.

The implanted temperature probes in the neck muscle captured body temperature data every minute. In order to evaluate postprandial metabolism differences among the groups and between the periods, blood sampling kinetics was done on d -2 (P1) and on d 1 (P2) in which the catheter was extended out of the respiration chamber to allow blood collection with the least disturbance to the animal. Blood (7 mL) was collected in 8-mL heparin tubes twice before the first meal at 0820h and 0840h (referred to as pre-prandial), at the beginning of the meal (0 min), and then at [START_REF] Tsanaktsidou | Titanium and chromium determination in feedstuffs using ICP-AES technique[END_REF][START_REF] Ratanpaul | Review: Effects of fibre, grain starch digestion rate and the ileal brake on voluntary feed intake in pigs[END_REF][START_REF] Durmaz | Probiotic Saccharomyces boulardii alleviates lung injury by reduction of oxidative stress and cytokine response induced by supraceliac aortic ischemia-reperfusion injury in rats[END_REF]90,120,150, and 180 min after the beginning of the meal. The blood samples were placed in ice before centrifugation (3,000 g; 10 min; 4ºC), and plasma was aliquoted and stored at -20°C until analysis. Hematocrit was measured at every sampling to correct for possible dilution of blood by saline solution that saturated the catheter line.

Laboratory analyses

The sampled feed per per iod and the feed refusals were oven-dried at 103°C overnight to subsequently determine their dry matter (DM) content. Representative samples of each experimental diet per feed fabrication were analyzed for ash, starch, ether extract, and crude fiber contents according to Association of Official Analytical Chemists (19) . Feed samples and total fecal samples (one per pig) were analyzed for DM, ash, nitrogen (N; Dumas method) and gross energy, and TiO2 content (20) . Periodic fecal samples were also analyzed for DM and TiO2 content. Urine was analyzed for N content using fresh samples and for gross energy content using urine (~30 mL) freeze-dried in polyethylene bags. Ammonia content in the solution where extracted air bubbled and in the condensed water was determined on fresh material using an enzymatic method (Enzytec fluid, Scil Diagnostics GmbH, Germany).

For plasma samples, commercially available kits were used to measure plasma levels of glucose [Glucose (HK), lactate (Lactic acid, ABX Pentra, Montpellier, France), NEFA (FUJIFILM Wako Chemicals Europe GmbH, Neuss, Germany), creatinine [Creatinine (Jaffe), Thermo Fisher Scientific Oy, Vantaa, Finland], triglyceride (Triglyceride, Thermo Fisher Scientific Oy, Vantaa, Finland) and urea (Urea, Thermo Fisher Scientific Oy, Vantaa, Finland).

Inter-assay CV were 2.48%, 1.64%, 6.17%, 3.87%, 2.98%, and 2.87%, respectively. Intraassay CV were 2.73%, 1.06%, 3.42%, 1.85%, 0.72%, and 2.32%, respectively. Plasma levels of α-amino N (Protéines-kit; bioMérieux, 182 Craponne, France), insulin (ST AIA-PACK IRI, Tosoh Corporation, Tokyo, Japan), total triiodothyronine or T3 (ST AIA-PACK T3, Tosoh Corporation, Tokyo, Japan), and total thyroxin or T4 (ST AIA-PACK T4, Tosoh Corporation, Tokyo, Japan) were also determined. Intra-assay CV were 5.20%, 2.3%, 3.8%, and 3.99%, respectively.

Calculations

Dry matter intake per period was measured as the difference between total DM feed offered per period (total feed offered × % DM content) and the sum of the DM feed refused per period. Feeding behavior parameters were calculated from the continuous data collected from the feeding trough. The meal criterion is the maximal duration between 2 successive feeding bouts owing to the same meal. From preliminary analyses, increasing durations have been tested that resulted to different total number of meals. The meal criterion of the present study was determined to be 10 min because this was the minimal duration for which the resulting number of meals did not increase any longer (P>0.05). Thus, two consecutive visits separated by a time interval shorter than 10 min were considered to belong to one meal. This adopted meal criterion was also used for calculation of daily feeding behavior parameters such as number of meals, feeding duration, inter-meal interval, and rate of feed intake (RFI)(as described by Renaudeau et al. 21 ) . Due to the imposed and prolonged feed access restriction in 2FW and 8FW groups, inter-meal intervals were considered from the first meal taken after 0900h (first feeding window of the day) and the last meal taken before 0900h of the following day.

Apparent fecal digestibility of DM, N, and energy was measured per pig and was split into 2 periods based on TiO2 content in the periodic fecal samples. Nitrogen retention was calculated per period as the difference between N intake and N lost in the feces, urine, and as ammonia. The resulting N retention was multiplied by 6.25 to determine protein deposition (PD) per period. Digestible energy and metabolizable energy (ME) intake were computed according to standard methods and considering CH4 production. Total HP was calculated from respiratory gas exchanges, CH4, and urinary N (including evaporated N) according to the formula of Brouwer (22) . Energy retention (RE) was the difference between ME intake and HP. . Components of HP were partitioned into activity heat production (AHP), thermic effect of feeding (TEF), and minimal heat production (MHP) calculated using simultaneous measurements of O2 and CO2 concentrations in the respiration chamber, of force sensor signals,

and of physical characteristics of the gas in the chamber based on the modeling approach by van Milgen et al. (18) but with the modifications described by Quemeneur et al. (23) . The first day of P1 (considered as adaptation) and of P2 (ambient temperature varied during the transition on d 0 and thus it could not be determined when pigs were still in TN or already in HS conditions) were removed in the HP and feeding behavior calculations. Evaporated water and latent evaporative heat losses were calculated using the RH and temperature measurements in the chambers based on the principle described by Renaudeau et al. (24) . The energy used to warm feed and water intakes to the body temperature level was determined by multiplying the amount ingested (kg), the temperature difference of the body and ingesta (°C), and the specific heat of the ingesta (2.000 and 4.184 kJ•kg -1 °C-1 , for feed and water, respectively). Since temperature of the feed was not measured, it was assumed to be 22°C for P1 and 28°C for P2.

Sensible heat loss was assumed to be the difference between total HP and the sum of latent heat losses and the heat dissipated by warming the ingesta to body temperature level.

Partitioning of HP were all expressed relative to the metabolic BW (BW 0.60 ) and the heat dissipation parameters were afterwards expressed as %HP.

Body temperature data was averaged per hour within each period (removing adaptation, transition, and sick day/s if any) and per period. Results of plasma metabolites and hormones were adjusted in proportion to the hematocrit level measured in a particular sampling to the basal hematocrit (calculated as the mean of pre-prandial hematocrits of the pig). For postprandial plasma kinetics, the values at time 0 was considered to be the mean of the values at min -40, -20, and 0.

Statistical analyses

The number of animals (minimum 5 pigs per treatment) was determined using GLMPOWER (25) procedure (based on previous measurements of ME intake of pigs submitted to a HS challenge (24) to detect a reduction when the diet is supplemented with LY by half of the decreased ME intake under HS with a statistical power of 0.80 and a 0.05 alpha level. The experimental design allowed measuring each pig at two periods, at TN and under HS. For the statistical analyses, P < 0.05 was considered as significant, and P < 0.10 was considered as a trend.

During the experiment, one pig died (N-8FW) and two pigs got sick (1 N-8FW and 1 Y-2FW) with reasons unrelated to the treatments, and two pigs (1 N-Unli and 1 Y-Unli) also had erroneous data due to disturbances in the respiratory chamber and were thus removed from calculations and data analysis. The animal (n=31) was considered as the experimental unit. For plasma parameters, pigs with signs of fever were not included in the blood sampling thus only individuals with samples taken during both periods were included in the data analysis (n=28, and n=27 for postprandial kinetics) and for the body temperature measurement, only pigs whose probes worked and were secured properly in place throughout the experiment were included (n=29).

Growth performance, feeding behavior, fecal digestibility coefficients, components of N and energy balance, thermoregulation responses, and body temperature were summarized per period and were analyzed using the REPEATED statement of the PROC MIXED procedure [START_REF]SAS/STAT® 9.1 User's Guide[END_REF] with diet (D), feeding window (FW), period (P), and their interactions as fixed effects, and replicate and block within replicate as random effects. For contrast analysis, the SLICE or the LSMESTIMATE statement of SAS was used. LSMESTIMATE statement was used to evaluate the diet effect in general [(N-Unli, N-2FW, N-8FW) vs. (Y-Unli, Y-Table 2. Effect of live yeast supplementation and feeding window on feed and water intake, growth, and feeding behavior of male finishing pigs exposed to high ambient temperature. 1 A total of 31 pigs were allocated to 6 experimental groups in 9 replicates with 2 blocks per replicate. All pigs were housed at 22°C from d -5 to -1 (Period 1 and then at 28°C from d 0 to 4 (Period 2). FW: feeding window; N: non-supplemented diet; Y: live yeast supplemented diet.

2 Data was analyzed using PROC MIXED model with feeding window (FW), diet (D), period (P), and their interactions as fixed effects, with pig as a repeated unit per period. a,b LSmeans with different superscripts letters differ according to the experimental group and period according to D×FW×P interaction (P < 0.05). **P < 0.01, * P < 0.05, T P < 0.10. 3 Contrast statements within period (P<0.05):

 [N-Unli, N-2FW, N-8FW] vs. [Y-Unli, Y-2FW, Y-8FW];  (N-Unli vs. Y-Unli);  (N-2FW vs. Y-2FW); and ‡ (N-8FW vs. Y-8FW)] for diet effect. # [N-Unli, Y-Unli] vs. [N-2FW, Y-2FW]; || [N-Unli, Y-Unli] vs. [N-8FW, Y-8FW]; § [N-2FW, Y-2FW] vs. [N-8FW, Y-8FW]
for FW effect. 4 RSD: residual standard deviation. 5 A total of 28 pigs out of the 33 pigs in the experiment. Data was analyzed using PROC MIXED model with feeding window (FW), diet (D), day of sampling (d), and their interactions as fixed effects, with pig as a repeated unit per period. Body temperature and postprandial plasma parameters were analyzed using the REPEATED statement of the PROC MIXED procedure (25) with D, FW, P, hour (h) or time of sampling (t) within period (depending on the variable), and their interactions as fixed effects.

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Meanwhile, hourly body temperature and evaporated water within period were also compared per experimental group using the REPEATED statement with P, h, and their interaction as fixed effects. The resulting Lsmeans were then compared to the maximum body temperature (Btmax) or evaporated water per group and per period. At P1, the Btmax was recorded at h 18

for Unli, at h 15 for 2FW, and at h 19 for 8FW groups. At P2, this was at h 2, 21, 19, 15, 21, and 20 for N-Unli, Y-Unli, N-2FW, Y-2FW, N-8FW, and Y-8FW, respectively.

There was only one sampling point for plasmatic concentration of T3 and T4 therefore only D, FW, P, and their interactions were considered as fixed effects. The size of the meal at min 0 was not included in the statistical analysis of the plasma measures but was separately analysed to assess the effect of D, FW, P, and their interactions.

Postprandial plasma kinetics of insulin, glucose, lactate, α-amino nitrogen followed a bell-shaped curve and a modified Erlang model developed by van Milgen et al. (26) was adapted (Supplementary fig. 1) in order to describe their kinetics. This takes into account four parameters: initial or pre-prandial concentration in the plasma (Cinitial), the postprandial concentration after 180 min (Cfinal), the shape factor of the curve (λ), and the area under the curve above Cinital and Cfinal (AUC). Maximum concentration (Cmax) and time at Cmax (Tmax in min) were calculated from the estimated parameters as described in the aforementioned paper. Data per pig per period was fitted into the model using PROC NLIN and the resulting estimates were subjected to REPEATED measure of the PROC MIXED procedure ( 25) with D, FW, P (based on the day of sampling), and their interactions as fixed effects.

Results

Growth performance and feeding behavior

Table 2 shows the summary of the growth performance and feeding behavior traits of the finishing pigs according to their experimental group. The effect of period on DMI and live Table 3. Effect of feed access and live yeast supplementation on energy and nitrogen metabolism of male finishing pigs exposed to high ambient temperature. 1 A total of 36 pigs were allocated to 6 experimental groups in 9 replicates with 2 blocks per replicate. All pigs were housed at 22°C from d -5 to -1 (Period 1 and then at 28°C from d 0 to 4 (Period 2). FW: feeding window; N: non-supplemented diet; Y: live yeast supplemented diet.

2 Data was analyzed using PROC MIXED model with feeding window (FW), diet (D), period (P), and their interactions as fixed effects, with pig as a repeated unit per period. **P < 0.01, * P < 0.05, T P < 0.10. BW was significant with lower DM intake (-527 g DM•d -1 ; P < 0. 01) and higher mean BW (+4.0 kg BW; P < 0. 01) in P2 than in P1. Supplementation of LY increased DMI during P2 (+205 g DM•d -1 ; P = 0.03) especially for the 2FW group (+551 g DM•d -1 ; contrast P < 0.01).

For water intake, interaction between diet, FW, and period was significant. Water intake typically increased during P2 (+3,848 g DM•d -1 ; P < 0.01) albeit being statistically significant only for the Y-2FW group (+8,919 g•d -1 ; P = 0.01). For feeding behavior traits, heat exposure decreased feeding duration (-16.4 min•d -1 ; P < 0.01) and increased RFI (+4.3 g•min -1 ; P < 0.01).

Interaction between FW and period was detected for the number of meals (P = 0.02) and meal size (trend at P = 0.05): exposure to HS reduced the number of meals of pigs in Unli groups (-1.3 meals•d -1 on average; P < 0.01) but not in 2FW and 8FW groups, and also affected meal size for pigs in 2FW (-237 g•meal -1 ; P < 0.01) but not in Unli and 8FW groups. For dietary treatment contrast comparison, supplemented pigs in the Unli group tended to have smaller meal size during P1 (P = 0.07). During P2, supplemented pigs had generally faster RFI (P = 0.04) regardless of FW. LY supplementation also decreased inter-meal intervals within the Unli group (P = 0.02) and decreased feeding duration within the 8FW group (P = 0.02) in P2.

N retention and energy utilization

As shown in Table 3, period significantly affected components of N retention and energy utilization (P < 0.01) with higher fecal digestibility coefficient (+0.52 points) and AHP (+40 kJ kg BW -0.60 •d -1 ), and lower ME intake, HP, and RE (-627, -166, -460 kJ kg BW -0.60 •d -1 , respectively) in P2 than in P1. Regardless of period, FW significantly affected AHP (P = 0.04) with 8FW pigs having higher AHP than Unli pigs (+54 kJ kg BW -0.60 •d -1 on average) while 2FW pigs had intermediate values. During P1, N-Unli and Y-Unli had higher ME intake (P = 0.04 and trend at P = 0.06, respectively) and RE (P < 0.04) than N-2FW. Unli groups also had higher MHP+TEF than both 2FW and 8FW groups (contrast at P < 0.04) during this period.

During heat exposure in P2, supplemented pigs had generally higher ME intake (P = 0.03) with the difference most pronounced between the 2FW groups (+570 kJ kg BW -0.60 •d -1 ; P < 0.01) but only numerical in the Unli pigs (+245 kJ/ kg BW -0.60 •d -1 ; P = 0.16). Supplemented pigs also had higher RE (P = 0.01) within the Unli (+204 kJ kg BW -0.60 •d -1 ; trend at P = 0.09) and the 2FW group (+461 kJ kg BW -0.60 •d -1 ; P < 0.01) but not in the 8FW group. LY supplementation also increased fat deposition (P = 0.02) but, like for ME intake, it was only significantly in the 2FW groups (+149 g•d -1 ; P < 0.01) and was only numerical in the Unli groups (+54 g•d -1 ; P = 2 Data was analyzed using PROC MIXED model with feeding window (FW), diet (D), period (P), and their interactions as fixed effects, with pig as a repeated unit per period.**P < 0.01, * P < 0.05, T P < 0.10. a,b,c,d,e,f LSmeans with different superscripts letters differ according to the experimental group and period according to D×FW×P interaction (P < 0.05). 3 Contrast statements within period (P<0. 0.16). Nevertheless, supplemented pigs had higher protein deposition regardless of FW (+18 g•d -1 ; P = 0.03) and most notably for pigs in the Unli group (+28 g•d -1 ; trend at P = 0.06).

Themoregulation responses

Table 4 shows the thermoregulation responses related to heat dissipation (expressed as %HP) and the average body temperature of the pigs per period. Latent evaporative heat loss represented 30% of HP and body temperature averaged 38.5°C in TN conditions. When exposed to high ambient temperature in P2, these increased (P < 0.01) to 56% of HP and to 39.0°C, respectively. Under HS, proportion of heat dissipated via the sensible route and used to heat the ingested feed to body temperature level decreased (P < 0.01) from 67 to 41% and from 0.44 to 0.25% of HP, respectively, while proportion used to heat ingested water to body temperature level increased from 1.9 to 2.5% HP (P < 0.01). Latent evaporative water losses increased on average from 2,537 to 4,272 g•d -1 (P < 0.01). Based on the contrast analyses in P2, LY-supplemented pigs had higher proportion of heat lost via evaporative route in the 2FW group (P = 0.04), but it was the opposite for those in the 8FW group (P = 0.02).

Figure 2 and 3 shows the pattern of hourly body temperature and evaporative water loss, respectively, for the experimental groups. There was a distinct diurnal variation of body temperature in Unli pigs (Fig. 2a) during P1 with peaks at around 0900h and 1400h, and this variation was more pronounced in the 2FW (Fig. 2b) and 8FW (Fig. 2c) groups. In the 2FW groups, during P1, body temperature peaked to similar level (BTmax) at 0900h and at 1500h (P > 0.10), whereas in 8FW groups, BTmax was measured in the evening with a significantly higher body temperature peak than the body temperature in the morning (P < 0.05). Meanwhile, latent evaporative water loss pattern was not correlated to the body temperature pattern and remained relatively constant throughout the day during P1 at around 131 g•h -1 .

Compared to P1, the diurnal variation of body temperature in Unli pigs was less apparent in P2 wherein body temperature was at BTmax 80% of the day (P > 0.10). During this period the body temperature in the 2FW groups remained at BTmax 5 h after the first meal in non-supplemented pigs and 2 h for the supplemented pigs, and both groups were at BTmax for 7 to 8 h after the second meal. Body temperature of N-2FW pigs was higher in P2 than in P1 throughout the day, while that of Y-2FW pigs were similar across period except during nonfeeding times when it was higher (at hours 0 to 9, 14, and 23 (P < 0.05)). For 8FW groups during P2, body temperature was at BTmax for 11 h (1400h to 0000h) in non-supplemented pigs while it was for 8 h (1700h to 0000h) for the supplemented pigs. In contrast to P1, pattern of evaporative water loss closely followed time of feeding and body temperature pattern during P2, except for the Y-8FW group for which it remained relatively constant compared to the other groups. Nevertheless, evaporative water loss increased for all groups in P2 (P < 0.05). In the 2FW groups, evaporative water losses was higher in the supplemented pigs starting from 0900h to 0000h (P < 0.05) while it was the opposite in the 8FW groups (higher in the nonsupplemented pigs from 1100h to 2100h (P < 0.05)). The maximum evaporated water was recorded for N-8FW pigs for 9 h starting at 1500h (1 h after they attained BTmax), while it was recorded only for 5 h for the Y-8FW starting at 2000h (3 h after they attained BTmax).

Plasma metabolite and hormone concentrations

The average meal size at time 0 of blood sampling on d -2 (P1) and d 1 (P2), shown in Table 2, followed a similar trend as the respective meal size reported per period (described in a previous subsection). The meal size was significantly different only on d 1 (P2) for 2FW pigs (+335 g for Y pigs; P = 0.04), while the difference was only numerical for Unli pigs for P1 and P2 (-264 g on average for Y pigs; P ≤ 0.15). Results of the postprandial insulin, glucose, lactate, and α-amino nitrogen are presented in Fig. 4 and the description of their kinetics is found in Supplementary table 1. Exposure to HS increased Cfinal (trend at P = 0.06) and Cmax (P < 0.01) for plasma glucose, and decreased AUC (trend at P = 0.06) and Cmax (P = 0.04) for plasma αamino nitrogen, but did not have a significant effect on plasma insulin parameters. The effect of FW on plasma insulin kinetics was significant with 2FW pigs having higher AUC on average than Unli and 8FW pigs during P1 (P ≤ 0.04), and than 8FW during P2 (P = 0.04), and higher Cmax than 8FW regardless of the period (P ≤ 0.03). Plasma insulin AUC and Cmax were generally lower in LY-supplemented pigs within the Unli groups (trend at P ≤ 0.08) during P1 and within the 2FW groups during P1 and P2 (P ≤ 0.04). Supplemented pigs also showed lower Cmax for plasma lactate (P = 0.02) and glucose (P = 0.06) than non-supplemented pigs in P1, especially for those in the 2FW group (P = 0.02). Between the 2FW groups, supplemented pigs also had lower AUC for lactate (P = 0.03) and lower Cmax for α-amino nitrogen (P = 0.03) in P1.

Figure 5 shows the postprandial plasma concentrations of urea, creatinine, and NEFA.

Regardless of the experimental group, there was an increase (P < 0.01) in plasma levels of creatinine and urea when exposed to hot conditions. The time of sampling was significant (P < 0.01) for the three metabolites with plasma level of urea increasing while those of creatinine and NEFA decreasing after a meal. There was an interaction between diet and FW wherein plasma levels of creatinine and NEFA were lower for supplemented pigs in 2FW group (P < 0.05) but not in other FW groups. The results of the analysis of plasma hormone T3 and prohormone T4 are presented in Fig. 6. Exposure to high ambient temperature resulted in a decrease in plasma T3 and T4 (P < 0.01). Regardless of the period, plasma levels of T3 tended to be affected by FW (higher in 2FW than Unli pigs; P = 0.08) while the diet significantly affected the level of plasma T4 (higher in supplemented diet; P = 0.02). The line (-) corresponds to the predicted values at minute (time; t) obtained with the non-linear model (Supplementary figure 1).

Discussion

The present study aimed to evaluate whether the improved HS response of pigs supplemented with S. cerevisiae var boulardii results from a modified feeding behavior and/or energy metabolism and if these responses can be replicated by imposing an increased meal frequency.

Effect of high ambient temperature in non-supplemented pigs fed ad-libitum

As homeothermic animals, pigs strive to maintain a constant body temperature regardless of the environmental conditions (27) . In high ambient temperature, heat loss via sensible routes becomes limited due to the lower temperature gradient between the body surface and the environment, thus pigs relies more on the latent evaporative route for losing heat. In the present study, latent heat losses accounted for 56% of HP in N-Unli pigs kept in HS conditions in agreement with the 60% already reported by Renaudeau et al. (24) for pigs kept at 33°C and 75% RH. Body water balance is vital for thermoregulation (28) thus evaporative losses must be replenished explaining the increased water consumption during P2. The water intake which increased by 274% from P1 to P2 (if expressed as L water intake/kg DMI) also served other heat loss purposes in hot conditions as indicated by the higher HP proportion used to warm ingested water to body temperature level.

Whereas water intake increased, DMI decreased in the N-Unli pigs by -753 g•d -1 (or -30%) to reduce metabolic HP related to nutrient utilization (TEF) (29) . This value is higher than the -400 g DM•d -1 reported in the literature (30) probably due to the pig's limited ability to adapt to the short duration of the heat challenge in the present study. This drop in feed intake was related to an altered feeding behavior such as less time spent in the feeder, faster RFI, and decreased number of meals (31,32) and resulted to lower HP, RE, and thus lower nutrient deposition. The reduced HP in P2 also accounts for the decreased thyroid hormones of HS pigs in the present and in previous studies (33,34) . Heat exposure did not affect plasma insulin levels in this study in contrast to previous reports (8,35) , but it slightly increased glucose levels suggesting reduced insulin sensitivity in agreement with previous studies (36) . It can be hypothesized that dietary glucose is a less preferred energy source in hot conditions (37) which could partly explain the need for higher muscle breakdown during HS indicated by higher postprandial creatinine and urea levels in P2 (38) .

Effects of live yeast supplementation in pigs fed ad libitum and subjected to heat stress Between the two Unli groups, LY supplementation attenuated HS effects on DMI reduction (-20% in Y-Unli vs. -30% in N-Unli) that tended to improve energy efficiency (+5.5 points) and PD (+26 g•d-1) in agreement with Labussière et al. (9) . Although effect of LY on meal frequency was not significant, shorter inter-meal intervals were observed in supplemented pigs in the present study similar to findings in dairy cows (39) . Differences in body temperature and postprandial responses between the Unli groups were only numerical and not clearly differentiated, possibly due to indeterminate feeding times and the fact that body temperature and insulin levels are known to also depend on the characteristics of the previous meal (meal size and when the last meal was taken). Nevertheless, this study confirms improved feed intake and energy efficiency in LY supplemented pigs under HS but in contrast to the results published by Labussière et al. (9) , these improved performances were not clearly related to changes in feeding behavior.

The following subsections focuses on the FW-restricted groups with the aim to look at the effects related to different meal frequencies and to LY supplementation. As showed in the present study, HP and body temperature are not constant during the day hence, maintaining a thermal balance between heat production and loss must also be a dynamic process. Thus, this theory is further explored with a particular focus on the dynamic changes in the thermoregulation and physiological responses during the day of pigs subjected to high ambient temperature.

Effect of different meal frequencies in non-supplemented heat stressed pigs

In the present study, the 2FW strategy worked to drive meal frequency down to ~2 meals a day. However, the 8FW strategy did not achieve the expected increase in the number of meals (N-8FW and N-Unli had similar number of meals per day). As undigested nutrients in the ileum can trigger ileal brake which signals satiety, and inhibits gastric emptying and feed intake (40) , the imposed 90-min meal interval may not have been enough time for satiety signals to end prompting the N-8FW to skip a meal or two. Nonetheless, N-8FW still had higher meal frequency than the N-2FW pigs in the present study.

In TN conditions, body temperature pattern closely followed feeding pattern, as highly evident in the 2FW and 8FW pigs. The immediate postprandial increase in body temperature was probably due to both the instantaneous AHP (feeding activity) and the TEF-related heat (energy needed for digestion, absorption, and from metabolic processes). Pre-and postprandial body temperature also increased with each additional meal (more gradual in N-8FW than in LSmeans within a row with different superscripts letters differ (P < 0.05) according to the experimental group.

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N-2FW) possibly from the cumulative heat increment following each meal. However, evaporative water loss remained relatively constant throughout the day and was clearly dissociated with body temperature, suggesting minimal need for latent heat losses as the sensible route was probably sufficient in TN conditions to maintain constant core body temperature immediately after a meal.

The observation that body temperature 1) remained at Btmax over a longer period after a meal in N-2FW pigs and 2) reached Btmax earlier in P2 than in P1 (1400h vs. 1700h) in N-8FW pigs, suggest that the ability to maintain thermal balance after a meal seemed to decrease upon heat exposure. Based on the higher pre-prandial body temperature of pigs in P2 than in P1, heat load was already high even before feeding. This result confirms previous data suggesting a reduced pig's ability to lose TEF-related heat in hot conditions (41) . In fact, as evaporated water loss pattern closely followed the feeding time and the body temperature peaks during HS, sensible heat losses seemed to be no longer sufficient and consequently pigs relied more on the latent route in P2. It is unclear why Btmax was maintained longer in P2 even during non-feeding times. One hypothesis could be related to the reported decrease of gastric emptying rate in HS conditions (42) possibly due to the restricted splanchnic blood flow during HS or because these processes entail energy and thermal load. Indeed, hyperthermia and its possible consequences on gastric emptying might have prolonged satiety signals which may have caused the observed longer inter-meal intervals in HS than in TN conditions in the present study. This can also help explain the exacerbated HS effects in the N-2FW pigs compared to the N-8FW pigs in terms of reduction of DMI and energy efficiency (-30% vs. -17% and -17.5 vs. -6.5 points, respectively). Large meal size and high insulin levels have been reported

to delay not just gastric emptying but also carbohydrate absorption (43,44) . Moreover, as insulin promotes glycolysis, it may have caused the prolonged and elevated lactate levels observed in N-2FW pigs from excess pyruvate production.

The N-8FW pigs in the present study seemed to have had better heat tolerance when compared to the other non-supplemented groups especially regarding ME intake and RE. This result did not seem to be related to the increased meal frequency per se but might be related to the imposed feeding management in this study. First, the imposed prolonged feed access restriction during the night (thus no further TEF-related HP) might have helped HS pigs to lower their body temperature. Indeed, a faster decrease in gastrointestinal temperature was reported when feed was withdrawn in pigs subjected to acute hyperthermia (45) . In addition, the higher water intake and evaporative heat losses observed in the N-8FW pigs when compared to the N-2FW pigs would also explain their better heat tolerance. Between these two groups, the imposed increased meal frequency may have stimulated the pigs to drink more water with each meal since pigs are "prandial drinkers", i.e. their drinking bouts are stimulated by feed ingestion (46) . As a result, the higher water intake may have contributed to the increased latent heat losses of N-8FW pigs since an animal's hydration status is closely linked to its evaporative heat loss capacity in order to maintain body water balance (47) . These mechanisms might have helped the N-8FW pigs to maintain a lower body temperature during most parts of the day (which was not evident when looking only at the average daily body temperature)

and thus allowed them to eat more in hot conditions compared to N-Unli and N-2FW pigs.

Despite the relatively higher RE measured in N-8FW compared to the other N groups during P2, their PD did not differ from N-Unli pigs and did not reach the highest PD observed for the Y-Unli group. The balance between protein synthesis and breakdown (protein turnover)

regulates whole body PD. Even if protein breakdown is reduced during fasting, protein synthesis rate remains comparatively lower resulting to protein loss (48) which is in line with the lower net PD observed in the present study for pigs subjected to extended feed withdrawal periods (2FW and 8FW) compared to the Unli pigs in TN or HS conditions. However, it does not explain the +35 g•d-1 higher fat deposition of the N-8FW compared to N-Unli pigs.

Protein deposition entails high thermal load thus capacity of pigs to reach their potential for PD is usually limited in hot conditions (49) . Since the N-8FW pigs already reached Btmax early at h 14 during HS conditions, it can be assumed that additional ME intake from here onwards was deposited as fat since the animals needed to maintain homeostasis, and fat deposition has a lower thermal load than that of protein deposition.

This present study therefore demonstrates that the thermal balance is a dynamic process highly influenced by the feeding level and behavior because of the large contribution of TEF and feeding-related AHP to instantaneous increases in HP. Feeding large meals impairs ability of pigs to cope with HS possibly related to combined changes in gastric emptying, energy metabolism, and saturation of heat loss pathways. The results of the present study also demonstrates that pre-prandial body temperature is a contributing factor, together with the ability of the animal to lose heat after a meal, to improve feed intake and protein deposition in hot conditions.

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Effect of live yeast supplementation in pigs subjected to different meal frequencies during heat stress

As discussed above the improvement in pig performance under hot conditions could not be associated to an increase in meal frequency in the present study. Comparing the N-8FW pigs to LY-supplemented pigs, the increased DMI during HS in the former group was not to the same extent as the latter (Y-Unli), neither did it result to an increase in PD. Furthermore, based on RE and PD measured in P2, supplemented pigs with low meal frequency (Y-2FW) tolerated heat better than N-8FW pigs. However, it remains unclear why Y-8FW pigs did not perform as well as the Y-Unli. As already mentioned, the feeding management employed in the present study may have been an oversimplified sense of "frequent feeding" in supplemented pigs. Duration of inter-meal intervals were similar among the supplemented groups in P2 (average 240 min), thus under hot conditions, FW did not affect the ability of supplemented pigs to start a new meal after the last one has ended. Nevertheless, Y-8FW still deposited more protein than the N-8FW despite having lower ME intake. The Y-8FW pigs attained Btmax at 1700h whether in TN or HS conditions, which means their reduced PD in P2

was not due to the effect of HS on PD potential, but rather due to lower feed intake. This inclination to prioritize and use higher proportion of ME intake for protein instead of lipid deposition has been observed in restricted-fed pigs in TN conditions (50) . One can thus hypothesize that Y-8FW pigs were experiencing a lower level of heat stress than N-8FW as supported by their relatively constant evaporative water loss in P2 suggesting that the sensible route was still their main route for maintaining a thermal balance. Hence, this raises the possibility that the positive effects of LY-supplementation might be related to improved thermoregulation responses either by reducing heat load or by dissipating heat more efficiently.

Pigs are reported to change feeding behavior with more meals taken during the cooler parts of the day in studies with cyclic or natural heat stress challenges (51,52) . Thus, the idea behind imposing a low meal frequency (2FW groups) in an environment with a constantly high ambient temperature was to increase the severity of the heat challenge by preventing pigs to modify their feeding behavior as an adaptation response. As demonstrated in the previous subsection, this resulted to inability of non-supplemented pigs to cope with HS. However, this was not the case with the Y-2FW group that showed the least severe reduction in DMI and energy efficiency (-10% and -3.6 points, respectively) which also translated into higher RE and PD. The principal differences between the 2FW groups seem to be linked to energy metabolism Results

105

(linked to insulin response), and latent heat loss capacity. In terms of energy metabolism, supplemented 2FW pigs showed enhanced insulin sensitivity. A yeast extract from S.

cerevisiae has already been reported to potentiate insulin action (53) and S. boulardii has been demonstrated to attenuate diabetic-related complications such as hyperglycemia (due to impaired insulin activity or production) by microbiota modulation and improved immune responses (54-56) . High insulin levels are associated with elevated lactate thus the improved insulin sensitivity in supplemented pigs may have contributed to their higher energy efficiency especially since lactate requires energy to be reconverted back to glucose before it can be used for nutrient deposition. Increased insulin sensitivity was also found to decrease de novo lipogenesis (57) which may also partly explain the higher PD of supplemented pigs especially in hot conditions. Nevertheless, these are just few hypotheses since insulin is a key hormone in many metabolic processes not only in pigs but also in humans and in other livestock species.

Thus, an improved insulin sensitivity may have caused many other cascades of reactions that contributed to the higher energy efficiency and thermoregulatory responses of supplemented pigs during HS.

As previously mentioned and as shown in the 2FW groups a low pre-prandial body temperature is important in alleviating effects of HS on feed intake. The lower pre-prandial body temperature and higher evaporative heat loss and water intake of Y-2FW pigs enabled them to decrease their body temperature immediately after a meal and maintain a relatively similar meal size in both periods, in contrast to the N-2FW pigs. The hypothesized impact of high insulin levels and high heat load on gastric emptying might have thus been mitigated by LY during hot conditions. This could further explain the shorter time spent at Btmax and their slightly shorter inter-meal intervals (-44 min•d -1 ) compared to N-2FW pigs. This shorter intermeal interval in Y-2FW pigs might have indirectly promoted water intake by prompting them to drink more often because of the prandial drinking behavior of pigs (46) . Thus, the improved evaporative heat loss in the supplemented 2FW pigs may have been partly due to their higher water intake because an improved latent heat loss capacity is closely linked to a better hydration status. Another hypothesis on the improved evaporative heat loss capacity of supplemented pigs might be related to the gut-lung axis theory (58) and the modulatory effects of S. boulardii on gut microbiota especially in HS conditions (9) . S. boulardii supplementation during inflammatory challenges has been previously reported to alleviate lung injury and oxidative stress (59,60) but there are currently no studies on LY effect on respiratory health during heat stress. Nevertheless, with a more efficient ability to dissipate heat via the latent pathway after a meal, one can assume that Y-2FW pigs had to rely less on a reduction in metabolic HP (alongside their T4 levels) to maintain homoeothermic status during hot conditions. Thus, the supplemented 2FW pigs were able to maintain higher DMI and potential for PD during HS which is also reflected in their lower plasma creatinine as there was less need to mobilize body protein for added energy. Finally, it is possible that the positive effects of LY during HS were most evident in the 2FW groups due to the increased severity of the heat challenge on these groups compared to other FW treatments.

Conclusion

To conclude, the present study demonstrates that improved heat tolerance of pigs lies in their ability to maintain the dynamic equilibrium between heat production and loss throughout the day. Live yeast supplementation increased insulin sensitivity and heat loss efficiency allowing the pigs to eat more (via shorter inter-meal intervals or increased meal frequency) and thus increasing their energy efficiency during heat stress. The lower thermal load in supplemented pigs helped them reach a higher maximum potential for protein deposition, which is otherwise limited in hot conditions. Imposing an increased meal frequency does not replicate the positive effects of LY supplementation during HS because the altered feeding behavior is not the cause but a mere consequence of the improved energy metabolism and thermoregulation responses of LY supplementation in pigs.

Chapter 4. Effect of sex and live yeast supplementation in heat stressed pigs

This chapter presents the results of Experiment 4 that aimed to evaluate the effect of biological sex and LY supplementation in HS pigs.

Introduction

Reduced growth performance of pigs during heat stress (HS) is mainly induced by a decrease in feed intake (FI). Live yeast (LY) supplementation was reported to improve HS responses by increasing FI and feed efficiency in entire male (EM) pigs (Labussière et al., 2021) but this has not been confirmed in females (F) (Chapter 2). Indeed, a sexual dimorphic effect of HS has been previously reported with barrows having lower feed efficiency than gilts during HS (Renaudeau et al., 2006) but no studies to date regarding the difference between EM and F. Thus, the objective of the study was to determine the effect of sex and LY supplementation in acclimated HS pigs.

Materials and methods

A total of 64 individually housed finishing pigs (32 EM and 32 F;80.8±0.9 kg BW) were used to evaluate the effects of sex and LY supplementation in HS pigs with 2 successive replicates. There were 2 levels of LY supplementation: 0 (N) vs. 100 g/ton of feed (Y; 1×10 6 CFU/g of feed of Saccharomyces cerevisiae var. boulardii CNCM I-1079) given to EM or F pigs. Thus, there were four experimental groups: N-EM, Y-EM, N-F, and Y-F. The pigs were equally distributed to two experimental rooms based on the treatments. Ambient temperature of both rooms were maintained at 22°C for 7 d and then at 32°C for 3 d during the adaptation period before the 27 d experimental period at 32°C. Feed and water were provided ad libitum. Growth and slaughter performance were measured. The pig was considered as the experimental unit and data was analyzed using PROC MIXED model with replicate I, sex (S), LY supplementation (D; diet), their interactions as fixed effects, and with litter origin as random effect.

Results and discussion

Table 1 shows the summary of the growth and slaughter performance. Replicate had a significant effect on live and slaughter BW wherein pigs in replicate 2 were heavier than pigs Table 1. Effect of live yeast supplementation and sex on growth and slaughter performance of finishing pigs exposed to high ambient temperature. 1,2

Rep 1

Rep 2 1 A total of 64 pigs were allocated to 4 experimental groups based on sex and live yeast supplementation in 2 successive replicates. Experimental period started after a 3-day acclimation of at 32°C was thereafter maintained for 28 d. N: non-supplemented diet; Y: live yeast supplemented diet; F-female; EM: entire male 2 Data was analyzed using PROC MIXED model with replicate (R), diet (D), sex (D), room (r), and their interactions as fixed effects and litter origin as random effect. **P < 0.01, * P < 0.05, a,b,c LSmeans within a row with different superscripts letters differ (P < 0.05) according to the experimental group.

Items N -EM Y -EM N -F Y -F N -EM Y -EM N -F Y -F RSD 3 Statistics Animals, n 8
3 RSD: residual standard deviation.

in replicate 1 (P<0.05). Regardless of the replicate, EM pigs were heavier and had higher average daily gain (ADG) than F pigs (P<0.05) and thus had higher final and slaughter BW (P<0.05). In TN conditions, boars were reported to grow faster and have higher feed efficiency than gilts (Blanchard et al., 1999;[START_REF] Ruiz-Ascacibar | Impact of amino acid and CP restriction from 20 to 140 kg BW on performance and dynamics in empty body protein and lipid deposition of entire male, castrated and female pigs[END_REF]. However, the improved efficiency in EM in this study was only observed on lighter (replicate 1) but not on heavier (replicate 2) pigs with significant interaction of replicate and sex on gain:feed of the pigs (P<0.05). This suggests that the negative effect of HS on feed efficiency is heightened in heavier EM but not F pigs probably due to exacerbated effect of HS on their ability to reach their maximal potential for protein deposition or growth (Le Bellego et al., 2002). This might be attributed to higher susceptibility of heavier pigs to HS (Renaudeau et al., 2011) coupled with a higher heat production of EM pigs due to their higher growth potential and protein deposition [START_REF] Moughan | A genetic upper limit to whole-body protein deposition in a strain of growing pigs[END_REF]Labussière et al., 2013). There was a significant interaction between replicate, diet, and sex for average daily feed intake (ADFI) expressed per BW 0.60 and ADG (P<0.05). Supplemented EM pigs in replicate 2 had higher ADFI per BW 0.60 (P<0.05) than supplemented EM pigs in replicate 1 and F pigs in replicates 1 and 2, and higher ADG than F pigs in replicate 1 and supplemented F pigs in replicate 2. Meanwhile, supplemented EM pigs only tended to have a higher ADFI per BW 0. 60 (P=0.08) than non-supplemented EM pigs in replicate 2. While this was only a tendency, this suggests that LY supplementation seemed to improve ADFI during HS but only in heavier EM pigs. Indeed, due to the high standard deviation in the ADFI response of the pigs in the study, it would have needed ~400 pigs to detect significance difference of P=0.05 between the diets according to PROC GLMPOWER. Thus, based on our results, the effect of LY supplementation on HS responses of pigs seem to depend on the BW and sex of the animal.

Conclusion

Entire male pigs had higher growth performance than female pigs in HS conditions.

Meanwhile, the effect of sex on feed efficiency (higher in EM than F pigs) seem to depend on the BW of the pigs (not observed in heavier pigs). LY supplementation also improved ADFI of HS pigs but only in heavier EM pigs. Thus, the effect of HS and of LY supplementation during HS seem to depend on the age or BW and on sex of the pig. Renaudeau, D., J. L. Gourdine, and N. R. St-Pierre. 2011. A meta-analysis of the effects of high ambient temperature on growth performance of growing-finishing pigs. J. Anim. Sci. 89.

Literature cited

Ruiz-Ascacibar, I., P. Stoll, M. Kreuzer, V. Boillat, P. Spring, and G. Bee. 2017. Impact of amino acid and CP restriction from 20 to 140 kg BW on performance and dynamics in empty body protein and lipid deposition of entire male, castrated and female pigs. Animal 11.

General discussion

Figure 24. Bullet point summary and conclusions of the chapters in the thesis

General discussion

The objective of this thesis project was to contribute to the understanding of HS effects in pigs and to evaluate the ability of some feeding strategies to mitigate them. A total of 4 experiments were conducted to:

• to investigate the effects of a chronic prenatal heat challenge,

• to distinguish direct and indirect HS effects,

• to determine whether or not positive effects of LY supplementation were linked to a modified feeding behavior,

• to evaluate possible interactions between LY supplementation and sex in HS pigs, and

• to further understand the dynamic process of how body heat balance (body temperature) changes and is maintained throughout the day within a narrow range.

The main results of the four experiments have already been extensively discussed in the previous chapters and summarized in Figure 24 and Table 10. Thus, this discussion will focus, first, on the possible sources of variation in HS responses among experiments. The second part discusses the dynamic responses of pigs to maintain their homeothermic body temperature in HS conditions and how these responses are influenced by the feeding behavior of the animals.

The third part focuses on the possible links among latent HL, water intake, feeding frequency, and insulin sensitivity that makes them important during HS. Finally, the fourth part discusses the possible mechanism of LY during HS in pigs.

Possible sources of variation in the severity of heat stress among experiments

The different heat challenge models employed in our experiments elicited different severity in HS responses. One illustration is the differences in ADFI drop of ad libitum-fed pigs as shown in Figure 25. In this section, some factors that might contribute to the differences among experiments are discussed.

Animal related factors

As mentioned in the literature review, heavier pigs are more susceptible to HS (Quiniou et al., 2000a;Renaudeau et al., 2011) due to their higher HP and their lower surface area to mass ratio that lowers their ability to dissipate heat. Our results agree with these conclusions wherein the effect of HS on ADFI was less important in Exp. 1 (-7%) in which pigs were lighter had lower feed efficiency than gilts during HS (Renaudeau et al., 2006b) while others reported higher susceptibility of gilts to heat-induced oxidative stress [START_REF] Rudolph | The effect of Mitoquinol (MitoQ) on heat stressed skeletal muscle from pigs, and a potential confounding effect of biological sex[END_REF][START_REF] Wesolowski | Heat stress increases mitochondrial complex I capacity in female pigs but favors reliance on complex II in males[END_REF]. Meanwhile in Exp. 4, heavier entire male pigs appeared to be more susceptible to HS. While feed efficiency is reported to be higher in entire males compared to females up to 140 kg BW in TN conditions (Ruiz-Ascacibar et al., 2017), we only observed this sex effect in relatively lighter pigs (initial BW of ~77 kg) but not in heavier pigs (initial BW of ~85 kg) subjected to HS. This lack of sex effect in heavier pigs must be because of a negative effect of HS on entire males but not in females at this BW. As discussed in the literature review, animals with higher growth potential are less able to tolerate HS (Renaudeau et al., 2006b;[START_REF] Rauw | Effects of diet and genetics on growth performance of pigs in response to repeated exposure to heat stress[END_REF][START_REF] Gourdine | The genetics of thermoregulation in pigs: A review[END_REF]. Thus, heightened effect of HS might be related to both the heavier BW (Renaudeau et al., 2011) and to the higher PD and fasting HP of entire male pigs [START_REF] Moughan | A genetic upper limit to whole-body protein deposition in a strain of growing pigs[END_REF]Labussière et al., 2013). This assumption has to be confirmed in further studies.

As another animal factor, early life fetal HS experience of the pig can also influence their future HS responses particularly their performance and their body composition (as reviewed by [START_REF] Johnson | Physiology symposium: Postnatal consequences of in utero heat stress in pigs[END_REF]and Liu et al., 2021). In addition, our results (Exp.

1) demonstrated that a chronic prenatal heat exposure resulted to a decreased ability of the pig to cope with a subsequent heat challenge in their postnatal life. Results from Exp. 1 also shows that even microbiota composition of growing pigs exposed to HS (same growing environment) tended to be influenced by their prenatal environment (Annex 3). Although there are increasing in utero HS studies, the repercussions of a prenatal HS are still largely unknown and remain to be explored. For example, our study only looked at a chronic (whole gestation) heat challenge but literature seems to suggest that different factors can even influence prenatal HS effects such as the previous HS experiences of the sire and the sows, the duration and pregnancy stage of the heat challenge (Boddicker et al., 2014), the physiological stage of the pig in their postnatal life (Johnson et al., 2015a;Johnson et al., 2015b), and their postnatal thermal environment (Exp. 1) among many others. Meanwhile, in our study (in Exp. 2, 3, and 4), we were not able to detect any prenatal effect of HS.

Housing condition, nature, and severity of the experimental heat challenge

Differences can also arise according to the type of housing conditions. Among the experiments, the highest drop in feed intake observed in Exp. 3 (-29%) indicates that HS level in the respiratory chambers at only 28°C of ambient temperature was greater than in larger rooms with even higher ambient temperatures. With a smaller confinement space and highly isolated equipment, radiant heat exchange which is dependent on distance between two objects [START_REF] Berman | Radiant heat loss, an unexploited path for heat stress reduction in shaded cattle[END_REF], i.e. between the pig and its surrounding environment, may have been higher in the respiratory chamber thus eliciting a stronger HS response than in the other experiments. The drop in ADFI in Exp. 3 (-753 g•DM•d -1 in ad-libitum fed pigs) was comparable with another study in the respiration chamber (-730 g DM•d -1 ) but using lighter pigs with higher ambient temperature (~50 kg; 32°C) (Renaudeau et al., 2013). Meanwhile, the ADFI drop in our study was higher than in another study using 100-kg finishing pigs (-410 g DM•d -1 ) (Labussière et al., 2021). This difference is probably explained by the more sudden increase of ambient temperature in our study compared to the aforementioned study (increase from 22 to 28°C in 3 h vs. in 2 d, respectively) which gave less chance for the pigs in our study to acclimate to the temperature increase.

Additionally, the nature of the heat challenge may influence the severity of observed HS responses. While the lower ADFI drop in Exp. 1 was probably related to the animal's lower BW, it might also be linked to the fact that a cyclic heat challenge was used compared to the constant ambient temperatures in the other experiments. A cyclic heat challenge was reported to allow pigs more opportunities to increase feed intake during the cooler parts of the day as reported previously (Xin and DeShazer, 1991;Quiniou et al., 2000c;[START_REF] Patience | Nutritional and physiological responses of growing pigs exposed to a diurnal pattern of heat stress[END_REF]. In fact, this was precisely why a constant heat challenge was chosen in our experiments because feeding behavior was of particular interest in this thesis (Exp. 2 and 3). Nevertheless, housing conditions influenced our results, specifically in Exp. 2 where, even if pigs were individually housed, feeding behavior of ad libitum-fed pigs were unintentionally modified as a result of stimulation from hearing or seeing pigs in other pens with different feeding distributions (Annex 4). Finally, our results emphasize the importance of housing conditions in evaluating HS effects and that, in line with this, the measured responses for one study may only be specific to the particular experimental conditions for that experiment.

Implications

Many factors influence how pigs respond to a heat challenge and a higher ambient temperature does not always necessarily mean a more severe HS response. The different heat challenge models used in this thesis project demonstrated that these differences might be related to the animals (at least BW, sex) and the housing conditions (room characteristics, stocking density, possible interactions among the pigs, heat challenge used). Additional complex factors that might cause differences in pig response to HS are certainly linked to the pig's prenatal thermal environment since there is still little known about its consequences on the postnatal life of the pig. Nonetheless, these factors should be considered when designing future HS experiments. Future studies might also be needed to elucidate the sexual dimorphic responses to HS in pigs taking into account its possible interaction with BW and age. On the other hand, while prenatal pig studies can be more complicated to design and very expensive, there are still many things to consider in furthering our understanding about the factors at play in a prenatal heat challenge such as the previous HS experiences of sires and sows, the duration of the challenge, the pregnancy stage, and even the possible interaction with the sex of the progeny.

Feeding behavior and its influence on dynamic regulation of body heat balance

One of the key ideas confirmed in this thesis project is related to the dynamic response of pigs in their attempt to maintain a thermal equilibrium (or relatively constant body temperature) during the day. With the pig being in constant heat exchange with its environment and experiencing variations in metabolic HP throughout the day, heat storage (difference of HP and HL) is never constant as reflected by the fluctuations in body temperature. While this is not a new concept, only few studies in pigs focused on the dynamic HS responses within the day (Xin and DeShazer, 1991;Cervantes et al., 2018). Owing to the continuous measurements of body core temperature in Exp. 2 and 3 and HP and HL components from the respiratory chambers in Exp. 3, our results have concretely demonstrated that HP, body temperature, and proportions of sensible and latent HL of growing pigs fluctuates during the day depending on their feeding behavior and ambient temperature. In this subsection, we will describe first how feeding behavior and meal characteristics influence the dynamics of body heat balance in TN conditions and then how this regulation is impacted when pigs are exposed to HS.

Thermoneutral conditions

In agreement with Labussière et al. (2013), a large part of HP variations observed in Exp. 2 and 3 in individually housed pigs was due to heat associated with the feeding activity.

As shown in Figure 26, this was also reflected in the body temperature fluctuations which increased and decreased according to when the pigs have or do not have a feeding activity.

These feeding-related fluctuations are comparable to the intestinal temperature pattern reported in a previous study in TN conditions [START_REF] Morales | Effect of feed intake level and dietary protein content on the body temperature of pigs housed under thermo neutral conditions[END_REF]. In fact, these increases in body temperature reflect temporary imbalances between HP and HL [START_REF] Taylor | Considerations for the measurement of core, skin and mean body temperatures[END_REF]. This transient hyperthermia might be related to the time it takes for heat to be exported (e.g. as the blood travels from the core to the skin and then to the environment) and/ or to the delay in the initiation of HL mechanisms which are activated only after a deviation from the set point temperature is detected [START_REF] Tan | Regulation of body temperature by the nervous system[END_REF].

While pigs are able to lose heat by both the sensible and latent routes, they rely mainly on the changes in the skin blood flow to maintain a constant body temperature in TN conditions.

Literature suggests sensible HL represent 70-80% of total HL in pigs in TN environment [START_REF] Kamada | Effects of environmental temperature, humidity and air movement on heat loss particularly that of latent heat, from the pig[END_REF]Renaudeau et al., 2013) which is comparable to our observations in Exp. 3 when looking at daily averages. However, as demonstrated in our study, HP and body temperature are not constant during the day and thus proportions of sensible and latent HL also fluctuate accordingly. Furthermore, since body temperature remains constant only when all heat produced is equal to heat lost (heat storage), this implies that net heat storage in the body also fluctuates since body temperature is not constant. Thus, for the sake of a more accurate graphical representation, we also tried to roughly estimate heat storage in the body using the changes in the measured body temperature, the mean BW of the pig, and the assumption that the specific heat of lean tissue is at 3.47 kJ•kg -1 •°C -1 [START_REF] Vallerand | How should body heat storage be determined in humans: by thermometry or calorimetry?[END_REF][START_REF] Giering | Specific heat capacities of human and animal tissues[END_REF] as shown in Figure 27 12 . As seen in the graph, sensible HL increases whenever there is an increase in body temperature caused by the feeding activity, and it can fluctuate from around 28 to 50 kJ•kg BW -0.60 •h -1 (or 53 to 73% of HP) during the day. Meanwhile, as previously discussed in Exp. 3, transient heat imbalances caused by feeding activity in TN conditions did not significantly increase the amount of heat lost by evaporation and it remained relatively constant at around 18 to 31 kJ•kg BW -0.60 •h -1 during the day representing around 42-48% of HP when HP was relatively low (i.e. no feed intake) and only up to 26-34% of HP during times of high HP (i.e. during or after a meal).

While there was a subsequent decrease in body temperature after a meal as result of these HL mechanisms, our results in Exp. 2 and 3 demonstrated that there is an incremental increase in the pre-and postprandial body temperature with each additional meal (also clearly shown in Figure 26). This incremental increase in HP per meal agrees with previous studies [START_REF] Schrama | Dietary betaine supplementation affects energy metabolism of pigs[END_REF][START_REF] Bolhuis | Effects of fermentable starch and straw-enriched housing on energy partitioning of growing pigs[END_REF][START_REF] Van Erp | Circadian misalignment imposed by nocturnal feeding tends to increase fat deposition in pigs[END_REF] and was likely from the cumulative heat increment due to nutrient utilization (long-term TEF) that follows each meal.

Meal characteristics such as meal size also influenced the degree of HP and body temperature increases. For example, using the data from pigs in Exp. 3 (Figure 26), HP and body temperature increased 13 on average by 361 kJ and 1.08°C, respectively, for the pigs with an average meal size of ~1.20 kg (N-2FW) after the 0900h meal while it was only by 222 kJ and 0.65°C for pigs with an average meal size of ~0.54 kg (N-8FW). Furthermore, HP and body temperature seemed to remain higher for a longer time in the pigs with larger meal sizes which is logical since duration of gastric emptying of larger meals takes longer than smaller meal sizes (Gregory et al., 1990) and thus there is longer influx of nutrients that contributes to the long-term TEF (nutrient utilization).

However, our results have also shown that it is not only the meal size but also the pattern of meal distributions that influenced the pattern of body temperature and heat balance. Looking at the feeding managements applied in this thesis project, specifically in Exp. 2 and 3, the degree of body temperature fluctuations were less apparent when pigs have unrestricted access to the feed or feeder as they were able to eat when and how much they want. This was in contrast to restricted pigs (in time or amount of feed) which were relatively in a "fasted state" during large periods of the day and had to eat the bulk of their meals within a shorter period of the day. An example is shown in 123 results of Exp. 2 and 3, our results thus demonstrated that meal distribution (alongside duration of meal intervals) can also influence variations in the body heat balance.

The feeding managements employed in the thesis project may admittedly have been oversimplified. With gastric satiation being volumetric and intestinal satiation being nutritive [START_REF] Powley | Gastric satiation is volumetric, intestinal satiation is nutritive[END_REF], one can imagine that it was the gastric volume that limited the pigs with only 2 meals to eat more per meal. On the other hand, it may have been nutrients still present in the intestinal tract which can trigger ileal brake (Ratanpaul et al., 2019) and the relatively short 90-min interval that restricted the pigs with 8 FW to eat at every feeding window. Nevertheless, our results clearly showed the importance of feeding behavior and meal characteristics (especially size and pattern) in looking at the dynamics of body heat balance during the day. Future studies might be needed to elucidate the effects of different meal sizes and interval durations on thermoregulatory and metabolic responses of pigs (e.g., how much time it takes to restore body temperature to its basal value based on the size of a previous meal).

Heat stressed conditions

Upon heat exposure, there was a reduction in metabolic HP (Figure 29) which was related to the feed intake reduction as observed across our experiments (Exp. 1, 2, 3, and 4) and in other studies (Collin et al., 2001c;Renaudeau et al., 2013). This reduction in metabolic HP may also be related to a lower metabolic rate [START_REF] Fausnacht | Heat stress reduces metabolic rate while increasing respiratory exchange ratio in growing pigs[END_REF] as indicated by decreased thyroid hormone levels which was consistently observed in HS pigs in Exp. 1,2,3 and in other studies [START_REF] Collin | Effects of high temperature on body temperature and hormonal adjustments in piglets[END_REF]Campos et al., 2014b;Sanz Fernandez et al., 2015a).

Despite this, body temperature of pigs under HS remained higher than those under TN conditions which suggested a higher heat load and that HL pathways are more saturated during HS regardless of the time of day (Exp. 2 and 3).

Similar to the pattern in TN conditions, each additional meal resulted to an incremental increase in the pre-and post-prandial body temperature of the pigs subjected to HS. However, these increases in body temperature were to a lesser degree but were maintained for a longer period compared to that observed in TN conditions (as presented in Exp. 2). This was in contrast to TN conditions wherein there was a clear and continuous body temperature decline after the last meal of the pigs with feed or feed access restriction, results of Exp. 2 and 3 showed that body temperature of the pigs remained high during HS even during periods with no feeding activity.

In both experiments, we hypothesized this to be related to a slowed down gastric emptying and motility as reported in animals under HS (Gonzalez-Rivas et al., 2020) possibly as a result of restricted splanchnic blood flow. The negative effect of HS on gastric emptying and motility remains a theory and has yet to be confirmed in pigs. However, our results support that delayed gastric emptying seems to be related to large meal size and/or high insulin levels (Gregory et al., 1990;Eliasson et al., 1995) since pigs with larger meal sizes and higher levels of insulin (N-2FW) maintained a high body temperature for longer periods than the pigs with smaller meal sizes (N-8FW). Nevertheless, body temperature fluctuations were less apparent during HS than during TN conditions and even more so when pigs were imposed to eat large meals per day.

Aside from the pattern of HP and body temperature, patterns of sensible and latent HL were also modified by heat exposure with increased proportions of latent HL during HS conditions (Figure 30). Pigs lost a maximum of only 28-34 kJ•kg BW -0.60 •h -1 via sensible route during HS even when the HP is high as compared to ~50 kJ•kg BW -0.60 •h -1 in TN conditions.

As discussed in the literature review, sensible HL is driven by the temperature gradient between the body and the environment and this gradient is decreased in high ambient temperature. Then, pigs start to dissipate more heat via the evaporative route [START_REF] Curtis | Environmental management in animal agriculture[END_REF]. According to our results, the ability of pigs to lose heat via the latent pathway seems to be influenced by their feeding behavior. While heat loss via the latent route can be as low as 25-28 kJ•kg BW -0.60 •h -1 (50-54% of HP) during periods of low HP, this can increase up to 40 kJ•kg BW -0.60 •h -1 (58% HP) in N-2FW pigs and up to 53 kJ•kg BW -0.60 •h -1 (71% of HP) in N-8FW pigs when HP is highest (e.g. during or after a meal). Moreover, the pattern of incremental increase in HP during the day was closely followed by an incremental increase In latent HL In the N-8FW pigs in contrast to N-2FW pigs. Our results have demonstrated that limited ability for sensible HL in hot conditions means an increased importance of the latent route during HS but especially during periods of high HP (such as during or after a meal). Furthermore, the feeding behavior of the pigs seems to influence their ability to dissipate heat via the latent route.

Implications

Body heat balance as reflected in the core body temperature varies throughout the day depending on the variations in metabolic HP and in the ability of the pigs to lose heat. Our results have shown that feeding behavior, meal characteristics, and meal distribution pattern highly influence fluctuations of HP, HL proportions, and body temperature. Our results also further demonstrated the importance of the latent route in maintaining thermal equilibrium during HS wherein the sensible HL may not be sufficient, especially during periods of high HP such as during and after a meal. It is thus important to consider these dynamic changes in the body heat balance during the day when analyzing how pigs respond to HS.

Importance of latent heat loss, water intake, feeding behavior, and insulin sensitivity during HS

In line with our results on the increased reliance of pigs to the latent route for heat dissipation during HS, this subsection focuses on the possible links between latent HL, water intake, feeding management, and insulin sensitivity and their importance during HS as demonstrated by our study. Principal component analysis (PCA) was applied to the data from non-supplemented TN pigs in Exp. 3 to look at the relationship between important parameters.

The data collected from HS conditions were then projected on the 2 latent variables of the PCA calculated from TN data. The loading plot shown in Figure 31 indicates that in TN conditions, latent HL, ME intake and retained energy have large positive loadings on component 1.

Meanwhile, low insulin sensitivity (or a high insulin: glucose ratio) and water intake have large positive loadings and PD has moderate negative loading on component 2.

High capacity for heat loss increases ability to maintain feed intake during HS

Projections in the PCA of data collected during TN and HS suggests that regardless of the ambient temperature, there is a positive link between latent HL and ME intake (Figure 31).

In TN conditions, this link may be due to higher metabolic water produced with a higher amount of substrate oxidation [START_REF] Van Es | Report to the subcommittee on constants and factors: Constants and factors regarding metabolic water[END_REF] as it represented ~40% of evaporated water 14 .

However, this was less likely the case in HS conditions wherein there was lower feed intake and the metabolic water only represented ~20% of evaporated water. It is more likely that the higher latent HL during HS is related to the increased panting [START_REF] Robertshaw | Mechanisms for the control of respiratory evaporative heat loss in panting animals[END_REF] as supported by the higher respiration rate observed in Exp. 2 and in other studies [START_REF] Cottrell | The effect of heat stress on respiratory alkalosis and insulin sensitivity in cinnamon supplemented pigs[END_REF][START_REF] Le | Betaine and isoquinoline alkaloids protect against heat stress and colonic permeability in growing pigs[END_REF]. 14 Metabolic water is the water produced by oxidizing energy-containing substances in the diet. Oxidized carbohydrate and fat were calculated based on the formula in Text box 2 using volumes of O2 consumed and CO2 produced while oxidized protein is from the total N lost in the urine, condensates, and air from the experiment in the respiratory chamber. Metabolic water was then calculated using the constants assuming that there is 60, 107, and 42 g water produced per 100 g of carbohydrates, fat, and protein, respectively.

The reduction of feed intake during HS is based on the fact that there is limited capacity for sensible HL in hot conditions and thus there is a need to rely on a reduced HP to still maintain thermal equilibrium of the body. It is thus logical to assume that in order for pigs to rely less on reducing HP and feed intake in hot conditions, one must increase the ability of the pig to dissipate heat via the only other HL route which is the latent pathway. Taking the N-2FW and N-8FW as examples, the improved ability of N-8FW to incrementally dissipate heat via the latent pathway allowed them to dissipate a total of 200 kJ•kg BW -0.60 more heat during the course of the day than the N-2FW pigs. This higher amount of heat lost from the pig's body can probably mean that the N-8FW pigs can eat at least 200 kJ•kg BW -0.60 more energy without increasing their body temperature. This can thus partly explain why N-8FW pigs had lower reduction in their ME intake compared to the N-2FW (-500 vs. -740 kJ•kg BW -0.60 , respectively) upon heat exposure.

High water intake increases capacity to dissipate heat via the latent route

Reasonably, an increased evaporative HL during HS means a higher body water loss (dehydration) which can trigger thirst [START_REF] Leib | [END_REF] which helps explain the increased water consumption during HS. While increased latent HL during HS disrupts body water balance, body water balance is vital for thermoregulation as demonstrated by a reduced ability for evaporative HL in dehydrated animals [START_REF] Baker | Effect of dehydration on hypothalamic control of evaporation in the cat[END_REF]McKinley et al., 2009).

Furthermore, dehydration can lead to plasma hyper osmolality 15 that not only increase water retention [START_REF] Mroz | Water in pig nutrition: physiology, allowances and environmental implications[END_REF] but has also been shown to delay vasodilation responses with subsequent negative effects on skin blood flow responses [START_REF] Nakajima | Plasma hyperosmolality and arterial pressure regulation during heating in dehydrated and awake rats[END_REF][START_REF] Shibasaki | Plasma hyperosmolality elevates the internal temperature threshold for active thermoregulatory vasodilation during heat stress in humans[END_REF]. Thus, the increased water intake of HS pigs may be necessary to replenish water lost and to restore body water homoeostasis thereby allowing the pigs to maintain or increase heat dissipation via the latent route [START_REF] Mckinley | Interaction between thermoregulation and osmoregulation in domestic animals[END_REF]. As shown in the PCA (Figure 31), in contrast to TN conditions, water intake has a stronger correlation with latent HL (as % HP) and with ME intake in HS conditions. From that, it is assumed that improving water intake increases the ability of pigs to maintain or increase capacity for latent HL and consequently for feed intake.

Figure 32. Three mechanisms on how the feeding management imposed for the N-8FW groups in Exp. 3 increased feed intake during HS.

Increasing meal frequency increases drinking frequency and thus water intake

One of the main objectives of this thesis project was to determine whether a modified feeding behavior, specifically an increased meal frequency, can help the pigs to improve feed intake, energy efficiency, and PD during HS. From our results, we have no clear conclusions since the effects of meal frequency on the ability to cope with HS varied according to the experiments (Exp. 2 and 3). However, we theorize that it is the type of feeding management rather than the increased meal frequency per se that helped N-8FW pigs in Exp. 3 to improve performance (or at least ME intake) during HS.

The imposed feeding management in the N-8FW group has three combined elements that may have helped them in improving feed intake during HS compared to the other groups (N-Unli and N-2FW) as summarized in Figure 32.

• First, the imposed increased meal frequency was theorized to have increased drinking bouts and thus water intake as a result of the prandial drinking behavior of pigs [START_REF] Yang | Effects of food on drinking behaviour of growing pigs[END_REF]Bigelow and Houpt, 1988) which seem to agree with our results when comparing N-8FW to N-2FW pigs (Figure 33a).

• Second, the imposed narrow feeding windows of 15-min each (that did not allow pigs to eat as much as they want per meal) in addition to the 13 h feed access restriction during the night might have left the pigs feel hungrier prompting them to drink more water as "abdominal fill" in order to feel satiety [START_REF] Yang | Effects of food on drinking behaviour of growing pigs[END_REF]. This does seem to be the case (Figure 33b) when comparing water intake (and thus latent HL) of the N-8FW pigs and N-Unli pigs who had unrestricted access to feed.

• Third, the 13 h feed access restriction itself may have allowed N-8FW to lose excess heat and go back to a lower baseline BT until the next meal in the following day. Indeed, feed withdrawal strategy has already been demonstrated to help pigs (Kpodo et al., 2020a) and poultry species [START_REF] Francis | Alleviation of acute heat stress by food withdrawal or darkness[END_REF][START_REF] Farghly | Feeding time can alleviate negative effects of heat stress on performance, meat quality and health status of turkey[END_REF] to cope with HS. It has also been suggested in our results in Exp. 3 and in another study (Cervantes et al., 2018) that a lower pre-prandial body temperature or a lower heat load before a meal is an important factor in the capacity of the animal to lose heat after a meal.

Nevertheless, the improved feed intake in non-supplemented pigs did not translate to an increase in PD (Exp. 3). Looking back in the PCA in Figure 31, heat exposure seemed to modify energy partitioning in pigs. Regardless of the period, LD was strongly correlated to ME intake while this was not the case for PD in which its correlation with ME intake seems to be weaker in HS than in TN conditions. This agrees with reports of a decreased maximum potential for PD of pigs subjected to HS. This effect was attributed to the higher energetic cost (and heat increment) of PD compared to LD (Le Bellego et al., 2002b). This could also help explain the relatively increased fatness (especially visceral adiposity) demonstrated in our results in Chapters 1 and 2 and in many studies [START_REF] Kouba | Influence of a high ambient temperature on lipid metabolism in the growing pig[END_REF]Le Bellego et al., 2002b;[START_REF] Wu | Effects of high ambient temperature on lipid metabolism in finishing pigs[END_REF]. Meanwhile, based on the position of the groups (N-Unli, N-2FW, and N-8FW) on the PCA, a higher PD also seem to be related to the shorter length of feed restriction.

Since the whole body PD is regulated by the balance between protein synthesis and breakdown, the imposed prolonged feed restriction may have decreased protein synthesis rate thereby contributing to a decreased net protein accretion [START_REF] Reeds | Protein turnover in growing pigs. Effects of age and food intake[END_REF]Arnal et al., 1987).

An increased insulin sensitivity improves PD in HS

Projections in the PCA of data collected under HS suggests a lower insulin: glucose ratio (signifying increased insulin sensitivity) seems to be more correlated with a higher PD during HS than during TN. Furthermore, some studies have attributed increased insulin sensitivity to a decreased fat deposition [START_REF] Dunshea | Effect of dietary protein on body composition and insulin resistance using a pig model of the child and adolescent[END_REF]Salgado et al., 2022) and to leaner pig breeds [START_REF] Albuquerque | Transcriptomic profiling of skeletal muscle reveals candidate genes influencing muscle growth and associated lipid composition in portuguese local pig breeds[END_REF][START_REF] Rodríguez-López | Determining insulin sensitivity from glucose tolerance tests in Iberian and landrace pigs[END_REF]. Many studies have already either demonstrated or hypothesized that an improved insulin sensitivity is important in coping with HS (Rhoads et al., 2013;Sanz Fernandez et al., 2015b;Liu et al., 2017;[START_REF] Cottrell | The effect of heat stress on respiratory alkalosis and insulin sensitivity in cinnamon supplemented pigs[END_REF]. One hypothesis is that insulin sensitivity helps improve capacity to lose heat via the sensible route since it has been reported to improve microvascular circulation and vasodilation [START_REF] Serné | Microvascular function relates to insulin sensitivity and blood pressure in normal subjects[END_REF][START_REF] De Jongh | Impaired Microvascular Function in Obesity[END_REF]. Another hypothesis may be related to the direct role of insulin on whole body temperature regulation. Insulin is reported to delay heat loss mechanisms by inhibiting warm-sensitive neurons in the hypothalamus [START_REF] Sanchez-Alavez | Insulin causes hyperthermia by direct inhibition of warm-sensitive neurons[END_REF]. From this, lower insulin levels in the blood (to maintain glucose homeostasis) can thus help reduce or maintain a lower body temperature.

Implications

Taking into account what has been discussed in this and in earlier subsections, maintaining body heat equilibrium and body water balance are closely linked. Therefore, improvement of water intake is also highly correlated to the ability of pigs to dissipate heat via the latent route. The ability of pigs to decrease body temperature and to lose heat after a meal seem to be major contributing factors in improving feed intake during HS. Evaluating feeding strategies that stimulate an increase in water intake (e.g., via modification of feeding behavior, liquid feeding) and that rapidly decrease body temperature (e.g., feed withdrawal) might help in maintaining feed intake during HS. However, the exact scheme of the feeding management (e.g., optimal inter-meal intervals, pattern of meal distribution throughout the day, the size of the meal, the duration of feed restriction, and many others) in order to maintain body heat balance and feed intake and performance during HS are also important to take into account when evaluating feeding strategies. With the many roles of insulin on metabolic processes and body temperature regulation, there seems to be potential in evaluating feeding strategies that improve insulin sensitivity to help pigs cope with heat challenges while maintaining their growth performance.

Live yeast mechanism during HS may be due to an improved insulin sensitivity

One of the main objectives of the thesis was to determine whether the positive effects of LY supplementation in HS pigs observed in a previous study, i.e. improved feed intake, energy efficiency, and PD (Labussière et al., 2021), were due to changes in feeding behavior and/or energy metabolism, and if these effects can be replicated by imposing an increased meal frequency. However, as our results demonstrated, increasing meal frequency can only explain improved feed intake and energy efficiency but failed to improve PD during HS. Rather, the altered feeding behavior was not the cause but is theorized to be one of the consequences of an improved insulin sensitivity. We thus theorize that the effect of LY on improved insulin sensitivity 1) allows the pigs to eat more (via shorter inter-meal intervals or increased meal frequency) and thus leads to increase in feed intake and energy efficiency, and 2) improves thermoregulation responses of pigs that mitigates HS effects and allows them to reach a higher maximal potential for PD.

Mechanisms of LY supplementation in pigs subjected to HS

Improved insulin sensitivity modifies feeding behavior and thus improves feed intake.

High insulin in the blood has been reported to delay gastric emptying (Eliasson et al., 1995) thus we hypothesize that lower levels of circulating insulin needed to regulate blood glucose levels can attenuate HS effect on gastric emptying allowing pigs to eat within shorter inter-meal intervals (as observed in Exp. 3) or to increase meal frequency (Labussière et al., 2021). This theorized effect on gastric emptying may also help to explain why supplemented pigs with only 2 feeding windows (Y-2FW) were able to drink more water (Figure 35) and improve both feed intake and PD compared to their non-supplemented counterpart during HS (N-2FW). Furthermore, as mentioned in Exp. 3, duration of inter-meal intervals were similar among the supplemented groups (240 min) during HS which suggests supplemented pigs can maintain their ability to shortly start a new meal after the last one has ended regardless of the imposed feeding behavior. The indirect effect of LY on an increased meal frequency may thus have helped pigs drink more water, increase latent HL especially during or after a meal, return to a lower body temperature, and thus improve feed intake and energy efficiency (Figure 34;

green box). Field observations reported by Lallemand and previous research work using the same yeast strain supports a better digestive transit in gestating sows and a higher feed intake in sows fed the live yeast during lactation [START_REF] Chevaux | Meta-analysis of the influence of live yeast addition on feed intake in lactating sows[END_REF][START_REF] Domingos | Saccharomyces Cerevisiae var. Boulardii CNCM I-1079 during late gestation and lactation improves voluntary feed intake, milk production and litter performance of mixed-parity sows in a tropical humid climate[END_REF] which could imply similar mechanisms. However, an improved feed intake does not always translate to an improvement in PD thus there must be another mechanism aside from the increased meal frequency.

Improved insulin sensitivity improves thermoregulation responses thereby attenuating

HS effects on PD.

We hypothesize that since ME intake was less correlated to PD during HS, then the effect of LY on PD must be related to mitigating the direct effect of heat exposure on the maximal potential for PD. It can thus be assumed that this effect might be related to improved thermoregulation effects caused by an improved insulin sensitivity that decreased severity of HS in LY supplemented pigs. Indeed, lower skin temperature were observed in supplemented pigs in a previous study (Labussière et al., 2021). Meanwhile, in our study, improved thermoregulation responses may have also allowed supplemented 8FW pigs in Exp. 3 (Y-8FW) to maintain similar body temperature with non-supplemented counterparts (N-8FW) in HS conditions without as much reliance on an evaporative HL. Furthermore, Y-8FW were also able to deposit more protein during HS than the N-8FW despite having lower ME intake, suggesting attenuated HS effect on a reduced maximal potential for PD when LY is supplemented.

So how does LY improve insulin sensitivity?

In humans, LY supplementation has been reported to attenuate diabetic-related complications such as hyperglycemia (due to impaired insulin activity or production) by microbiota modulation and improved immune responses (Brandão et al., 2018;Albuquerque et al., 2019;Cunha et al., 2020). Moreover, yeast extract from S. cerevisiae has also been reported to potentiate insulin action (Edens et al., 2002). Possible mechanisms in pigs may be related to improved intestinal integrity, bile acid metabolism, and SCFA production as reported in LY supplemented pigs [START_REF] Bontempo | Live yeast dietary supplementation acts upon intestinal morpho-functional aspects and growth in weanling piglets[END_REF][START_REF] Oswald | Mycoplasma vaccination responses in immunodepressed weanling pigs supplemented with S. cerevisiae boulardii[END_REF][START_REF] Gresse | Saccharomyces Cerevisiae var Boulardii CNCM I-1079 reduces expression of genes involved in inflammatory response in porcine cells challenged by enterotoxigenic E. Coli and influences bacterial communities in an in vitro model of the weaning piglet colon[END_REF]Labussière et al., 2021;[START_REF] Bravo De Laguna | Effect of Saccharomyces cerevisiae boulardii CNCM I-1079 on bile acid signalling in post-weaning piglets[END_REF] since these have been previously correlated to help improve insulin sensitivity [START_REF] Caricilli | The Role of Gut Microbiota on Insulin Resistance[END_REF][START_REF] Khan | Microbial Modulation of Insulin Sensitivity[END_REF]. However, the exact mechanism by which LY supplementation improves insulin sensitivity (or at least potentiate insulin action) has not yet been established in pigs so far.

Possible sources of variation in responses among experiments

Similar to the discussed differences in HS severity among experiments, there were also variation in the effect of LY supplementation in pigs subjected to HS that seems to be related to both animal and environmental factors. Exp. 4 has demonstrated that the effect of LY supplementation during HS might be dependent on the age or BW and sex of the animal and its interactions. Improved responses to HS seem to be more consistent in LY-supplemented heavier entire male pigs as shown in Exp. 3, Exp. 4 (Replicate 2), and in a previous study (Labussière et al., 2021). This suggests that the beneficial effects of LY supplementation in high ambient temperature may be more important for more susceptible animals. Indeed, published literature on the improvement of HS responses by LY supplementation is usually in animals susceptible to HS due to high productivity or generally higher HP such as finishing pigs, lactating sows, and dairy cows [START_REF] Brewster | Saccharomyces cerevisiae boulardii improves performance of pigs fed low and high energy diets in summer[END_REF][START_REF] Lee | Effect of saccharomyces boulardii supplementation on performance and physiological traits of holstein calves under heat stress conditions[END_REF]Perdomo et al., 2020;[START_REF] Domingos | Saccharomyces Cerevisiae var. Boulardii CNCM I-1079 during late gestation and lactation improves voluntary feed intake, milk production and litter performance of mixed-parity sows in a tropical humid climate[END_REF]Labussière et al., 2021).

However, in this thesis project, variation in responses due to the animal factors might be confounded with the feeding management employed and the housing conditions. For example in Exp. 2, the effect of LY supplementation may not have been observed not only because female pigs were used but may be due to the pair-feeding management employed and to the unprecedented effect of the housing conditions and the experimental design on the feeding behavior of the ad libitum fed pigs. Indeed, the feeding management applied can be a source of variation. For example, differences in ME intake response were observed even between two LY supplemented groups such as the Y-2FW and Y-8FW (difference of 358 kJ•kg BW -0.60 •d -1 ) in Exp. 3 using entire male pigs. When it comes to housing conditions, more consistent results were also observed in experiments in the respiratory chambers which seemed to elicit stronger HS responses than in larger experimental rooms. Similar results were observed in terms of improved feed intake, energy efficiency, and PD (or N retention) in supplemented HS pigs in the previous study in the respiratory chambers (Labussière et al., 2021) and with our results in Exp. 3 using the supplemented HS pigs with the unlimited FW. However, the degree of improvement may have been slightly different such as a higher feed intake improvement by LY in the aforementioned study wherein heavier entire male pigs were used (~100 kg vs. ~80 kg in our study). These variations suggest that the positive effects of LY supplementation during HS seem to be more evident when the stress or challenge is more severe.

Implications

Insulin is key to many metabolic processes especially in energy metabolism that is highly impacted by heat exposure. Although our results have demonstrated that feeding management strategies can help pigs to cope with HS, nutritional interventions that increase insulin sensitivity and increase latent HL capacity of pigs seem to be more efficient not only in helping pigs cope with HS but in helping them maintain their performance during HS. The actual mechanisms of how LY supplementation improve insulin sensitivity, how this increased insulin sensitivity improves pig performance during HS, and the seeming interaction with sex and/or severity of heat challenge can be venues for future research.

Conclusions Conclusions

The main objectives of this thesis work were to contribute in bridging knowledge gaps in some aspects of heat stress in pigs such as 1) to investigate the effects of a chronic prenatal heat challenge on heat stress susceptibility during the growing-finishing period 2) to distinguish direct and indirect effects of heat stress on growth performance, hormones, and metabolism

3) to determine whether the positive effects of live yeast supplementation were caused by a modified feeding behavior 4) to evaluate interactions between live yeast supplementation and sex in HS pigs, and 5) to further understand the dynamic process of how body heat balance (body temperature) changes and is maintained throughout the day within a narrow range while ambient temperature is increasing.

The different heat challenge models used in this project demonstrated that many factors could influence pig's ability to cope with heat stress such as animal related factors (body weight, sex, feeding behavior as estimated by the alternation of feeding and fasting periods), the environment (housing conditions, the nature and the severity of the heat challenge), and even the interactions among them. Furthermore, it is not just the immediate surroundings but also the prenatal thermal environment which can affect pig's postnatal performance. This was demonstrated by the decreased ability of the pigs born from heat stressed sows to tolerate a postnatal heat challenge which has consequences on their growth performance and on their body composition. This effect of an altered body composition especially visceral adiposity was also shown to be a direct effect of elevated temperature on energy partitioning between lean and fat deposition.

By using continuous measurements of heat production and losses and of body temperature, our results have demonstrated how feeding behavior (meal characteristics and pattern) and its interaction with ambient temperature influences how the pigs try to maintain homeothermy throughout the day. In line with this, we have also demonstrated the importance of the latent heat loss pathway in maintaining thermal equilibrium in high ambient temperature, in particular during periods coinciding with high heat production (i.e., after a meal).

Data obtained from the thesis project highlights the importance of feeding strategies that stimulate water intake and rapid decrease in body temperature in order to improve feed intake during heat stress. Live yeast supplementation was found to promote insulin sensitivity, modified feeding behavior, improved thermoregulation responses, and increased latent heat loss capacity of pigs. Thus, nutritional interventions that improve insulin sensitivity seem to be more efficient in helping pigs cope and maintain performance during heat stress.

Perspectives Perspectives

Improving our knowledge about heat stress and how it deteriorates animal performance must be further understood in order to find ways to mitigate it. While there are increasing studies about prenatal heat stress in pigs, results are still unclear and sometimes inconsistent.

Our study only considered a whole gestation heat stress but further studies are needed to determine other factors at play such as the most susceptible stage of pregnancy for the sow and the fetus, duration of a heat challenge (Boddicker et al., 2014), previous heat stress experiences of sires and sows [START_REF] Gourdine | Effects of breed and season on performance of lactating sows in a tropical humid climate[END_REF][START_REF] Rosé | Interactions between sire family and production environment (temperate vs. tropical) on performance and thermoregulation responses in growing pigs[END_REF], and even the possible interaction with the sex of the progeny. Our results show that prenatal heat stress increase basal body temperature and this was hypothesized to be related to a modified set point due to exposition to increased cytokines in the fetal environment [START_REF] Glover | Prenatal stress and the programming of the HPA axis[END_REF]. However, literature suggests that there may also be an alteration of the energy metabolism in pigs as suggested by reports of a higher fasting heat production of in in utero heat stressed pigs [START_REF] Chapel | Determining the effects of early gestation in utero heat stress on postnatal fasting heat production and circulating biomarkers associated with metabolism in growing pigs[END_REF], and higher insulin of pigs born from heat stressed sows and subjected to a postnatal heat challenge (Johnson et al., 2015b). Future research can look more into the possible effects of prenatal heat stress on the energy metabolism and insulin sensitivity of pigs and their ability to cope with a postnatal challenge.

Sexual dimorphic heat stress responses also have not yet been established in pigs. We speculate that, in growing pigs, entire males may be more susceptible due to their higher growth potential and heat production (Labussière et al., 2013;[START_REF] Ruiz-Ascacibar | Impact of amino acid and CP restriction from 20 to 140 kg BW on performance and dynamics in empty body protein and lipid deposition of entire male, castrated and female pigs[END_REF] when compared to females and surgically castrated males. Future studies are needed to determine how sex affects pig responses to heat stress not just in terms of its impact on growth but especially in terms of body composition, and taking into consideration its possible interaction with body weight and age. Furthermore, the effect of sex is usually studied using barrows as the male representative (Renaudeau et al., 2006b;[START_REF] Rudolph | The effect of Mitoquinol (MitoQ) on heat stressed skeletal muscle from pigs, and a potential confounding effect of biological sex[END_REF] but future studies should also consider both immunocastrated and entire male pigs especially with the increasing ban on surgical castration in the European Union. Research suggests sex effect on feeding behavior of pigs which might help explain their sensitivity to heat stress [START_REF] Cross | Feeding behavior of grow-finish swine and the impacts of heat stress[END_REF].

Future studies on the effect of sex and feeding behavior during heat stress, probably along with hormonal differences might be needed.

Since maintaining body heat balance is a dynamic process, management strategies that involves improving heat losses (e.g., fogging, floor cooling, misting) only during periods with high heat production (after a meal) might also warrant further studies especially on cost and energy savings point of view. In line with the timing, feeding during the night wherein there is relatively low metabolic rate [START_REF] Ingram | Circadian rhythms in the pig[END_REF] can also be evaluated. While this has already been shown to improve lactating sow performance (Choi et al., 2019b), it has not yet been studied in growing pigs. In evaluating feeding management strategies, many factors can also be considered such as optimal inter-meal intervals that allows pigs to return to a lower body temperature but still minimize protein losses.

Many studies have postulated about the importance of improving insulin sensitivity in coping with heat stress (Rhoads et al., 2013;Sanz Fernandez et al., 2015b;Liu et al., 2017;[START_REF] Cottrell | The effect of heat stress on respiratory alkalosis and insulin sensitivity in cinnamon supplemented pigs[END_REF]. Our results on the positive effects of live yeast supplementation exhibited its ability to improve insulin sensitivity but mechanism is still unknown and remains to be elucidated. One can imagine that this is the link or maybe one of the links between the gut microbiota modulation and modified feeding behavior by live yeast during heat stress (Labussière et al., 2021), especially since there have been reports of microbiota being linked with feeding behavior [START_REF] He | Exploring the role of gut microbiota in host feeding behavior among breeds in swine[END_REF]. Recent preliminary research suggests that S. cerevisiae boulardii CNCM-I 1079 can alter intestinal microbial bile acid metabolism in piglets, leading to an impact on Fibroblast Growth Factor19 expression which could explain postprandial attenuation of both energy expenditure and satiety signals [START_REF] Bravo De Laguna | Effect of Saccharomyces cerevisiae boulardii CNCM I-1079 on bile acid signalling in post-weaning piglets[END_REF]. At the same time, there are still many gaps about the improvement of pig performance during heat stress that is driven by insulin sensitivity, one of which is gastric emptying. Effects of heat stress on the kinetics of gastric emptying and its highly likely relationship with feeding behavior can also be further studied.

Finally, since our results suggest that live yeast supplementation during heat stress seems to be more beneficial for susceptible animals, it can be interesting to evaluate precision feeding approaches that can provide feed to certain animals that are deemed susceptible to heat stress (e.g., based on BW, sex, growth potential). Since precision feeding systems can also provide feeding behavior data, it would be interesting in the future to set up a system that can detect changes in the feeding behavior patterns of the pigs that signal heat stress (or any perturbation) and couple them with tailored solutions to each individual pig (e.g. provision of live yeast supplemented feed).

References Introduction

Climatic environment is one of the main limiting factors of production efficiency in the swine industry especially when ambient temperature is above the zone of thermal comfort.

Although thermal heat stress (HS) is an occasional problem in temperate countries during the 2 to 3 summer months and/or during hot spells, it is a more permanent problem in many tropical countries where pig production has grown at a high rate during the last two decades.

Reproductive sows are very susceptible to HS. These effects include anoestrus, delayed puberty, reduced farrowing rates, increase in abortion rate and reduced litter size [START_REF] Prunier | Environmental and seasonal influences on the return-to-oestrus after weaning in primiparous sows: a review[END_REF]. In addition, HS during pregnancy can induce structural and functional changes in the foetus that may persist through the productive life with subsequent effects on growth performance and body composition. The aim of this study was to evaluate the effects of heat exposure during almost whole gestation on the growth performance and carcass composition of the offspring.

Material and methods

A total of 24 mixed parity pregnant sows were housed at thermoneutrality (PTN,[START_REF] Van Milgen | Dynamic aspects of oxygen consumption and carbon dioxide production in swine[END_REF][START_REF]Official methods of analysis[END_REF][START_REF] Tsanaktsidou | Titanium and chromium determination in feedstuffs using ICP-AES technique[END_REF][START_REF] Renaudeau | Effects of high ambient temperature and dietary protein level on feeding behavior of multiparous lactating sows[END_REF][START_REF] Brouwer | Report of sub-committee on constants and factors[END_REF](23)[START_REF] Renaudeau | Effect of thermal heat stress on energy utilization in two lines of pigs divergently selected for residual feed intake[END_REF]n=12) or in HS conditions (PHS, 28-34 °C, n=12) from the 7th to the 106th day of gestation.

From the 107th of gestation to the 28th days of lactation, all sows and their progeny were 2 BW and ADG data were analysed with a model including the effects of the temperature treatment during pregnancy (PTN vs PHS) and growing period (GTN vs GHS), the interaction, and litter origin. The pen effect was considered as a random effect. The experimental unit was the animal. ADFI and FCR data were analysed with a model including the effects of the temperature treatment during pregnancy (PTN vs PHS) and growing period (GTN vs GHS), interactions, and litter origin. The experimental unite was the pen within each room.

The interaction between prenatal and postnatal HS effect was significant for initial BW in period 1 and 2 (P<0.05, Table 1). In P1, growth performance was similar for the PTN and the PHS groups. These results are in agreement with those previously reported by Boddicker et al., (2014) and Johnson et al. (2015). In P2, elevated temperature significantly reduced growth performance whatever the prenatal temperature treatment. HS pigs fed-AL as the reference group. Thus, there were four experimental groups: TN-AL, HS-AL, TN-PF8, and HS-PF8. In PF8 groups, the calculated daily ration was divided into 8

feed provisions distributed at every 90-min intervals from 09h00 to 19h30. In P2, HS-AL and PF8 pigs had lower ADFI (-19% on average; P < 0.01), ADG (-25% on average; P < 0.01), and a reduced final BW (-6.1 kg on average; P < 0.01) than TN-AL pigs. Rectal temperature was increased by HS (+0.32°C on average; P < 0.05) but did not significantly differ within HS nor within TN groups (P > 0.05). The PF8 groups had lower total viscera percentage than TN-AL pigs (-2.1% on average; P > 0.05), while HS-AL had intermediate results. Pigs in HS-AL and in PF8 groups had similar slaughter BW (P < 0.05) but HS-PF8 had more perirenal fat than TN-PF8 (+1.6%; contrast at P < 0.05). Plasma insulin on d 8 of P2 was higher in HS-PF8 than in TN-PF8 pigs (P < 0.05). Heat stressed pigs had lower plasma TT3 (P < 0.05) even compared to TN-PF8 with the same feeding level. Plasma creatinine was higher in HS pigs within AL (P < 0.05) and within PF8 groups (P < 0.05). In AL groups, reduced growth performance in HS pigs was mostly due to a reduced feed intake. Increasing feed provision frequency during HS did not seem to improve pig performance. However, the increased fat deposition in internal depot in HS-PF8 could be an adaptation response related to direct effect of HS on pig metabolism.

Annex figure 1. Non-metric multidimensional scaling (NMDS) analysis using Bray-Curtis distances of samples according to pregnancy (PTN vs. PHS) and growing (GTN vs. GHS) environment.

Permutational multivariate analysis of variance (PERMANOVA) pairwise test was used to detect signifance in the distance between each group. Fecal samples were collected from growing pigs (as described in Chapter 3.01 c 1 All pigs were housed at thermoneutrality (Period 1; d -7 to -1) before being subjected to either thermoneutral (TN) or heatstressed (HS) conditions (Period 2; d 0 to 19). In each environment, diet was provided either ad libitum (AL; 2 feed provisions/day) or pair-fed with two (PF2) or eight (PF8) feed provisions/day). N: non-supplemented diet. Y: live yeast supplemented diet. 2 Residual standard deviation. 3 The pig was considered as the experimental unit. Data was analyzed using PROC MIXED model with replicate (R), experimental group (G), day (D) or period (P), their interactions as fixed effects. *P < 0.05, **P < 0.01. a,b,c LSmeans within a row with different superscripts letters differ (P < 0.05) according to the experimental group Comme la plupart des animaux d'élevage, les porcs sont considérés comme des animaux homéothermes qui régulent leur température corporelle centrale dans une plage étroite en maintenant un équilibre entre la production et la perte de chaleur. Les porcs sont notamment sensibles au stress thermique (HS), non seulement en raison de leur incapacité à transpirer [START_REF] Ingram | Stimulation of cutaneous glands in the pig[END_REF], mais aussi parce que la sélection d'une croissance maigre plus rapide a entraîné une production de chaleur métabolique plus élevée chez les porcs modernes [START_REF] Brown-Brandl | Heat and moisture production of modern swine[END_REF]. Dans des conditions chaudes, le maintien de la température corporelle par des ajustements physiologiques et/ou comportementaux réduit généralement les traits économiquement importants liés à la reproduction et à la production (DeShazer et al., 2009;Renaudeau et al., 2012). Le HS est donc l'un des principaux facteurs limitants de la production porcine, en particulier avec les pertes économiques associées à la réduction de la croissance, de l'efficacité alimentaire, et de la qualité de la carcasse chez les porcs en croissance et à la réduction des performances de reproduction chez les truies, entre autres [START_REF] Mayorga | Heat stress adaptations in pigs[END_REF][START_REF] Liu | Review: What have we learned about the effects of heat stress on the pig industry?[END_REF]. Alors que l'effet le plus largement rapporté du HS chez les porcs en croissance est la réduction de la consommation alimentaire qui se traduit par une réduction de la croissance (Renaudeau et al., 2011), des études de la dernière décennie ont démontré qu'il existe d'autres conséquences à prendre en compte, telles que effets directs de l'exposition à la chaleur et même de l'environnement thermique prénatal des porcs [START_REF] Ross | Physiological mechanisms through which heat stress compromises reproduction in pigs[END_REF][START_REF] Johnson | Physiology symposium: Postnatal consequences of in utero heat stress in pigs[END_REF][START_REF] Liu | Review: What have we learned about the effects of heat stress on the pig industry?[END_REF]. En effet, afin de développer des stratégies pour atténuer ses effets de stress, les mécanismes par lesquels les porcs maintiennent l'équilibre thermique et par lesquels le HS détériore les performances des animaux doivent être mieux compris.

Par conséquent, l'objectif général de ce projet de thèse est de contribuer aux connaissances sur le HS chez les porcs. Ainsi, la thèse vise à déterminer comment un stress thermique prénatal peut influencer la réponse d'un porc à un stress thermique postnatal et à distinguer les effets directs de l'exposition à la chaleur (i.e. ceux qui ne sont pas liés à la fois à la consommation et au comportement alimentaire). De plus, le projet vise également à comprendre comment les porcs maintiennent l'équilibre thermique au cours de la journée, en particulier celui lié au comportement alimentaire. Enfin, les stratégies d'alimentation (i.e. la modification de la fréquence d'alimentation et la supplémentation en levures vivantes) sont évaluées afin d'atténuer les effets du HS chez les porcs.

Sommaire de la revue de la littérature

Des études dans la littérature ont démontré que quelles que soient les conditions environnementales, les porcs essaient de maintenir une température corporelle constante propice aux processus biologiques. Le non-respect de cette consigne peut entraîner des conséquences négatives dues à la chaleur et, dans des cas extrêmes, la mort. Au cours d'un challenge thermique, les animaux maintiennent un équilibre thermique via des ajustements biochimiques, physiologiques et/ou comportementaux appelés collectivement réponses de thermorégulation. L'exposition à une température ambiante élevée limite leur capacité pour dissiper la chaleur, perturbant ainsi l'équilibre thermique. Lorsque leur charge thermique dépasse leur capacité de dissipation thermique et qu'ils ne peuvent plus maintenir leur température corporelle, ils entrent dans un état du stress thermique (HS). La diminution de la croissance lors d'un HS est principalement liée à la réduction de la consommation alimentaire puisqu'il s'agit de la principale réponse d'adaptation du porc pour limiter la production de chaleur.

Des études récentes ont montré que ce n'est pas seulement l'environnement immédiat mais aussi l'environnement thermique prénatal qui affecte les performances des porcs en termes de température corporelle centrale, de composition corporelle, et de leurs réponses à un HS postnatal. Cependant, les résultats sont incohérents, peut-être en raison des différences de durée et de stade du challenge prénatal d'une étude à l'autre. Un nombre croissant d'études a également examiné les effets directs de l'exposition à la chaleur (non liés à la consommation alimentaire), comme l'augmentation de l'adiposité et la diminution de la flexibilité métabolique.

Ces travaux ont utilisés des stratégies d'alimentation appelées « pair-feeding ». Dans ce modèle, un groupe dans un environnement thermoneutre (TN) est rationné sur le niveau de consommation de ses congénères exposés à la chaleur. Cependant, il est difficile d'évaluer quels effets sont dus à l'exposition à la chaleur et lesquels sont confondus avec la stratégie d'alimentation (nombre de repas, heures de distribution, etc.).

Le niveau d'alimentation et le comportement contribuent largement aux pics instantanés de la production de chaleur (HP) dus à l'activité et à l'effet thermique du repas. Ces variations de HP et de température corporelle impliquent que le maintien de l'équilibre thermique est un processus dynamique tout au long de la journée. Pourtant, la plupart des stratégies nutritionnelles visant à atténuer les effets du HS se concentrent principalement sur la réduction 180 de la charge thermique globale (par exemple, l'amélioration de l'utilisation de l'énergie ou la réduction de l'augmentation de la HP). Il n'y a que quelques études sur les porcs qui étudient les stratégies de l'alimentation (par exemple, la fréquence de l'alimentation). Cependant, il a été récemment démontré que la supplémentation avec des levures vivantes (LY) modifiait le comportement alimentaire des porcs et des vaches laitières. Ces modifications du comportement alimentaire pourraient expliquer pourquoi les animaux résistent mieux à la chaleur comparativement à ceux ne recevant pas de levure.

Il existe certainement de nombreux autres éléments concernant le HS qui doivent être compris. L'objectif principal de ce projet de thèse est de contribuer aux connaissances sur certains aspects émergents de l'HS chez les porcs en quatre parties : Les porcs avaient une consommation alimentaire journalière (CMJ) similaire pendant P1 (P > 0,05). Dans P2, les porcs HS-AL et PF8 avaient une CMJ (-19 %), un gain quotidien moyen (-25 %) et un poids corporel final (-6,1 kg) inférieurs à ceux des porcs TN-AL (P < 0,01). Les porcs TN-AL avaient une épaisseur de gras dorsal plus importante que les porcs TN-PF8 (P < 0,05), et les porcs HS avaient des résultats intermédiaires. Les porcs HS avaient un pourcentage de graisse périrénale plus élevé sur la base de l'analyse de contraste entre les porcs PF8 (P < 0,05). Les réponses de thermorégulation des porcs augmentaient avec l'exposition au HS mais ne différaient pas entre les groupes HS ou TN (P > 0,05). Pour les porcs TN, la variation de la température musculaire (Tmuscle) dépendait de l'alimentation et de l'activité physique, comme pour les porcs HS, Tmuscle augmentait progressivement tout au long de la journée. La Tmuscle des porcs PF8 a augmenté avec chaque repas supplémentaire, mais a atteint un plateau plus tôt pour HS-PF8 que pour les porcs TN-PF8 ; une augmentation de Tmuscle par repas était également plus faible dans HS-PF8 que dans TN-PF8 (P < 0,05). L'exposition au HS a diminué la T3 et la T4 plasmatiques (P < 0,05) et augmenté la créatinine plasmatique (P < 0,05). Entre les groupes PF8, les porcs HS ont également eu une augmentation transitoire de l'insuline plasmatique au jour 8 (P < 0,05). L'effet du HS sur la CMJ a diminué la croissance des porcs, mais il existe des effets induits par la chaleur, tels que des réponses physiologiques altérées, qui pourraient expliquer les effets directs du HS observés dans d'autres publications, notamment en termes d'augmentation de l'adiposité. L'augmentation de la fréquence d'alimentation dans la présente étude n'a pas amélioré la réponse HS des porcs.

Chapitre 4. Effet du sexe et de la supplémentation en levures vivantes chez les porcs stressés par la chaleur

La réduction des performances de croissance des porcs pendant le stress thermique (HS) est principalement induite par une diminution de la consommation alimentaire (CMJ). Il a été rapporté que la supplémentation en levures vivantes (LY) améliorait les réponses HS en augmentant la consommation alimentaire et l'efficacité alimentaire chez les porcs mâles entiers (EM) (Labussière et al., 2021), mais cela n'a pas été confirmé chez les femelles (F) (Chapitre 2). En effet, un effet du dimorphisme sexuel a le stresse thermique a déjà été rapporté avec des castrats ayant une efficacité alimentaire inférieure à celle des femelles pendant l'HS (Renaudeau et al., 2006) mais aucune étude n'existe à ce jour concernant la différence entre EM et F. Ainsi, l'objectif de l'étude était de déterminer l'effet du sexe et de la supplémentation en LY chez des porcs exposés à un stress thermique. Un total de 64 porcs de finition logés individuellement (32 EM et 32 F ;80,8 ± (Blanchard et al., 1999 ;[START_REF] Ruiz-Ascacibar | Impact of amino acid and CP restriction from 20 to 140 kg BW on performance and dynamics in empty body protein and lipid deposition of entire male, castrated and female pigs[END_REF]. Cependant, l'amélioration de l'efficacité alimentaire des porcs EM dans cette étude n'a été observée que sur des porcs plus légers (répétition 1) mais pas sur des porcs plus lourds (répétition 2) avec une interaction significative de la répétition et du sexe sur le ratio gain de poids: alimentation des porcs (P <0, 05). Cela suggère que l'effet négatif de l'HS sur l'efficacité alimentaire est accru chez les porcs EM plus lourds, mais pas chez les porcs F, probablement en raison de l'effet exacerbé de l'HS sur leur capacité à atteindre leur potentiel maximal de dépôt de protéines ou de croissance (Le Bellego et al., 2002). Cela pourrait être attribué à une sensibilité plus élevée des porcs plus lourds à un HS (Renaudeau et al., 2011) associée à une production de chaleur plus élevée des porcs EM en raison de leur potentiel de croissance et de leur dépôt de protéines plus élevés [START_REF] Moughan | A genetic upper limit to whole-body protein deposition in a strain of growing pigs[END_REF]Labussière et al., 2013). Il y avait une interaction significative entre la répétition, le régime alimentaire et le sexe pour la consommation alimentaire (CMJ) exprimé par BW 0,60 et ADG (P <0,05). Lors de la répétition 2, les porcs EM supplémentés avaient un CMJ plus élevé par BW 0,60 (P<0,05) que les porcs EM supplémentés de la répétition 1 et les porcs F des répétitions 1 et 2. Leur GMQ était également plus élevé que les porcs F dans la répétition 1 et les porcs F supplémentés dans la répétition 2. Les porcs EM supplémentés avaient seulement tendance à avoir un CMJ plus élevé par BW 0,60 (P = 0,08) que les porcs EM non supplémentés dans la répétition 2. Bien qu'il ne s'agisse que d'une tendance, cela suggère que la supplémentation en LY semble améliorer la CMJ pendant le HS, mais uniquement chez les porcs EM et en particulier quand ils sont lourds. En effet, en raison de l'écart type élevé de la réponse CMJ des porcs de l'étude, il aurait fallu environ 400 porcs pour détecter une différence significative de P = 0,05 entre les régimes selon PROC GLMPOWER. Ainsi, sur la base de nos résultats, l'effet de la supplémentation en LY sur les réponses HS des porcs semble dépendre du poids corporel et du sexe de l'animal.

Conclusions

Les principaux objectifs de ce travail de thèse étaient de 1) étudier les effets d'une exposition à la chaleur en période prénatale chronique sur la sensibilité au stress thermique des animaux pendant la période de croissance-finition 2) distinguer les effets directs et indirects du stress thermique sur les performances de croissance, les réponses physiologiques et le métabolisme Avec l'augmentation de la température mondiale et des vagues de chaleur plus fréquentes, le stress thermique réduit la production porcine dans de nombreux pays. La diminution de la croissance est principalement liée à une réduction de la consommation alimentaire mais d'autres aspects du stress thermique chez les porcs sont apparus au cours des dernières années.

La thèse vise à contribuer aux connaissances actuelles avec les objectifs suivants : 1) étudier l'effet d'un stress thermique prénatal chronique sur la tolérance des porcs à la chaleur, 2) confirmer les effets directs et indirects de l'exposition à la chaleur, 3) évaluer les stratégies d'alimentation qui peuvent aider les porcs pendant le stress thermique, et 4) explorer l'idée que le maintien de l'équilibre thermique est un processus dynamique qui varie tout au long de la journée. Ce projet de thèse a montré qu'un déséquilibre entre la production et la perte de chaleur (charge thermique) et la capacité de perte de chaleur entraîne une réduction des performances, non seulement une croissance réduite, mais également une altération du métabolisme énergétique, des réponses physiologiques et une composition corporelle exacerbée par une exposition au stress thermique pendant la période foetale.De plus, l'amélioration de la tolérance à la chaleur des porcs est liée dans leur capacité à maintenir un équilibre dynamique entre la production et la perte de chaleur tout au long de la journée, en particulier pendant et après un repas en raison de la forte contribution du comportement alimentaire à la production de chaleur. Nos résultats ont également confirmé la tolérance à la chaleur est en partie déterminée par la capacité des animaux à perdre de la chaleur par la voie évaporative. Les stratégies nutritionnelles liées à un comportement alimentaire modifié ont été évaluée. La supplémentation en levures vivantes a amélioré la sensibilité à l'insuline et l'efficacité de la perte de chaleur qui a aidé les porcs à faire face au stress thermique en modifiant le comportement alimentaire avec des effets positifs sur l'efficacité énergétique lors d'un défi thermique.

Title :

Dynamic responses of growing pigs to heat stress modulated by prenatal life and feeding practices Keywords : heat stress, pigs, energy metabolism, thermoregulation feeding behaviour, live yeast

Abstract :

Heat stress remains to be one of the major limiting factors in swine production in many countries and this concern will rise in the future especially with increasing global temperature and more frequent heat waves due to warming climate. While decreased growth is mainly related to a reduction in feed intake, other aspects of heat stress in growing pigs have emerged in the recent decades. The thesis thus aims to contribute to the current knowledge with the following objectives: 1) to investigate effect of a chronic prenatal heat challenge on pig's heat tolerance, 2) to confirm direct and indirect effects of heat exposure on pig performance, 3) to evaluate feeding practices that can help pigs cope to a heat challenge, and 4) to explore the idea of maintaining thermal balance as a dynamic process that varies throughout the day. This thesis project has demonstrated that an imbalance between heat production and loss (heat load) results in reduced growth and in altered energy metabolism, physiological responses, and body composition. This later effect is exacerbated in pigs exposed to a chronic prenatal heat challenge. Improved heat tolerance in pigs was found to depend on their ability to maintain dynamic equilibrium between heat production and losses within a day, especially during and after a meal. Our results also confirmed the importance of water intake in improving evaporative heat losses in the ability of pigs to cope with heat stress. Nutritional management practices linked to modified feeding behavior were evaluated. Live yeast supplementation improved insulin sensitivity, increased heat loss efficiency under heat stress, and modified feeding behavior with positive effects on energy efficiency and protein deposition during a heat challenge.

  Title page .......................................................................................................................... i

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 2 .............................................................. Table Critical ambient temperature according to age, body weight, physiological stage, floor type, air speed, and skin wetness. .................................................................................................................. Table

  1 . ............................. Table 10. Summary of the effects of HS only on ad libitum-fed non-supplemented growing pigs in the experiments of the thesis project. ........................................................................................................

  Figure 1. Simplified representation of where heat is generated in animal cells: ATP production and ATP utilization. ATP is the chemical link between catabolism and anabolism and is the energy currency of the living cell. (Adapted from Lehninger et al., 2005) ..................................................................... 17Figure 2. Example of heat production partitioning between components due to the basal metabolic rate (fasting heat production; FHP), the short-and long-term thermic effect of feeding (TEFst and TEFlt), and the physical activity (AHP). MHP (or RHP; minimal or resting HP) is the sum of the FHP and the TEFlt. (Adapted from Labussière et al., 2013). ..................................................................................... 20 Figure 3. The core and shell model for heat transfer driven by (1) the internal and (2) the external thermal gradient. ................................................................................................................................... 22 Figure 4. Proportion of heat loss via the sensible route decreases and via the latent route increases with increasing ambient temperature. Adapted from a) Bond et al. (1952) and b) Renaudeau (unpublished results). .................................................................................................................................................. 23 Figure 5. Relative heat loss modeled as a function of relative humidity at different ambient temperatures. (Adapted from Fialho et al., 2004). ................................................................................ 24 Figure 6.Model of thermoregulation as a negative feedback system for maintaining a homeothermic body temperature. If there is a deviation sensed in the body temperature, the system makes alterations to restore it toward its set point value. BAT: brown adipose tissue. (Adapted from[START_REF] Sawka | Physiological responses to acute exercise-heat stress[END_REF][START_REF] Weller | Body temperature and its regulation[END_REF][START_REF] Modell | A physiologist's view of homeostasis[END_REF][START_REF] Fossion | A physicist's view of homeostasis: how time series of continuous monitoring reflect the function of physiological variables in regulatory mechanisms[END_REF][START_REF] Tan | Regulation of body temperature by the nervous system[END_REF]. ................ 25 Figure 7. General relationship of the core body temperature, heat production, and heat losses according to the ambient temperature in growing pigs. LCT: Lower critical temperature, ECT: evaporative critical temperature, UCT: upper critical temperature. Adapted from Mount (1979), Renaudeau et al. (2004), Renaudeau et al. (2012), and Gourdine et al. (2021). ........................................................................... 26 Figure 8. Heat production, and sensible and latent heat losses of pigs subjected to a mean air temperature of 30.8°C but with different temperature fluctuations: a) CON or constant at 30.8°C, b) RPK or reduced peak temperature of 26 to 33°C, and c) RNT or reduced night temperature of 23.4 to 40°C. (Adapted from Xin and DeShazer, 1991) ................................................................................... 30 Figure 9. Body (intestinal) temperature of pigs with a high (400 g/meal) or low (320 g/meal) feed intake recorded at 5-min intervals. Pigs received their meals (0600, 1400, and 2200 h) and were exposed to a natural ambient temperature from around 29 to 36°C. (Adapted from Cervantes et al., 2018). ........... 30 Figure 10. Postprandial plasma kinetics of insulin and glucose exposed to different ambient temperatures after 17 d at 24°C or 30°C (Adapted from Campos et al., 2019). ................................... 31 Figure 11. Relationship between heat stress and the autonomic nervous system (ANS), and the hypothalamic-pituitary-adrenal (HPA) axis which are implicated in thermoregulation and thermal acclimation responses. (Adapted from Gonzalez-Rivas et al., 2020) ................................................... 33 Figure 12. Effect of temperature and ME intake on protein and lipid deposition in 24 to 65 kg live weight barrows. Predicted data for protein deposition (PD, g/day) at 23°C (-), at 30°C (-) and lipid deposition (LD, g/day) at 23°C (----), at 30°C (----). (Adapted from Le Bellego et al., 2002b). ..... 34 Figure 13. Some animal and environment related factors affecting severity of heat stress in pigs. .... 36 Figure 14. Effect of ambient temperature and body weight on ADFI and ADG of growing pigs. (Adapted from Renaudeau et al., 2011) ................................................................................................ 36 Figure 15. Absolute drop in body weight gain (a), and feed intake (b) between a constant TN (21.9°C) and cyclical HS (31.8°C average) HS environments for three pig lines subjected thrice to a 4-day heat exposure (HS1, HS2, and HS3). HS1 was preceded by a 9-day TN period and HS2 and HS3 by a 7-day TN period. (Adapted from Rauw et al., 2017). ..................................................................................... 37

Figure 2 .

 2 Figure 1. Simplified representation of where heat is generated in animal cells: ATP production and ATP utilization. ATP is the chemical link between catabolism and anabolism and is the energy currency of the living cell. (Adapted from Lehninger et al., 2005) ..................................................................... 17Figure 2. Example of heat production partitioning between components due to the basal metabolic rate (fasting heat production; FHP), the short-and long-term thermic effect of feeding (TEFst and TEFlt), and the physical activity (AHP). MHP (or RHP; minimal or resting HP) is the sum of the FHP and the TEFlt. (Adapted from Labussière et al., 2013). ..................................................................................... 20 Figure 3. The core and shell model for heat transfer driven by (1) the internal and (2) the external thermal gradient. ................................................................................................................................... 22 Figure 4. Proportion of heat loss via the sensible route decreases and via the latent route increases with increasing ambient temperature. Adapted from a) Bond et al. (1952) and b) Renaudeau (unpublished results). .................................................................................................................................................. 23 Figure 5. Relative heat loss modeled as a function of relative humidity at different ambient temperatures. (Adapted from Fialho et al., 2004). ................................................................................ 24 Figure 6.Model of thermoregulation as a negative feedback system for maintaining a homeothermic body temperature. If there is a deviation sensed in the body temperature, the system makes alterations to restore it toward its set point value. BAT: brown adipose tissue. (Adapted from[START_REF] Sawka | Physiological responses to acute exercise-heat stress[END_REF][START_REF] Weller | Body temperature and its regulation[END_REF][START_REF] Modell | A physiologist's view of homeostasis[END_REF][START_REF] Fossion | A physicist's view of homeostasis: how time series of continuous monitoring reflect the function of physiological variables in regulatory mechanisms[END_REF][START_REF] Tan | Regulation of body temperature by the nervous system[END_REF]. ................ 25 Figure 7. General relationship of the core body temperature, heat production, and heat losses according to the ambient temperature in growing pigs. LCT: Lower critical temperature, ECT: evaporative critical temperature, UCT: upper critical temperature. Adapted from Mount (1979), Renaudeau et al. (2004), Renaudeau et al. (2012), and Gourdine et al. (2021). ........................................................................... 26 Figure 8. Heat production, and sensible and latent heat losses of pigs subjected to a mean air temperature of 30.8°C but with different temperature fluctuations: a) CON or constant at 30.8°C, b) RPK or reduced peak temperature of 26 to 33°C, and c) RNT or reduced night temperature of 23.4 to 40°C. (Adapted from Xin and DeShazer, 1991) ................................................................................... 30 Figure 9. Body (intestinal) temperature of pigs with a high (400 g/meal) or low (320 g/meal) feed intake recorded at 5-min intervals. Pigs received their meals (0600, 1400, and 2200 h) and were exposed to a natural ambient temperature from around 29 to 36°C. (Adapted from Cervantes et al., 2018). ........... 30 Figure 10. Postprandial plasma kinetics of insulin and glucose exposed to different ambient temperatures after 17 d at 24°C or 30°C (Adapted from Campos et al., 2019). ................................... 31 Figure 11. Relationship between heat stress and the autonomic nervous system (ANS), and the hypothalamic-pituitary-adrenal (HPA) axis which are implicated in thermoregulation and thermal acclimation responses. (Adapted from Gonzalez-Rivas et al., 2020) ................................................... 33 Figure 12. Effect of temperature and ME intake on protein and lipid deposition in 24 to 65 kg live weight barrows. Predicted data for protein deposition (PD, g/day) at 23°C (-), at 30°C (-) and lipid deposition (LD, g/day) at 23°C (----), at 30°C (----). (Adapted from Le Bellego et al., 2002b). ..... 34 Figure 13. Some animal and environment related factors affecting severity of heat stress in pigs. .... 36 Figure 14. Effect of ambient temperature and body weight on ADFI and ADG of growing pigs. (Adapted from Renaudeau et al., 2011) ................................................................................................ 36 Figure 15. Absolute drop in body weight gain (a), and feed intake (b) between a constant TN (21.9°C) and cyclical HS (31.8°C average) HS environments for three pig lines subjected thrice to a 4-day heat exposure (HS1, HS2, and HS3). HS1 was preceded by a 9-day TN period and HS2 and HS3 by a 7-day TN period. (Adapted from Rauw et al., 2017). ..................................................................................... 37

20 Figure 3 . 22 Figure 4 . 23 Figure 5 .

 203224235 Figure 1. Simplified representation of where heat is generated in animal cells: ATP production and ATP utilization. ATP is the chemical link between catabolism and anabolism and is the energy currency of the living cell. (Adapted from Lehninger et al., 2005) ..................................................................... 17Figure 2. Example of heat production partitioning between components due to the basal metabolic rate (fasting heat production; FHP), the short-and long-term thermic effect of feeding (TEFst and TEFlt), and the physical activity (AHP). MHP (or RHP; minimal or resting HP) is the sum of the FHP and the TEFlt. (Adapted from Labussière et al., 2013). ..................................................................................... 20 Figure 3. The core and shell model for heat transfer driven by (1) the internal and (2) the external thermal gradient. ................................................................................................................................... 22 Figure 4. Proportion of heat loss via the sensible route decreases and via the latent route increases with increasing ambient temperature. Adapted from a) Bond et al. (1952) and b) Renaudeau (unpublished results). .................................................................................................................................................. 23 Figure 5. Relative heat loss modeled as a function of relative humidity at different ambient temperatures. (Adapted from Fialho et al., 2004). ................................................................................ 24 Figure 6.Model of thermoregulation as a negative feedback system for maintaining a homeothermic body temperature. If there is a deviation sensed in the body temperature, the system makes alterations to restore it toward its set point value. BAT: brown adipose tissue. (Adapted from[START_REF] Sawka | Physiological responses to acute exercise-heat stress[END_REF][START_REF] Weller | Body temperature and its regulation[END_REF][START_REF] Modell | A physiologist's view of homeostasis[END_REF][START_REF] Fossion | A physicist's view of homeostasis: how time series of continuous monitoring reflect the function of physiological variables in regulatory mechanisms[END_REF][START_REF] Tan | Regulation of body temperature by the nervous system[END_REF]. ................ 25 Figure 7. General relationship of the core body temperature, heat production, and heat losses according to the ambient temperature in growing pigs. LCT: Lower critical temperature, ECT: evaporative critical temperature, UCT: upper critical temperature. Adapted from Mount (1979), Renaudeau et al. (2004), Renaudeau et al. (2012), and Gourdine et al. (2021). ........................................................................... 26 Figure 8. Heat production, and sensible and latent heat losses of pigs subjected to a mean air temperature of 30.8°C but with different temperature fluctuations: a) CON or constant at 30.8°C, b) RPK or reduced peak temperature of 26 to 33°C, and c) RNT or reduced night temperature of 23.4 to 40°C. (Adapted from Xin and DeShazer, 1991) ................................................................................... 30 Figure 9. Body (intestinal) temperature of pigs with a high (400 g/meal) or low (320 g/meal) feed intake recorded at 5-min intervals. Pigs received their meals (0600, 1400, and 2200 h) and were exposed to a natural ambient temperature from around 29 to 36°C. (Adapted from Cervantes et al., 2018). ........... 30 Figure 10. Postprandial plasma kinetics of insulin and glucose exposed to different ambient temperatures after 17 d at 24°C or 30°C (Adapted from Campos et al., 2019). ................................... 31 Figure 11. Relationship between heat stress and the autonomic nervous system (ANS), and the hypothalamic-pituitary-adrenal (HPA) axis which are implicated in thermoregulation and thermal acclimation responses. (Adapted from Gonzalez-Rivas et al., 2020) ................................................... 33 Figure 12. Effect of temperature and ME intake on protein and lipid deposition in 24 to 65 kg live weight barrows. Predicted data for protein deposition (PD, g/day) at 23°C (-), at 30°C (-) and lipid deposition (LD, g/day) at 23°C (----), at 30°C (----). (Adapted from Le Bellego et al., 2002b). ..... 34 Figure 13. Some animal and environment related factors affecting severity of heat stress in pigs. .... 36 Figure 14. Effect of ambient temperature and body weight on ADFI and ADG of growing pigs. (Adapted from Renaudeau et al., 2011) ................................................................................................ 36 Figure 15. Absolute drop in body weight gain (a), and feed intake (b) between a constant TN (21.9°C) and cyclical HS (31.8°C average) HS environments for three pig lines subjected thrice to a 4-day heat exposure (HS1, HS2, and HS3). HS1 was preceded by a 9-day TN period and HS2 and HS3 by a 7-day TN period. (Adapted from Rauw et al., 2017). ..................................................................................... 37

26 Figure 8 .

 268 Figure 1. Simplified representation of where heat is generated in animal cells: ATP production and ATP utilization. ATP is the chemical link between catabolism and anabolism and is the energy currency of the living cell. (Adapted from Lehninger et al., 2005) ..................................................................... 17Figure 2. Example of heat production partitioning between components due to the basal metabolic rate (fasting heat production; FHP), the short-and long-term thermic effect of feeding (TEFst and TEFlt), and the physical activity (AHP). MHP (or RHP; minimal or resting HP) is the sum of the FHP and the TEFlt. (Adapted from Labussière et al., 2013). ..................................................................................... 20 Figure 3. The core and shell model for heat transfer driven by (1) the internal and (2) the external thermal gradient. ................................................................................................................................... 22 Figure 4. Proportion of heat loss via the sensible route decreases and via the latent route increases with increasing ambient temperature. Adapted from a) Bond et al. (1952) and b) Renaudeau (unpublished results). .................................................................................................................................................. 23 Figure 5. Relative heat loss modeled as a function of relative humidity at different ambient temperatures. (Adapted from Fialho et al., 2004). ................................................................................ 24 Figure 6.Model of thermoregulation as a negative feedback system for maintaining a homeothermic body temperature. If there is a deviation sensed in the body temperature, the system makes alterations to restore it toward its set point value. BAT: brown adipose tissue. (Adapted from[START_REF] Sawka | Physiological responses to acute exercise-heat stress[END_REF][START_REF] Weller | Body temperature and its regulation[END_REF][START_REF] Modell | A physiologist's view of homeostasis[END_REF][START_REF] Fossion | A physicist's view of homeostasis: how time series of continuous monitoring reflect the function of physiological variables in regulatory mechanisms[END_REF][START_REF] Tan | Regulation of body temperature by the nervous system[END_REF]. ................ 25 Figure 7. General relationship of the core body temperature, heat production, and heat losses according to the ambient temperature in growing pigs. LCT: Lower critical temperature, ECT: evaporative critical temperature, UCT: upper critical temperature. Adapted from Mount (1979), Renaudeau et al. (2004), Renaudeau et al. (2012), and Gourdine et al. (2021). ........................................................................... 26 Figure 8. Heat production, and sensible and latent heat losses of pigs subjected to a mean air temperature of 30.8°C but with different temperature fluctuations: a) CON or constant at 30.8°C, b) RPK or reduced peak temperature of 26 to 33°C, and c) RNT or reduced night temperature of 23.4 to 40°C. (Adapted from Xin and DeShazer, 1991) ................................................................................... 30 Figure 9. Body (intestinal) temperature of pigs with a high (400 g/meal) or low (320 g/meal) feed intake recorded at 5-min intervals. Pigs received their meals (0600, 1400, and 2200 h) and were exposed to a natural ambient temperature from around 29 to 36°C. (Adapted from Cervantes et al., 2018). ........... 30 Figure 10. Postprandial plasma kinetics of insulin and glucose exposed to different ambient temperatures after 17 d at 24°C or 30°C (Adapted from Campos et al., 2019). ................................... 31 Figure 11. Relationship between heat stress and the autonomic nervous system (ANS), and the hypothalamic-pituitary-adrenal (HPA) axis which are implicated in thermoregulation and thermal acclimation responses. (Adapted from Gonzalez-Rivas et al., 2020) ................................................... 33 Figure 12. Effect of temperature and ME intake on protein and lipid deposition in 24 to 65 kg live weight barrows. Predicted data for protein deposition (PD, g/day) at 23°C (-), at 30°C (-) and lipid deposition (LD, g/day) at 23°C (----), at 30°C (----). (Adapted from Le Bellego et al., 2002b). ..... 34 Figure 13. Some animal and environment related factors affecting severity of heat stress in pigs. .... 36 Figure 14. Effect of ambient temperature and body weight on ADFI and ADG of growing pigs. (Adapted from Renaudeau et al., 2011) ................................................................................................ 36 Figure 15. Absolute drop in body weight gain (a), and feed intake (b) between a constant TN (21.9°C) and cyclical HS (31.8°C average) HS environments for three pig lines subjected thrice to a 4-day heat exposure (HS1, HS2, and HS3). HS1 was preceded by a 9-day TN period and HS2 and HS3 by a 7-day TN period. (Adapted from Rauw et al., 2017). ..................................................................................... 37

31 Figure 11 .

 3111 Figure 1. Simplified representation of where heat is generated in animal cells: ATP production and ATP utilization. ATP is the chemical link between catabolism and anabolism and is the energy currency of the living cell. (Adapted from Lehninger et al., 2005) ..................................................................... 17Figure 2. Example of heat production partitioning between components due to the basal metabolic rate (fasting heat production; FHP), the short-and long-term thermic effect of feeding (TEFst and TEFlt), and the physical activity (AHP). MHP (or RHP; minimal or resting HP) is the sum of the FHP and the TEFlt. (Adapted from Labussière et al., 2013). ..................................................................................... 20 Figure 3. The core and shell model for heat transfer driven by (1) the internal and (2) the external thermal gradient. ................................................................................................................................... 22 Figure 4. Proportion of heat loss via the sensible route decreases and via the latent route increases with increasing ambient temperature. Adapted from a) Bond et al. (1952) and b) Renaudeau (unpublished results). .................................................................................................................................................. 23 Figure 5. Relative heat loss modeled as a function of relative humidity at different ambient temperatures. (Adapted from Fialho et al., 2004). ................................................................................ 24 Figure 6.Model of thermoregulation as a negative feedback system for maintaining a homeothermic body temperature. If there is a deviation sensed in the body temperature, the system makes alterations to restore it toward its set point value. BAT: brown adipose tissue. (Adapted from[START_REF] Sawka | Physiological responses to acute exercise-heat stress[END_REF][START_REF] Weller | Body temperature and its regulation[END_REF][START_REF] Modell | A physiologist's view of homeostasis[END_REF][START_REF] Fossion | A physicist's view of homeostasis: how time series of continuous monitoring reflect the function of physiological variables in regulatory mechanisms[END_REF][START_REF] Tan | Regulation of body temperature by the nervous system[END_REF]. ................ 25 Figure 7. General relationship of the core body temperature, heat production, and heat losses according to the ambient temperature in growing pigs. LCT: Lower critical temperature, ECT: evaporative critical temperature, UCT: upper critical temperature. Adapted from Mount (1979), Renaudeau et al. (2004), Renaudeau et al. (2012), and Gourdine et al. (2021). ........................................................................... 26 Figure 8. Heat production, and sensible and latent heat losses of pigs subjected to a mean air temperature of 30.8°C but with different temperature fluctuations: a) CON or constant at 30.8°C, b) RPK or reduced peak temperature of 26 to 33°C, and c) RNT or reduced night temperature of 23.4 to 40°C. (Adapted from Xin and DeShazer, 1991) ................................................................................... 30 Figure 9. Body (intestinal) temperature of pigs with a high (400 g/meal) or low (320 g/meal) feed intake recorded at 5-min intervals. Pigs received their meals (0600, 1400, and 2200 h) and were exposed to a natural ambient temperature from around 29 to 36°C. (Adapted from Cervantes et al., 2018). ........... 30 Figure 10. Postprandial plasma kinetics of insulin and glucose exposed to different ambient temperatures after 17 d at 24°C or 30°C (Adapted from Campos et al., 2019). ................................... 31 Figure 11. Relationship between heat stress and the autonomic nervous system (ANS), and the hypothalamic-pituitary-adrenal (HPA) axis which are implicated in thermoregulation and thermal acclimation responses. (Adapted from Gonzalez-Rivas et al., 2020) ................................................... 33 Figure 12. Effect of temperature and ME intake on protein and lipid deposition in 24 to 65 kg live weight barrows. Predicted data for protein deposition (PD, g/day) at 23°C (-), at 30°C (-) and lipid deposition (LD, g/day) at 23°C (----), at 30°C (----). (Adapted from Le Bellego et al., 2002b). ..... 34 Figure 13. Some animal and environment related factors affecting severity of heat stress in pigs. .... 36 Figure 14. Effect of ambient temperature and body weight on ADFI and ADG of growing pigs. (Adapted from Renaudeau et al., 2011) ................................................................................................ 36 Figure 15. Absolute drop in body weight gain (a), and feed intake (b) between a constant TN (21.9°C) and cyclical HS (31.8°C average) HS environments for three pig lines subjected thrice to a 4-day heat exposure (HS1, HS2, and HS3). HS1 was preceded by a 9-day TN period and HS2 and HS3 by a 7-day TN period. (Adapted from Rauw et al., 2017). ..................................................................................... 37

Figure 13 . 36 Figure 14 . 36 Figure 15 .

 1336143615 Figure 1. Simplified representation of where heat is generated in animal cells: ATP production and ATP utilization. ATP is the chemical link between catabolism and anabolism and is the energy currency of the living cell. (Adapted from Lehninger et al., 2005) ..................................................................... 17Figure 2. Example of heat production partitioning between components due to the basal metabolic rate (fasting heat production; FHP), the short-and long-term thermic effect of feeding (TEFst and TEFlt), and the physical activity (AHP). MHP (or RHP; minimal or resting HP) is the sum of the FHP and the TEFlt. (Adapted from Labussière et al., 2013). ..................................................................................... 20 Figure 3. The core and shell model for heat transfer driven by (1) the internal and (2) the external thermal gradient. ................................................................................................................................... 22 Figure 4. Proportion of heat loss via the sensible route decreases and via the latent route increases with increasing ambient temperature. Adapted from a) Bond et al. (1952) and b) Renaudeau (unpublished results). .................................................................................................................................................. 23 Figure 5. Relative heat loss modeled as a function of relative humidity at different ambient temperatures. (Adapted from Fialho et al., 2004). ................................................................................ 24 Figure 6.Model of thermoregulation as a negative feedback system for maintaining a homeothermic body temperature. If there is a deviation sensed in the body temperature, the system makes alterations to restore it toward its set point value. BAT: brown adipose tissue. (Adapted from[START_REF] Sawka | Physiological responses to acute exercise-heat stress[END_REF][START_REF] Weller | Body temperature and its regulation[END_REF][START_REF] Modell | A physiologist's view of homeostasis[END_REF][START_REF] Fossion | A physicist's view of homeostasis: how time series of continuous monitoring reflect the function of physiological variables in regulatory mechanisms[END_REF][START_REF] Tan | Regulation of body temperature by the nervous system[END_REF]. ................ 25 Figure 7. General relationship of the core body temperature, heat production, and heat losses according to the ambient temperature in growing pigs. LCT: Lower critical temperature, ECT: evaporative critical temperature, UCT: upper critical temperature. Adapted from Mount (1979), Renaudeau et al. (2004), Renaudeau et al. (2012), and Gourdine et al. (2021). ........................................................................... 26 Figure 8. Heat production, and sensible and latent heat losses of pigs subjected to a mean air temperature of 30.8°C but with different temperature fluctuations: a) CON or constant at 30.8°C, b) RPK or reduced peak temperature of 26 to 33°C, and c) RNT or reduced night temperature of 23.4 to 40°C. (Adapted from Xin and DeShazer, 1991) ................................................................................... 30 Figure 9. Body (intestinal) temperature of pigs with a high (400 g/meal) or low (320 g/meal) feed intake recorded at 5-min intervals. Pigs received their meals (0600, 1400, and 2200 h) and were exposed to a natural ambient temperature from around 29 to 36°C. (Adapted from Cervantes et al., 2018). ........... 30 Figure 10. Postprandial plasma kinetics of insulin and glucose exposed to different ambient temperatures after 17 d at 24°C or 30°C (Adapted from Campos et al., 2019). ................................... 31 Figure 11. Relationship between heat stress and the autonomic nervous system (ANS), and the hypothalamic-pituitary-adrenal (HPA) axis which are implicated in thermoregulation and thermal acclimation responses. (Adapted from Gonzalez-Rivas et al., 2020) ................................................... 33 Figure 12. Effect of temperature and ME intake on protein and lipid deposition in 24 to 65 kg live weight barrows. Predicted data for protein deposition (PD, g/day) at 23°C (-), at 30°C (-) and lipid deposition (LD, g/day) at 23°C (----), at 30°C (----). (Adapted from Le Bellego et al., 2002b). ..... 34 Figure 13. Some animal and environment related factors affecting severity of heat stress in pigs. .... 36 Figure 14. Effect of ambient temperature and body weight on ADFI and ADG of growing pigs. (Adapted from Renaudeau et al., 2011) ................................................................................................ 36 Figure 15. Absolute drop in body weight gain (a), and feed intake (b) between a constant TN (21.9°C) and cyclical HS (31.8°C average) HS environments for three pig lines subjected thrice to a 4-day heat exposure (HS1, HS2, and HS3). HS1 was preceded by a 9-day TN period and HS2 and HS3 by a 7-day TN period. (Adapted from Rauw et al., 2017). ..................................................................................... 37

  Figure 1. Simplified representation of where heat is generated in animal cells: ATP production and ATP utilization. ATP is the chemical link between catabolism and anabolism and is the energy currency of the living cell. (Adapted from Lehninger et al., 2005) ..................................................................... 17Figure 2. Example of heat production partitioning between components due to the basal metabolic rate (fasting heat production; FHP), the short-and long-term thermic effect of feeding (TEFst and TEFlt), and the physical activity (AHP). MHP (or RHP; minimal or resting HP) is the sum of the FHP and the TEFlt. (Adapted from Labussière et al., 2013). ..................................................................................... 20 Figure 3. The core and shell model for heat transfer driven by (1) the internal and (2) the external thermal gradient. ................................................................................................................................... 22 Figure 4. Proportion of heat loss via the sensible route decreases and via the latent route increases with increasing ambient temperature. Adapted from a) Bond et al. (1952) and b) Renaudeau (unpublished results). .................................................................................................................................................. 23 Figure 5. Relative heat loss modeled as a function of relative humidity at different ambient temperatures. (Adapted from Fialho et al., 2004). ................................................................................ 24 Figure 6.Model of thermoregulation as a negative feedback system for maintaining a homeothermic body temperature. If there is a deviation sensed in the body temperature, the system makes alterations to restore it toward its set point value. BAT: brown adipose tissue. (Adapted from[START_REF] Sawka | Physiological responses to acute exercise-heat stress[END_REF][START_REF] Weller | Body temperature and its regulation[END_REF][START_REF] Modell | A physiologist's view of homeostasis[END_REF][START_REF] Fossion | A physicist's view of homeostasis: how time series of continuous monitoring reflect the function of physiological variables in regulatory mechanisms[END_REF][START_REF] Tan | Regulation of body temperature by the nervous system[END_REF]. ................ 25 Figure 7. General relationship of the core body temperature, heat production, and heat losses according to the ambient temperature in growing pigs. LCT: Lower critical temperature, ECT: evaporative critical temperature, UCT: upper critical temperature. Adapted from Mount (1979), Renaudeau et al. (2004), Renaudeau et al. (2012), and Gourdine et al. (2021). ........................................................................... 26 Figure 8. Heat production, and sensible and latent heat losses of pigs subjected to a mean air temperature of 30.8°C but with different temperature fluctuations: a) CON or constant at 30.8°C, b) RPK or reduced peak temperature of 26 to 33°C, and c) RNT or reduced night temperature of 23.4 to 40°C. (Adapted from Xin and DeShazer, 1991) ................................................................................... 30 Figure 9. Body (intestinal) temperature of pigs with a high (400 g/meal) or low (320 g/meal) feed intake recorded at 5-min intervals. Pigs received their meals (0600, 1400, and 2200 h) and were exposed to a natural ambient temperature from around 29 to 36°C. (Adapted from Cervantes et al., 2018). ........... 30 Figure 10. Postprandial plasma kinetics of insulin and glucose exposed to different ambient temperatures after 17 d at 24°C or 30°C (Adapted from Campos et al., 2019). ................................... 31 Figure 11. Relationship between heat stress and the autonomic nervous system (ANS), and the hypothalamic-pituitary-adrenal (HPA) axis which are implicated in thermoregulation and thermal acclimation responses. (Adapted from Gonzalez-Rivas et al., 2020) ................................................... 33 Figure 12. Effect of temperature and ME intake on protein and lipid deposition in 24 to 65 kg live weight barrows. Predicted data for protein deposition (PD, g/day) at 23°C (-), at 30°C (-) and lipid deposition (LD, g/day) at 23°C (----), at 30°C (----). (Adapted from Le Bellego et al., 2002b). ..... 34 Figure 13. Some animal and environment related factors affecting severity of heat stress in pigs. .... 36 Figure 14. Effect of ambient temperature and body weight on ADFI and ADG of growing pigs. (Adapted from Renaudeau et al., 2011) ................................................................................................ 36 Figure 15. Absolute drop in body weight gain (a), and feed intake (b) between a constant TN (21.9°C) and cyclical HS (31.8°C average) HS environments for three pig lines subjected thrice to a 4-day heat exposure (HS1, HS2, and HS3). HS1 was preceded by a 9-day TN period and HS2 and HS3 by a 7-day TN period. (Adapted from Rauw et al., 2017). ..................................................................................... 37

Figure 16 .

 16 Figure16. Effects of heat challenge(T at 22°C vs. H at 32°C) in the first (P1), second (P2), and third (P3) experimental periods on internal body temperature (Tcore, °C) profiles throughout the experiment. The challenge, which was repeated in P1, P2, and P3, consisted of a 3-d prechallenge period (22°C), a 5-d HS challenge (32°C), and a 7-d recovery period (22°C) (Adapted from[START_REF] Renaudeau | Impact of single or repeated short-term heat challenges mimicking summer heat waves on thermoregulatory responses and performances in finishing pigs[END_REF].......... 37 Figure17. Relationship between ambient temperature and respiration rate (a), rectal temperature (b), feed intake (c), and water to feed ratio (d) according different relative humidities (50% RH, 65% RH, and 80% RH) (Adapted fromHuynh et al., 2005). ............................................................................... 39 Figure 18. Potential modes of action of Saccharamyces cerevisiae var. boulardii ............................. 41 Figure 19. Summary of the experimental designs of the trials performed in the thesis project. .......... 45 Figure 20. Ambient temperature of the rooms in the experiments. In Exp. 1, 2, and 3, period 1 was considered to be where all groups were in TN environment and period 2 when the HS challenge already started with d 0 considered as the transition between the periods. ....................................................... 47 Figure 21. Diagram of the set up in the experimental pen for Experiments 1, 2, and 4 where pigs were individually housed. A makeshift automatic feed distributor was provided only for Experiment 2 for pigs with 8 feeding distributions. .......................................................................................................... 48 Figure 22. Diagram of the set up in the respiratory chamber for Experiment 3. ................................. 48 Figure 23. Energy and nitrogen balance .............................................................................................. 51 Figure 24. Bullet point summary and conclusions of the chapters in the thesis ................................ 117 Figure 25. Effects of HS after 1 week of heat exposure on ADFI drop (expressed as % of pre-challenge ADFI) according to the experiments (different colors) and sex of pigs used (solid line for females, broken lines for entire males). After 7 d of HS for Exp. 1 (PTN-GHS), Exp. 2 (Trt 5; HS-AL), and Exp. 4 (N -EM and N -F) and after 5 d for Exp. 3 (N -Unli). .................................................. 119 Figure 26. Hourly variations of feed intake, heat production, and body core temperature in growing pigs with restricted feeding windows kept in TN conditions of 22°C (mean±SE). Data from N-2FW (grey) and N-8FW (violet) pigs in Exp. 3. ....................................................................................... 121 Figure 27. Hourly variations in latent and sensible heat losses, estimated heat storage, and body temperature in TN conditions (22°C) according to the feeding management. Data used from pigs in a) N-2FW and b) N-8FW group in Exp. 3. ........................................................................................ 121 Figure 28. Example of the relationship of feeding management and body temperature fluctuations in growing pigs raised in TN conditions (22°C). Data from pigs N063693 for N-Unli (blue) and N062538 for N-8FW (violet) from Exp. 3 ....................................................................................................... 122 Figure 29. Hourly variations of feed intake, heat production, and body temperature in growing pigs according to the frequency of meal distribution (mean±SE). Data from (a) N-2FW and (b) N-8FW pigs raised in TN (blue; 22°C) and HS (red; 28°C) conditions from Exp. 3. ..................................... 123 Figure 30. Latent and sensible HL and estimated heat storage according to the ambient temperature (a. TN at 22°C and b. HS at 28°C) and the feeding management (Unli, 2FW, 8FW). Data used from Exp. 3. ......................................................................................................................................................... 124 Figure 31. Principal component analysis accounting for 82.5% of the total variance among nonsupplemented pigs in Experiment 3 (N-Unli, N-2FW, and N-8FW) according to ambient temperature: TN conditions in black (22°C) and HS conditions in blue (28°C). Parameters included were ME intake, retained energy (retained E), water intake, protein deposition (PD), lipid deposition (LD), and plasma levels of insulin:glucose (InsGlc). ...................................................................................................... 125 Figure 32. Three mechanisms on how the feeding management imposed for the N-8FW groups in Exp. 3 increased feed intake during HS. ............................................................................................. 127 Figure 33. Hourly variations of meal frequency, water intake, and latent heat loss capacity in high ambient temperature (28°C) (mean±SE). Data from Exp. 3 wherein a) non-supplemented 2FW pigs are

39 Figure 18 . 41 Figure 19 . 45 Figure 20 . 47 Figure 21 . 48 Figure 22 . 48 Figure 23 . 51 Figure 24 .Figure 25 .Figure 26 .Figure 27 .Figure 28 .Figure 29 .Figure 30 . 124 Figure 31 .Figure 32 .Figure 33 .Figure 34 .Figure 35 .

 39184119452047214822482351242526272829301243132333435 Figure16. Effects of heat challenge(T at 22°C vs. H at 32°C) in the first (P1), second (P2), and third (P3) experimental periods on internal body temperature (Tcore, °C) profiles throughout the experiment. The challenge, which was repeated in P1, P2, and P3, consisted of a 3-d prechallenge period (22°C), a 5-d HS challenge (32°C), and a 7-d recovery period (22°C) (Adapted from[START_REF] Renaudeau | Impact of single or repeated short-term heat challenges mimicking summer heat waves on thermoregulatory responses and performances in finishing pigs[END_REF].......... 37 Figure17. Relationship between ambient temperature and respiration rate (a), rectal temperature (b), feed intake (c), and water to feed ratio (d) according different relative humidities (50% RH, 65% RH, and 80% RH) (Adapted fromHuynh et al., 2005). ............................................................................... 39 Figure 18. Potential modes of action of Saccharamyces cerevisiae var. boulardii ............................. 41 Figure 19. Summary of the experimental designs of the trials performed in the thesis project. .......... 45 Figure 20. Ambient temperature of the rooms in the experiments. In Exp. 1, 2, and 3, period 1 was considered to be where all groups were in TN environment and period 2 when the HS challenge already started with d 0 considered as the transition between the periods. ....................................................... 47 Figure 21. Diagram of the set up in the experimental pen for Experiments 1, 2, and 4 where pigs were individually housed. A makeshift automatic feed distributor was provided only for Experiment 2 for pigs with 8 feeding distributions. .......................................................................................................... 48 Figure 22. Diagram of the set up in the respiratory chamber for Experiment 3. ................................. 48 Figure 23. Energy and nitrogen balance .............................................................................................. 51 Figure 24. Bullet point summary and conclusions of the chapters in the thesis ................................ 117 Figure 25. Effects of HS after 1 week of heat exposure on ADFI drop (expressed as % of pre-challenge ADFI) according to the experiments (different colors) and sex of pigs used (solid line for females, broken lines for entire males). After 7 d of HS for Exp. 1 (PTN-GHS), Exp. 2 (Trt 5; HS-AL), and Exp. 4 (N -EM and N -F) and after 5 d for Exp. 3 (N -Unli). .................................................. 119 Figure 26. Hourly variations of feed intake, heat production, and body core temperature in growing pigs with restricted feeding windows kept in TN conditions of 22°C (mean±SE). Data from N-2FW (grey) and N-8FW (violet) pigs in Exp. 3. ....................................................................................... 121 Figure 27. Hourly variations in latent and sensible heat losses, estimated heat storage, and body temperature in TN conditions (22°C) according to the feeding management. Data used from pigs in a) N-2FW and b) N-8FW group in Exp. 3. ........................................................................................ 121 Figure 28. Example of the relationship of feeding management and body temperature fluctuations in growing pigs raised in TN conditions (22°C). Data from pigs N063693 for N-Unli (blue) and N062538 for N-8FW (violet) from Exp. 3 ....................................................................................................... 122 Figure 29. Hourly variations of feed intake, heat production, and body temperature in growing pigs according to the frequency of meal distribution (mean±SE). Data from (a) N-2FW and (b) N-8FW pigs raised in TN (blue; 22°C) and HS (red; 28°C) conditions from Exp. 3. ..................................... 123 Figure 30. Latent and sensible HL and estimated heat storage according to the ambient temperature (a. TN at 22°C and b. HS at 28°C) and the feeding management (Unli, 2FW, 8FW). Data used from Exp. 3. ......................................................................................................................................................... 124 Figure 31. Principal component analysis accounting for 82.5% of the total variance among nonsupplemented pigs in Experiment 3 (N-Unli, N-2FW, and N-8FW) according to ambient temperature: TN conditions in black (22°C) and HS conditions in blue (28°C). Parameters included were ME intake, retained energy (retained E), water intake, protein deposition (PD), lipid deposition (LD), and plasma levels of insulin:glucose (InsGlc). ...................................................................................................... 125 Figure 32. Three mechanisms on how the feeding management imposed for the N-8FW groups in Exp. 3 increased feed intake during HS. ............................................................................................. 127 Figure 33. Hourly variations of meal frequency, water intake, and latent heat loss capacity in high ambient temperature (28°C) (mean±SE). Data from Exp. 3 wherein a) non-supplemented 2FW pigs are

Figure 1 .Figure 2 .Figure 3 .Figure 4 .

 1234 FiguresFigure 1. Description of experimental design and the timing of the measurements……….…………90 Figure 2. Effect of diet [0 (N) vs. 100 (Y) g/ton LY supplementation] within each feeding window [(a)Unli vs. (b) 2FW vs. (c) 8FW] on the hourly body temperature (°C) of heat-stressed entire male finishing pigs (means ± SE)… ………………………………………………………………………..98 Figure 3. Effect of diet [0 (N) vs. 100 (Y) g/ton LY supplementation] within each feeding window [(a)Unli vs. (b) 2FW vs. (c) 8FW] on the hourly evaporated latent water (g) of heat-stressed entire male finishing pigs (means ± SE).….….…………………………...……………………………………......99 Figure 4. Effect of diet [0 (N) vs. 100 (Y) g/ton LY supplementation] and feeding window (Unli vs. 2FW vs. 8FW) on the postprandial plasma concentrations of (a) insulin, (b) glucose, (c) lactate, and (d) α-amino nitrogen of heat-stressed entire male finishing pigs (means ± SE)…………..………………100 Figure 5. Effect of diet [0 (N) vs. 100 (Y) g/ton LY supplementation] and feeding window (Unli vs. 2FW vs. 8FW) on the postprandial plasma concentrations of (a) urea, (b) creatinine, and (c) NEFA of heat-stressed entire male finishing pigs (means ± SE)…………………………………….....……….101 Figure 6. Effect of diet [0 (N) vs. 100 (Y) g/ton LY supplementation] and feeding window (Unli vs. 2FW vs. 8FW) on the plasma concentrations of (a) T3 and (b) T4 of heat-stressed entire male finishing pigs (means ± SE). ……………………………………………………………………………….......102

Figure 2 .

 2 Figure 2. Example of heat production partitioning between components due to the basal metabolic rate (fasting heat production; FHP), the short-and long-term thermic effect of feeding (TEFst and TEFlt), and the physical activity (AHP). MHP (or RHP; minimal or resting HP) is the sum of the FHP and the TEFlt. (Adapted from Labussière et al., 2013).

Figure 3 .

 3 Figure 3. The core and shell model for heat transfer driven by (1) the internal and (2) the external thermal gradient.

Figure 4 .

 4 Figure 4. Proportion of heat loss via the sensible route decreases and via the latent route increases with increasing ambient temperature. Adapted from a) Bond et al. (1952) and b) Renaudeau (unpublished results).

Figure 5 .

 5 Figure 5. Relative heat loss modeled as a function of relative humidity at different ambient temperatures. (Adapted from Fialho et al., 2004).

Figure 7 .

 7 Figure 7. General relationship of the core body temperature, heat production, and heat losses according to the ambient temperature in growing pigs. LCT: Lower critical temperature, ECT: evaporative critical temperature, UCT: upper critical temperature. Adapted from Mount (1979), Renaudeau et al. (2004), Renaudeau et al. (2012), and Gourdine et al. (2021).

Figure 8 .

 8 Figure 8. Heat production, and sensible and latent heat losses of pigs subjected to a mean air temperature of 30.8°C but with different temperature fluctuations: a) CON or constant at 30.8°C, b) RPK or reduced peak temperature of 26 to 33°C, and c) RNT or reduced night temperature of 23.4 to 40°C. (Adapted fromXin and DeShazer, 1991)

Figure 9

 9 Figure 9. Body (intestinal) temperature of pigs with a high (400 g/meal) or low (320 g/meal) feed intake recorded at 5-min intervals. Pigs received their meals (0600, 1400, and 2200 h) and were exposed to a natural ambient temperature from around 29 to 36°C. (Adapted from Cervantes et al., 2018).

Figure 10 .

 10 Figure 10. Postprandial plasma kinetics of insulin and glucose exposed to different ambient temperatures after 17 d at 24°C or 30°C (Adapted from Campos et al., 2019).

Figure 11 .

 11 Figure 11. Relationship between heat stress and the autonomic nervous system (ANS), and the hypothalamic-pituitary-adrenal (HPA) axis which are implicated in thermoregulation and thermal acclimation responses. (Adapted from Gonzalez-Rivas et al., 2020)

Figure 12 .

 12 Figure12. Effect of temperature and ME intake on protein and lipid deposition in 24 to 65 kg live weight barrows. Predicted data for protein deposition (PD, g/day) at 23°C (-), at 30°C (-) and lipid deposition (LD, g/day) at 23°C (----), at 30°C (----). (Adapted fromLe Bellego et al., 2002b).

Figure 13 .

 13 Figure 13. Some animal and environment related factors affecting severity of heat stress in pigs.

Figure 14 .

 14 Figure 14. Effect of ambient temperature and body weight on ADFI and ADG of growing pigs.(Adapted fromRenaudeau et al., 2011)

Figure 15 .

 15 Figure 15. Absolute drop in body weight gain (a), and feed intake (b) between a constant TN (21.9°C) and cyclical HS (31.8°C average) HS environments for three pig lines subjected thrice to a 4-day heat exposure (HS1, HS2, and HS3). HS1 was preceded by a 9-day TN period and HS2 and HS3 by a 7-day TN period. (Adapted from Rauw et al., 2017).

Figure 16 .

 16 Figure 16. Effects of heat challenge (T at 22°C vs. H at 32°C) in the first (P1), second (P2), and third (P3) experimental periods on internal body temperature (Tcore, °C) profiles throughout the experiment. The challenge, which was repeated in P1, P2, and P3, consisted of a 3-d prechallenge period (22°C), a 5-d HS challenge (32°C), and a 7-d recovery period (22°C) (Adapted from Renaudeau, 2020).

Figure 17 .

 17 Figure 17. Relationship between ambient temperature and respiration rate (a), rectal temperature (b), feed intake (c), and water to feed ratio (d) according different relative humidities (50% RH, 65% RH, and 80% RH) (Adapted from Huynh et al., 2005).

Figure 18 .

 18 Figure 18. Potential modes of action of Saccharamyces cerevisiae var. boulardii (Source: McFarland, 2010)

  evidenced-based mode of action of S. boulardii is mainly in the GIT and has been reviewed by McFarland (2010) (Figure18). In brief, S. boulardii has anti-toxin (blocking pathogen receptor site or acting as decoy receptor for pathogens), and antimicrobial effect (inhibition of pathogenic invasion by improvement of tight junctions and direct inhibition of pathogenic growth). Aside from gut microbiota modulation, LY has been shown to support normal microbiota establishment and to improve gut fermentation by increasing short chain fatty acids in times of disruption and disease. Other mechanisms also include improvement in nutrient utilization by secreting brush border enzymes and in immune responses by reducing proinflammatory responses and increasing antibody secretion such as secretory IgA.

Figure 20 .

 20 Figure 20. Ambient temperature of the rooms in the experiments. In Exp. 1, 2, and 3, period 1 was considered to be where all groups were in TN environment and period 2 when the HS challenge already started with d 0 considered as the transition between the periods. (a) Exp. 1: period 1 from d -7 to -1; period 2 was from d 0 to 57. (b) Exp. 2: period 1 from d -7 to d-1; period 2 was from d 0 to 19. (c) Exp. 3: period 1 from d -5 to -1; period 2 was from d 0 to 4. (d) Exp. 4: Experimental period started after a 3-day adaptation to the HS challenge from d 1 to d 27.

Figure 21 .

 21 Figure 21. Diagram of the set up in the experimental pen for Experiments 1, 2, and 4 where pigs were individually housed. A makeshift automatic feed distributor was provided only for Experiment 2 for pigs with 8 feeding distributions.

Figure 22 .

 22 Figure 22. Diagram of the set up in the respiratory chamber for Experiment 3. Not pictured: drinking bowl (wherein drinking water temperature was continuously measured).

5 ) 2 .

 52 and in supplemented (Trt. 3 and 7) pigs, Ad libitum groups were also used to compare the effects of LY supplementation in TN (Trt 1 vs. 3) and HS (Trt 5 vs. 7) conditions, 3. Pair-fed groups were used to determine the direct effects of HS regardless of the feeding level and behavior [(Trt 2 vs. 6) and (Trt. 4 vs. 7)],

Figure 23 .

 23 Figure 23. Energy and nitrogen balance

  2 and 3. A dynamic view of the partitioning of evaporative and sensible HL were also made possible by the continuous measurement of RH and air in the chambers based on the principle described byRenaudeau et al. (2013) and the measurement of water temperature.Animal metabolismheat production calculated based on respiratory gas exchanges, CH4, and urinary nitrogen (including evaporated nitrogen) (Text boxes 1 and 2). Dynamic measurement of the HP partitioning into AHP, TEF, and MHP were also possible due to the simultaneous measurements of O2 and CO2 concentrations in the respiration chamber, of force sensor signals, and of physical characteristics of the gas in the chamber based on the modeling approach byvan Milgen et al. (1997) but with the modifications described byQuemeneur et al. (2020).

  1330h in Exp. 1 and 2 once in TN conditions(d -4 and d-1, respectively) and on certain days of HS(d 3 and 49 of HS in Exp.1 and d 1, 8 and 20 of Hs in Exp. 2) in order to evaluate the effect of heat exposure duration on the plasma parameters. In Exp. 3, a catheter was inserted in the external jugular vein through a collateral vein under general anesthesia as described byMelchior et al. (2004) to be able to do successive periprandial blood samplings (on d -1 and d 1 of HS) with the least disturbance to the animal. This allowed evaluation of the metabolic status of the animal during its fasted and fed state, and the kinetics of the plasma hormones and metabolites during and after a meal.MicrobiotaSpot fecal samples were collected in all experiments but only those in Exp. 1 were thus far analyzed and presented in this thesis project. In order to evaluate if prenatal and postnatal HS affects the microbiota of the pigs, spot fecal samples were collected on d 4 and d 105 of gestation in the sows (pre-challenge and after 96 d of chronic HS exposure, respectively), and on 78 and 136 d of age (pre-challenge and after 58 d of chronic HS exposure, respectively) in the growing pigs. The samples were then analyzed for 16S rRNA analyses of microbiota composition.

Chapter 1 . 2 . 3 . 4 .Figure 1 . 58 Figure 2 .Figure 3 .

 123415823 Figure 1. Framework of the experimental study from gestation to growing period.

Figure 4 .

 4 Figure 4. Effect of climatic environment (TN or HS) during prenatal development (PE; PTN vs. PHS) and during growing (GE; GTN vs. GHS) on the skin (a) and rectal (b) temperatures of growing pigs.

Figure 5 . 64 Figure 6 .

 5646 Figure 5. Effect of climatic environment (TN or HS) during prenatal development (PE; PTN vs. PHS) and during growing (GE; GTN vs. GHS) on ADFI per metabolic BW (a) and ADG (b) in growing pigs using TN performance as covariate (LSMeans ± SEM).

Results 76 Figure 2 .

 762 Figure 2. Effect of the climatic environment and feeding management on acute response of finishing pigs in terms of Tskin (a), respiration rate (b), Trectal (c), and Tmuscle (d) (means ± SE).

Figure 3 .

 3 Figure 3. Effect of the climatic environment and feeding management on the Tskin (a), respiration rate (b), Trectal (c), and Tmuscle (d) of finishing pigs (means ± SE).

Figure 4 .

 4 Figure 4. Effect of the climatic environment and feeding management on plasma metabolites [creatinine (a), NEFA (b), glucose (c)] and plasma hormones [insulin (d), T3 I, T4 (f), T3:T4 (g), and insulin:glucose (h)] of finishing pigs: (means ± SE).

Figure 5 .

 5 Figure 5. Effect of the environmental treatment and feeding management on the nycthemeral pattern of Tmuscle (°C) and presence at the feeder (min) of finishing pigs.

Figure 6 .Chapter 3 .

 63 Figure 6. Effect of the environmental treatment and order of meal in terms of Tmuscle [baseline (a) and maximum (b)], total presence at the feeder (c), and area under the Tmuscle curve (calculated based on Figure 1) of PF8 finishing pigs.

Figure 1 .

 1 Figure 1. Description of experimental design and the timing of the measurements.

Fat

  deposition was then determined based on the energy balance, assuming that the RE was only deposited as protein (Rep = PD × 23.6 kJ•g-1) or as fat [lipid deposition = (RE -Rep) / 39.5 kJ•g-1]

  2FW, Y-8FW)] and within each FW [(N-Unli vs. Y-Unli), (N-2FW vs. Y-2FW), and (N-8FW vs. Y-8FW)], and to compare the FW: [(N-Unli, Y-Unli) vs. (N-2FW, Y-2FW)], [(N-Unli, Y-Unli) vs. (N-8FW, Y-8FW)], and [(N-2FW, Y-2FW) vs. (N-8FW, Y-8FW)].

3

 3 Contrast statements within period (P<0.05):  [N-Unli, N-2FW, N-8FW] vs. [Y-Unli, Y-2FW, Y-8FW];  (N-Unli vs. Y-Unli);  (N-2FW vs. Y-2FW); and ‡ (N-8FW vs. Y-8FW)] for diet, and # [N-Unli, Y-Unli] vs. [N-2FW, Y-2FW]; || [N-Unli, Y-Unli] vs. [N-8FW, Y-8FW]; § [N-2FW, Y-2FW] vs. [N-8FW, Y-8FW] for FW. 4 RSD: residual standard deviation.

  05):  [N-Unli, N-2FW, N-8FW] vs. [Y-Unli, Y-2FW, Y-8FW];  (N-Unli vs. Y-Unli);  (N-2FW vs. Y-2FW); and ‡ (N-8FW vs. Y-8FW)] for diet, and # [N-Unli, Y-Unli] vs. [N-2FW, Y-2FW]; || [N-Unli, Y-Unli] vs. [N-8FW, Y-8FW]; § [N-2FW, Y-2FW] vs. [N-8FW, Y-8FW] for FW. 4 RSD: residual standard deviation.

Figure 2 .

 2 Figure 2. Effect of diet [0 (N) vs. 100 (Y) g/ton LY supplementation] within each feeding window [(a)Unli vs. (b) 2FW vs. (c) 8FW] on the hourly body temperature (°C) of heat-stressed entire male finishing pigs (means ± SE). The broken line (---) represents pigs with N diet and the solid line (-) pigs fed with Y diet in TN (blue line) and HS (red line) conditions. Pigs were housed at thermoneutrality from d -5 to -1 (P1; 22°C) and then in heat-stressed conditions from d 0 to 4 (P2; 28°C). **P < 0.01.

Figure 3 .

 3 Figure 3. Effect of diet [0 (N) vs. 100 (Y) g/ton LY supplementation] within each feeding window [(a)Unli vs. (b) 2FW vs. (c) 8FW] on the hourly evaporated latent water (g) of heat-stressed entire male finishing pigs (means ± SE). The broken line (---) represents pigs with N diet and the solid line (-) pigs fed with Y diet in TN (blue line) and HS (red line) conditions. Pigs were housed at thermoneutrality from d -5 to -1 (P1; 22°C) and then in heat-stressed conditions from d 0 to 4 (P2; 28°C). T P < 0.10, **P < 0.01.

Figure 4 .

 4 Figure 4. Effect of diet [0 (N) vs. 100 (Y) g/ton LY supplementation] and feeding window (Unli vs. 2FW vs. 8FW) on the postprandial plasma concentrations of (a) insulin, (b) glucose, (c) lactate, and (d) α-amino nitrogen of heat-stressed entire male finishing pigs (means ± SE). Pigs were housed at thermoneutrality from d -5 to -1 (P1; 22°C) and then in heat-stressed conditions from d 0 to 4 (P2; 28°C). Blood sampling was done on d -2 for Period 1 and on d 1 for Period 2.The line (-) corresponds to the predicted values at minute (time; t) obtained with the non-linear model (Supplementary figure1).

Figure 5 .

 5 Figure 5. Effect of diet [0 (N) vs. 100 (Y) g/ton LY supplementation] and feeding window (Unli vs. 2FW vs. 8FW) on the postprandial plasma concentrations of (a) urea, (b) creatinine, and (c) NEFA of heat-stressed entire male finishing pigs (means ± SE). Pigs were housed at thermoneutrality from d -5 to -1 (P1; 22°C) and then in heat-stressed conditions from d 0 to 4 (P2; 28°C). Blood sampling was done on d -2 for Period 1 and on d 1 for Period 2.

Figure 6 .

 6 Figure 6. Effect of diet [0 (N) vs. 100 (Y) g/ton LY supplementation] and feeding window (Unli vs. 2FW vs. 8FW) on the plasma concentrations of (a) T3 and (b) T4 of heat-stressed entire male finishing pigs (means ± SE). Pigs were housed at thermoneutrality from d -5 to -1 (P1; 22°C) and then in heat-stressed conditions from d 0 to 4 (P2; 28°C). Blood sampling was done on d -2 for Period 1 and on d 1 for Period 2. a,b

  BW) compared to the other experiments (-16 to -29% drop in ADFI) with heavier pigs (>70 kg BW) upon heat exposure. Meanwhile, the effect of sex on the severity of HS was less clear. Female pigs decreased their ADFI by only 16% (in Exp. 2 and 4) while the entire male pigs by 29% or 19%(Exp. 3 and 4, respectively). To our knowledge, sexual dimorphic responses to HS has not yet been established in pigs. Previous reports suggested that barrows

Figure 25 .

 25 Figure 25. Effects of HS after 1 week of heat exposure on ADFI drop (expressed as % of prechallenge ADFI) according to the experiments (different colors) and sex of pigs used (solid line for females, broken lines for entire males). After 7 d of HS for Exp. 1 (PTN-GHS), Exp. 2 (Trt 5; HS-AL), and Exp. 4 (N -EM and N -F) and after 5 d for Exp. 3 (N -Unli).

Figure 26 .

 26 Figure 26. Hourly variations of feed intake, heat production, and body core temperature in growing pigs with restricted feeding windows kept in TN conditions of 22°C (mean±SE). Data from N-2FW (grey) and N-8FW (violet) pigs in Exp. 3.

Figure 27 .

 27 Figure 27. Hourly variations in latent and sensible heat losses, estimated heat storage, and body temperature in TN conditions (22°C) according to the feeding management. Data used from pigs in a) N-2FW and b) N-8FW group in Exp. 3.

Figure 28 .

 28 Figure 28. Example of the relationship of feeding management and body temperature fluctuations in growing pigs raised in TN conditions (22°C). Data from pigs N063693 for N-Unli (blue) and N062538 for N-8FW (violet) from Exp. 3

Figure 28 Figure 29 .

 2829 Figure 29. Hourly variations of feed intake, heat production, and body temperature in growing pigs according to the frequency of meal distribution (mean±SE). Data from (a) N-2FW and (b) N-8FW pigs raised in TN (blue; 22°C) and HS (red; 28°C) conditions from Exp. 3.

Figure 30 .

 30 Figure 30. Latent and sensible HL and estimated heat storage according to the ambient temperature (a. TN at 22°C and b. HS at 28°C) and the feeding management (Unli, 2FW, 8FW). Data used from Exp. 3.

Figure 31 .

 31 Figure 31. Principal component analysis accounting for 82.5% of the total variance among nonsupplemented pigs in Experiment 3 (N-Unli, N-2FW, and N-8FW) according to ambient temperature: TN conditions in black (22°C) and HS conditions in blue (28°C). Parameters included were ME intake, retained energy (retained E), water intake, protein deposition (PD), lipid deposition (LD), and plasma levels of insulin:glucose (InsGlc).

Figure 33 .

 33 Figure 33. Hourly variations of meal frequency, water intake, and latent heat loss capacity in high ambient temperature (28°C) (mean±SE). Data from Exp. 3 wherein a) non-supplemented 2FW pigs are represented in grey and 8FW pigs in violet b) non-supplemented Unli pigs are represented in blue and 8FW pigs in violet.

Figure 34 .

 34 Figure 34. Possible mechanism of how live yeast S. boulardii CNCM I-1079 supplementation (and increased meal frequency within the box in broken green lines) improves pig feed intake, energy efficiency, and protein deposition during heat stress. The text and lines in blue are parameters that were measured during the thesis project, in black are the reported effects of LY based on the literature, and in grey are hypothesized links among the elements based on published literature.

Figure 34

 34 summarizes the hypothesized mechanisms by which the increased insulin sensitivity with LY would improve pig performance during HS as discussed extensively in Exp. 3 and in the previous subsections.

Figure 35 .

 35 Figure 35. Hourly variations of feed intake, water intake, and latent heat loss capacity of 2FW pigs in high ambient temperature (28°C) (mean±SE). Data from Exp. 3 wherein nonsupplemented pigs (N) are represented in grey and supplemented pigs (Y) are represented in yellow.

Annex 4 .

 4 Continuation…Annex figure 2. Presence at the feeder (min) of finishing pigs according to the environmental treatment, feeding management, and live yeast supplementation. From d-7 to d-1 (P1), pigs were housed at thermoneutrality and then from d 0 to 21 (P2), either in thermoneutral (TN; 22°C) or in heat-stressed (HS; 32°C) conditions. In each environment, the diet (N: non-supplemented; Y: live yeast supplemented) was provided either ad libitum (AL; 2 distributions/day) or pairfed with two (PF2) or eight (PF8) distributions/day. Data included are from d 6 to 19 of the experimental period with d 0 as the transition period of HS groups from TN to HS conditions.IntroductionLe changement climatique reste l'un des défis majeurs du 21 ème siècle, en particulier avec des vagues de chaleur plus fréquentes et plus sévères, et une température mondiale qui devrait augmenter d'au moins1,5°C entre 20301,5°C entre et 20501,5°C entre (IPCC, 2018)). Cela constitue une challenge pour la production animale dans de nombreux pays en raison de ses conséquences sur la santé et le bien-être des animaux et sur la quantité et la qualité des ressources telles que les ressources utilisées pour l'alimentation animale[START_REF] Rojas-Downing | Climate change and livestock: Impacts, adaptation, and mitigation[END_REF][START_REF] Lacetera | Impact of climate change on animal health and welfare[END_REF] Renaudeau and Dourmad, 2021) et particulièrement pour la production porcine en raison de sa demande croissante et de sa croissance dans les pays subtropicaux et tropicaux (OECD-FAO, 2021).

1 .Chapitre 1 .Chapitre 2 . 3 .

 1123 Étudier les effets d'un HS prénatal chronique sur la croissance, la composition corporelle et les réponses physiologiques des porcs dans des conditions postnatales de TN ou de HS, 2. Évaluer et confirmer les effets indirects (causés par une consommation alimentaire réduite) et directs de l'HS (indépendamment de la consommation et du mode d'alimentation) sur les performances des porcs, 3. Déterminer si les effets positifs de la supplémentation en LY chez les porcs HS sont dus à des modifications du comportement alimentaire ou du métabolisme énergétique, et si ces effets peuvent être reproduits en imposant une fréquence de repas accrue, et 4. Pour mieux comprendre le processus dynamique de maintien de l'équilibre thermique corporel en examinant les variations des réponses de thermorégulations et physiologiques au cours de la journée des porcs soumis à une température ambiante élevée. Stratégie expérimentale générale Quatre expériences ont été planifiées pour répondre aux principales questions de la thèse. Les effets d'un HS prénatal chronique ont été évalués dans l'Expérience 1. Les mécanismes et facteurs possibles affectant les effets positifs de la supplémentation en LY pendant l'HS ont été évalués dans les Expériences 2, 3, et 4. L'Expérience 2 a été conçue pour tester si les effets positifs de LY étaient liés à une modification du comportement aliment (i.e., un nombre plus élevé de repas). Cette expérience a également été conçue pour distinguer les effets directs et indirects d'une température élevée en utilisant une approche d'alimentation « pair-fed » basée à la fois sur la restriction alimentaire et le changement dans la distribution de la fréquence des repas. L'Expérience 3 a été menée dans les chambres respiratoires afin de déterminer si l'effet LY sur le métabolisme énergétique des porcs était dû ou non à l'altération du comportement alimentaire et/ou à la modification du métabolisme énergétique (Labussière et al., 2021). Contrairement à l'Expérience 2, la restriction alimentaire dans l'Expérience 3 n'était limitée qu'au temps et à la durée de d'accès à la mangeoire, mais pas à la quantité d'aliment disponible. Enfin, l'Expérience 4 a été conçue pour avoir un lien entre l'Expérience 2 (toutes femelles) et l'Expérience 3 (tous des mâles entiers) et avec l'idée que la tolérance à la chaleur et la réponse à l'adaptation à la supplémentation en LY peuvent différer entre les sexes. Résultats Cette section se compose de quatre chapitres correspondant à chacune des quatre expériences : Le stress thermique prénatal chronique modifie la croissance, la composition de la carcasse et la réponse physiologique des porcs en croissance soumis au stress thermique postnatal Effet du stress thermique et de la gestion de l'alimentation sur les performances de croissance et les réponses physiologiques des porcs en finition Chapitre Effet de la supplémentation en levures vivantes et de la fréquence d'alimentation chez les porcs stressés par la chaleur Chapitre 4. Effet du sexe et de la supplémentation en levures vivantes chez les porcs stressés par la chaleur comparativement aux autres groupes. Dans l'ensemble, l'augmentation de la température corporelle, l'altération des fonctions thyroïdiennes et des réponses physiologiques au stress suggèrent une diminution de la tolérance à la chaleur et de la capacité de dissipation des porcs soumis à un HS prénatal chronique pendant toute la gestation. Un HS postnatal a diminué les performances de croissance, augmenté l'adiposité de la carcasse et diminué les traits métaboliques et les fonctions thyroïdiennes, en particulier chez les porcs précédemment soumis à un HS prénatal. Chapitre 2. Effet du stress thermique et de la conduite alimentaire sur les performances de croissance et les réponses physiologiques des porcs en finition Cette étude visait à déterminer si les réponses des porcs au stress thermique (HS) étaient directement dues à l'exposition à la chaleur (indépendamment du niveau et de la stratégie d'alimentation) ou étaient indirectement dues à la réduction de la consommation alimentaire mais également de déterminer si l'augmentation de la fréquence d'alimentation (fractionnement des repas) pouvait améliorer la réponse du porc au chaud. Un total de 48 porcs (66,1 ± 1,7 kg) a été réparti en quatre groupes en trois répétitions. Après 7 jours dans des conditions thermoneutres (TN) (22°C; période 1 [P1 ; jours -7 à -1]), les porcs ont été placés dans des conditions TN ou HS (32°C) pendant 20 jours (période 2 [P2 ; jour 0 à 19]). Le régime alimentaire a été fourni soit ad libitum (AL ; 2 distributions/j) soit en utilisant la stratégie du pair-feeding avec la distribution de 8 repas (PF8 ; 8 distributions/j) en utilisant des porcs HS-AL comme groupe de référence. Ainsi, les quatre groupes expérimentaux étaient TN-AL, HS-AL, TN-PF8 et HS-PF8. La ration quotidienne des porcs PF8 a été distribuée toutes les 90 minutes de 0900 à 1930 heures. Les données ont été analysées à l'aide de la procédure PROC MIXED avec répétition (n = 3), groupe expérimental (n = 4) et leurs interactions en tant qu'effets fixes, et le fonction REPEATED a été utilisée pour les données de mesures répétées.

Chapitre 3 .

 3 Effet de la supplémentation en levures vivantes et de la fréquence d'alimentation chez les porcs exposés la chaleur Chez les porcs en croissance, une croissance réduite pendant le stress thermique (HS) est principalement liée à une diminution de la consommation d'aliments. L'étude visait à déterminer si les effets positifs rapportés de la supplémentation en levures vivantes (LY) chez les porcs HS étaient dus à une modification du comportement alimentaire et/ou du métabolisme énergétique, et si ces effets peuvent être reproduits en imposant une augmentation de la fréquence de repas. L'effet de la supplémentation en LY [0 (NS) vs. 100 (LY) g/tonne d'aliment], et de la fenêtre d'alimentation (FW) [illimité ou Unli, 2FW de 1 h chacun et 8FW de 15 min chacun] ont été mesurés chez des porcs mâles entiers en finition (n=36). La température ambiante était de 22°C pendant la période thermoneutre (TN; 5 j) et de 28°C pendant la période HS (5 j). L'exposition à la chaleur a diminué la consommation en matière sèche (DMI) et la quantité d'énergie retenue (RE) (-627g et -460 kJ•kg PC -0,60 •j -1 , P<0,01). Au cours du HS, la supplémentation en LY chez les porcs Unli a diminué les intervalles entre les repas (P = 0, 02), atténuant l'effet du HS sur le DMI qui avait tendance à améliorer la RE (P = 0, 09). Le lot NS-8FW avait une DMI et une RE plus élevées que NS-2FW (P <0, 05) mais le dépôt de protéines était similaire. Les porcs supplémentés avaient un dépôt de protéines plus élevé pendant HS indépendamment de FW (+18 g•d -1 ; P = 0,03). En comparant les groupes 2FW, l'amélioration de la tolérance à la chaleur des porcs supplémentés en LY était due à une meilleure sensibilité à l'insuline (P<0,05) et à une capacité accrue à perdre de la chaleur par voie latente après un repas (P<0,05) leur permettant d'augmenter leur DMI (via un nombre accru de repas) et donc leur efficacité énergétique. L'augmentation de la fréquence des repas a amélioré le DMI chez les porcs HS mais n'a pas reproduit les effets positifs de LY sur le dépôt de protéines.

  0,9 kg de poids corporel) a été utilisé pour évaluer les effets de la supplémentation en LY [0 (N) vs 100 g/tonne d'aliment (Y ; 1×10 6 CFU/g d'aliment de Saccharomyces cerevisiae var. boulardii CNCM I-1079)] et chez les porcs selon leur sexe (EM ou F)et soumis à HS au cours de 2 répétitions successives. Ainsi, il y avait quatre groupes expérimentaux : N-EM, Y-EM, N-F et Y-F. Les porcs ont été répartis dans deux salles expérimentales en fonction des traitements. La température ambiante des deux pièces a été maintenue à 22°C pendant 7 jours puis à 32°C pendant 3 jours pendant la période d'adaptation avant la période expérimentale de 27 jours à 32°C. La croissance et les performances d'abattage ont été mesurées. Le porc a été considéré comme l'unité expérimentale et les données ont été analysées à l'aide du modèle PROC MIXED avec répétition (R), sexe (S), supplémentation en LY (D ; régime), leurs interactions comme effets fixes et avec l'origine de la portée comme effet aléatoire. La répétition a eu un effet significatif sur le poids vif et à l'abattage, les porcs de la répétition 2 étant plus lourds que les porcs de la répétition 1 (P < 0,05). Indépendamment des effets de la répétition les porcs EM étaient plus lourds et avaient un gain quotidien moyen (GMQ) plus élevé que les porcs F (P <0,05) Dans les conditions TN, les porcs mâle entiers croissent plus vite et ont une meilleure efficacité alimentaire que les femelles

3 )

 3 mieux comprendre les mécanismes expliquant les effets positifs de la supplémentation en levures vivantes sur les porcs exposés au chaud 4) évaluer les interactions entre la supplémentation en levures vivantes et le sexe chez les porcs exposés à la chaleur, et 5) caractériser la dynamique des réponses de thermorégulation au cours de la journée et évaluer les conséquences sur la tolérance à la chaleur chez le porc Les différents modèles de stresse thermique utilisés dans ce projet ont démontré que de nombreux facteurs peuvent influer sur la capacité du porc à faire face au stress thermique, tels que les facteurs liés à l'animal (poids corporel, sexe, comportement alimentaire tel qu'estimé par l'alternance des périodes d'alimentation et de jeûne), l'environnement (conditions de logement, la nature et la sévérité du challenge thermique), et les interactions entre tous ces facteurs. De plus, notre travail montre que l'environnement thermique prénatal peut avoir des conséquences à long terme sur les performances et les réponses physiologiques des porcs En utilisant des mesures continues de la production et des pertes de chaleur et de la température corporelle, nos résultats ont démontré comment le comportement alimentaire et son interaction avec la température ambiante influencent la façon dont les porcs tentent de maintenir l'homéothermie tout au long de la journée. Dans cette optique, nous avons également démontré l'importance des pertes de chaleur latente dans le maintien de l'équilibre thermique au chaud, en particulier pour compenser les pics de production de chaleur liés à l'effet thermique des repas. Les données obtenues à partir du projet de thèse soulignent l'importance des stratégies d'alimentation destinées à stimuler la consommation d'eau afin d'améliorer la prise alimentaire pendant le stress thermique. Il a été constaté que la supplémentation en levures vivantes favorisait la sensibilité à l'insuline, ce qui pourrait induire une modification du comportement alimentaire et des conséquences positives sur les réponses de thermorégulation chez les porcs. dynamiques des porcs en croissance au stress thermique modulées par la vie prénatale et les pratiques d'alimentation Mots clés : stress thermique, porcs, métabolisme énergétique, thermorégulation, comportement alimentaire, levures vivantes

Table 1 .

 1 Examples of factors that affect energy partitioning of growing pigs such as previous feeding level, sex, diet composition, and high ambient temperature.1

						Heat production				Heat production
	Factor	Category	Ave. BW, kg	ME intake	FHP	TEFlt (kJ kg BW -0.TEFst TEFtotal AHP	RE	FHP	TEFlt	TEFst	TEFtotal	AHP	RE

60

Table 2 .

 2 Critical ambient temperature according to age, body weight, physiological stage, floor type, air speed, and skin wetness.

		Age					Skin	Temperature, °C
	(week	Wt	Floor	Air		wetness			UC
	Animal	)	(kg)	type	speed	Feeding	,%	LCT ECT	T
	Piglet	1	2	mesh	still	ad lib	15	26	35
		4	5					24	33
	Weaners	5	7	mesh	still	ad lib	15	26	35
		6	10					24	33
		8	16					22	30
		5	7	concrete	still	ad lib	15	27	36
		6	10					25	34
		8	16					25	31
	Growers	9	20 concrete	still	ad lib	15	16	30
		15	50					12	28
		21	90					9	27
		9	20 concrete draughty	ad lib	15	18	32
		15	50					15	30
		21	90					14	29
		9	20 concrete draughty	ad lib	60	20	34
		15	50		+ spray			17	32
		21	90					16	31
	Dry
	sows
	Individual housing	concrete	still	restricted	15	15	27
					draughty restricted	15	18	29
					draught+
					spray	restricted	60	20	33
	Group housing		concrete	still	restricted	15	12	26
					draughty restricted	15	15	28
					draught+
					spray	restricted	60	18	32
	Lactating sow	150	mesh	still	ad lib	15	8	22
					still+drip		30	9	26
				concrete	still	ad lib	15	10	23
					still+drip		30	11	25
	Adapted from

Table 3 .

 3 Summary of gut microbiota alterations induced by heat stress in monogastric animals.

	Animal	Age	Sampling location	Treatment description	Phylum changes	Genus changes	References
		11 weeks	Feces	28-30°C vs. 25°C 3 weeks	Firmicutes (+) Bacteroidetes (-) Proteobacteria (+) Spirochaetes (+) Actinobacteria (-)	Not reported	(Le Sciellour et al., 2019)
	Pig	BW) Not mentioned (29-31 kg	Feces	34-36°C vs. 24-26°C	Proteobacteria (+) Bacteroidetes (-)	Acinetobacter (+)	(Xiong et al., 2020)
						Coprococcus 3 (+), Coprostanoli-
		Sow in 85d of pregnancy	Feces	28-32 °C vs. 22°C	No differences	genes group (+), Halomonas (+), Ruminococcaceae UCG-005 (+), Ruminococcaceae UCG-013 (+), Bacteroidales RF16 group (-),	(He et al., 2019a)
						Streptococcus (-), Treponema 2 (-
						)
				34-38°C vs.
		2 weeks	Cecum	24-26°C
				4 weeks
	Broiler

Table 5 .

 5 Examples of nutritional and management strategies to mitigate heat stress in pigs.

	Strategy	Principle and benefits	References
	Reduce heat production
	Increase diet density
	(Decreased crude protein and fiber,
	use of synthetic amino acids,
	addition of fat)

Table 6 .

 6 Summary of the experiments included in this thesis project1

		Exp. 1	Exp. 2	Exp. 3	Exp. 4
	General idea	Prenatal HS	Live yeast and feeding management	Live yeast and feeding frequency	Live yeast and sex
	Animal
		24 mixed parity
	Number	sows and 48	128 growing pigs	36 growing pigs	64 growing pigs
		growing pigs
	Sex	Females	Females	Entire males	Females & Entire males
	Initial BW, kg	35.9 ± 0.8	66.1 ± 0.6	62.7 ± 1.1	60.9 ± 0.6
	Final BW, kg	101.1 ± 1.1	92.7 ± 0.7	85.7 ± 1.2	106.1± 1.2
	Housing	Group-housed 3 pigs in a 4 m 2 pen 1 pig in a 2 m 2 pen Individual	Individual 3.3 m 2	Individual 1 pig in a 2 m 2 pen
	Environmental temperature
	Type	Cyclic 18 to 24°C vs. 28 to 34°C	Constant 22°C vs. 32°C	Constant 22°C then 28°C	Constant 32°C
	Heat challenge duration	109 d for the sows; pigs 58 d for the growing	20 d	5 d	27 d
	Treatments
		Pregnancy	Ambient	all groups	all groups were
		environment	temperature	experienced both TN	subjected to HS
		(TN vs. HS)	(TN vs. HS)	and HS conditions	conditions
	Factors	Growing environment (TN vs. HS)	Feeding management PF2 vs. PF8) (Ad libitum vs.	Feeding windows: Unlimited vs. 2 vs. 8	Female vs. Entire male
		-	LY supplementation (0 vs. 100 g/ton of feed provided on top)	LY supplementation (0 vs. 100 g/ton of feed)	LY supplementation (0 vs. 100 g/ton of feed provided on top)

  1,2

		Exp. 1	Exp. 2	Exp. 3	Exp. 4
	Ingredients, % as-fed
	Wheat	26.20	34.00	26.73	33.82
	Barley	25.55	25.00	26.73	25.00
	Corn	16.00	15.00	26.73	15.00
	Soybean meal	19.00	5.06	4.66	5.00
	Rapeseed meal	-	9.00	4.62	9.00
	Soybean hulls	-	-	1.71	-
	Wheat bran	5.00	8.00	1.71	8.00
	Vegetable oil	2.00	1.00	0.63	1.00
	Micro ingredients	6.25	2.94	6.13	3.18
	Stearin	-	-	0.05	-
	TiO2	-	-	0.30	-
	Chemical composition, % as fed 4
	Dry matter	88.4	88.7	88.9	88.8
	Crude Protein	16.3	14.8	12.4	15.6
	Crude Ash	4.6	4.5	4.4	4.1
	Crude Fat	3.3	2.8	2.7	3.2
	NDF	12.3	14.6	11.7	14.7
	ADF	3.9	5.0	4.1	4.6
	ADL	1.10	1.5	0.8	1.2
	Starch	45.3	40.6	47.8	-
	Formulated values, % as fed
	Net energy, MJ/kg	9.66	9.62	9.60	9.62
	SID lysine : NE (g/MJ NE)	0.83	0.84	0.65	0.84

1

All diets fed in pellet form.

2

Diet formulation shown for experiments 2, 3, and 4 are the control (non-supplemented) diets. For the supplemented diet, live yeast was added on top (0.10%) for the diets of Exp. 2 and 4, and was included in the formulation for Exp. 3 (0.10% live yeast instead of 0.05% stearin).

3

Including synthetic amino acids, vitamins and minerals.

Table 8 .

 8 Summary of the measurements taken during the experiments in this thesis project1 The non-supplemented groups in the HS room (Trt 5 vs. 6) were used to assess if increasing meal frequency can improve HS response of pigs, 5. The PF2 supplemented group in the HS room was used to evaluate the effect of LY supplementation regardless of feed intake and without allowing them to increase feeding

		Exp. 1	Exp. 2	Exp. 3	Exp. 4
	General idea	Prenatal HS	Live yeast and feeding management	Live yeast and feeding frequency	Live yeast and sex
	Live BW	Manual	Manual	Automatic and manual	Manual
	Growth parameters	Yes	Yes	Yes	Yes
	Slaughter	Yes, comprehensive	Yes, comprehensive	n/a	Yes, only carcass weight & lean meat %
	Nitrogen balance	n/a	n/a	Yes	n/a
				Weight of feeding
	Feeding and drinking behavior	n/a	Infrared barrier (feeder presence)	trough and water container (weight	n/a
				differences)
	Physical activity	n/a	n/a	Yes	n/a
	Thermoregulation	Rectal and skin temperature	Rectal and skin temperature Continuous	Heat loss partitioning (evaporative, sensible, etc.) Continuous	n/a
		-	measure of body	measure of body	-
			temperature	temperature
		-	Respiratory frequency	-	-
	Energy utilization	n/a	n/a	Yes	n/a
		Spot sampling	Spot sampling at	Periprandial
	Blood sampling	at 1330h on d	1330h on d -1, 1	kinetics	n/a
		-4, 3, 49	8, 20	on d -1 and 1
	Microbiota	Spot fecal sampling	Spot fecal sampling	Spot fecal sampling	Spot fecal sampling
	1 TN= thermoneutral; HS=heat stress; LY=live yeast

Table 9 .

 9 Plasma hormones and metabolites analyzed in Experiments 1, 2, and 3 1 .

	Ingredients, % as-fed	Exp. 1	Exp. 2	Exp. 3 2
	Hormones
	Insulin	-	Yes	Yes
	Thyroid hormones (T3, T4)	Yes	Yes	Yes
	Metabolites
	Glucose	Yes	Yes	Yes
	Lactate	-	-	Yes
	Creatinine	Yes	Yes	Yes
	NEFA	Yes	Yes	Yes
	Urea	-	-	Yes
	BAP	Yes	-	-
	SOD	Yes	-	-
	Enzyme activity
	Creatinine kinase	Yes	-	-
	Lactate dehydrogenase	Yes	-	-

Table 1 .

 1 Composition of the experimental diets.1

	Ingredients, % as-fed	Control (N)	Supplemented (Y)
	Wheat	26.732	26.728
	Barley	26.732	26.728
	Corn	26.732	26.728
	Soybean meal	4.659	4.658
	Rapeseed meal	4.621	4.624
	Soybean hulls	1.709	1.709
	Soft wheat bran	1.709	1.709
	Sunflower oil	0.631	0.634
	L-Lysine HCl	0.296	0.296
	L-Threonine	0.067	0.067
	Molasses	3.000	3.000
	Salt	0.450	0.450
	Calcium carbonate	0.993	0.993
	Monocalcium phosphate	0.866	0.866
	Vitamin-Mineral Premix 2	0.500	0.500
	Stearin	0.005	-
	LSB	-	0.010
	Indigestible marker (TiO2)	0.300	0.300
	Analyzed chemical composition, % as fed 3
	Actual dry matter	89.0	88.9
	Dry Matter	89.0	89.0
	Crude Protein	12.6	12.4
	Crude Ash	4.4	4.4
	Crude Fat	2.7	2.7
	NDF	11.5	11.7
	ADF	4.1	4.1
	ADL	0.8	0.8
	Starch	47.9	47.9
	Formulated values, % as fed
	Net energy, MJ/kg	9.60	9.60
	SID lysine : NE (g/MJ NE)	0.65	0.65

1

Diet fed in pellet form.

2

Provided per kilogram of complete diet: vitamin A, 1,000,000 IU; vitamin D3, 200,000 IU; vitamin E, 4,000 mg; vitamin B1, 400 mg; vitamin B2, 800 mg; calcium pantothenate, 2,170 mg; niacin, 3,000 mg; vitamin B12, 4 mg; vitamin B6, 200 mg; vitamin K3, 400 mg; folic acid, 200 mg; biotin, 40 mg; choline chloride, 100,000 mg; iron (sulfate), 11,200 mg; iron (carbonate), 4,800 mg; copper (sulfate), 2,000 mg; zinc (oxide), 20,000 mg; manganese (oxide), 8,000 mg; iodine (iodate), 40 mg; cobalt (carbonate), 20 mg; and selenium (selenite), 30 mg.

3

As-fed basis. Aside from actual dry matter content, values are calculated for the same dry matter content (89.0%).

Table 4 .

 4 Effect of feed access and live yeast supplementation on thermoregulation responses of male finishing pigs exposed to high ambient temperature.1,2,3

		Unli		2FW	8FW
	Items	N	Y	N	Y	N	Y	RSD 4	Statistics
	Evaporated latent water (g•d -1 )
	Period 1 Period 2  ‡	2659 cde 4158 ab	2676 cde 4061 ab	2249 d 3729 abc	2592 bcd 5215 a	2628 bcd 4864 a	2417 cde 3609 abcd	592	P**, D×FW*, D×FW×P*
	Latent heat loss (% HP)
	Period 1 Period 2  ‡	31.0 c 55.6 a	30.8 c 51.8 a	28.7 c 51.9 a	31.1 c 65.1 a	32.5 bc 63.3 a	29.6 c 49.0 ab	5.1	P**, D×FW*, D×FW×P**
	Sensible heat loss (% HP)
	Period 1 Period 2  ‡	66.3 ab 42.0 de	66.8 ab 46.0 cde 45.9 cde 69.0 a	66.7 ab 30.9 e	65.2 abc 32.7 de	68.2 a 49.2 bcd	5.7	P**, D×FW*, D×FW×P**
	Heat loss due to water intake (% HP)
	Period 1 Period 2  ‡	2.32 2.25	1.95 1.94	1.89 1.94	1.77 3.75	1.78 3.71	1.80 1.50	1.01	P*, D×FW T , D×FW×P*
	Heat loss due to feed intake (% HP)
	Period 1 # || Period 2  ‡	0.45 0.24	0.47 0.27	0.43 0.22	0.44 0.27	0.44 0.25	0.44 0.24	0.02	D*, FW T , P**
	Body temperature, °C
	Period 1 Period 2	38.58 abcd 38.70 bcd 38.42 cd 38.57 abcd 38.89 abcd 39.11 ab 39.18 a 38.90 abcd 38.84 abcd 38.89 abc 38.47 cd 38.39 d	0.18	P**, D×FW×P*

1 A total of 36 pigs were allocated to 6 experimental groups in 9 replicates with 2 blocks per replicate. All pigs were housed at 22°C from d -5 to -1 (Period 1 and then at 28°C from d 0 to 4 (Period 2). FW: feeding window; N: non-supplemented diet; Y: live yeast supplemented diet.

  Blanchard, P. J., M. Ellis, C. C. Warkup, J. P. Chadwick, and M. B. Willis. 1999. The influence of sex (boars and gilts) on growth, carcass and pork eating quality characteristics. Anim. Sci.Le Bellego, L., J. Van Milgen, and J. Noblet. 2002. Effect of high ambient temperature on protein and lipid deposition and energy utilization in growing pigs. Anim. Sci. 75. Moughan, P. J., L. H. Jacobson, and P. C. H. Morel. 2006. A genetic upper limit to whole-body protein deposition in a strain of growing pigs. J. Anim. Sci. 84. Renaudeau, D., M. Giorgi, F. Silou, and J. Weisbecker. 2006. Effect of breed (lean or fat pigs) and sex on performance and feeding behaviour of group housed growing pigs in a tropical climate. Asian-Australasian J. Anim. Sci. 19.

	68.
	Labussière, E., C. S. Achard, S. Dubois, S. Combes, M. Castex, and D. Renaudeau. 2021.
	Saccharomyces cerevisiae boulardii CNCM I-1079 supplementation in finishing male pigs
	helps to cope with heat stress through feeding behavior and gut microbiota modulation. Br. J.
	Nutr. 127.

Table 10 .

 10 Summary of the effects of HS only on ad libitum-fed non-supplemented growing pigs in the experiments of the thesis project.

				Exp. 1	Exp. 2	Exp. 3	Exp. 4
	Sex			Females	Females	Entire males	Females & Entire males
	Growth
	Pre-challenge mean BW, kg	39.2	74.1	83.2	74.4
	Challenge duration, d	58	19	5	27
	ADFI, g•d	-1	-332	-547	-846	-328 (vs. adaptation)
	ADFI, g DM•d -1	-295	-489	-753	-368 (vs. adaptation)
	ADG, g•d	-1	-127	-279	-737	-
	Hormones
	Insulin			-	increase on d 8 of HS	maintain	-
	Insulin: Glucose	-	-	maintain
				decrease	decrease
	T3			(more pronounced on	(more pronounced on
				d 3 than 49 of HS)	d 1 than 20 of HS)

Table 1 .

 1 housed at 24 °C. At 70 d of age, 48 females from the 24 litters were selected based on the climatic environment during pregnancy and transferred to the two climatic rooms (8 pens/room, 3 pigs/pen, 12 pigs/temperature treatment during pregnancy). After a 7-d adaptation period, the first room was maintained at an ambient temperature of 18-24 °C (GTN) during all the growing period. The second room (GHS) was kept at 18-24 °C for 7 d (P1) and thereafter at 28-34 °C for 56 d (P2). During the experimental period, feed and water were offered ad libitum, Pigs were weighted weekly and feed intake was measured daily. All the pigs were slaughtered at 140 d of age for measurements of carcass traits. Data were analysed with a mixed model including the effects of temperature during pregnancy (2 levels), temperature during growth period (2 levels) and interactions. Effects

	Results and discussion
	of climatic conditions (TN vs. HS) during pregnancy (P) and growing
	periods (G) on pig performance (least square means) 1,2
		GTN		GHS
		PTN	PHS	PTN	PHS	RMSE	Statistics
	No. of pigs	12	12	12	12
	Period 1
	Initial BW, kg 35.2ab	37.1a	36.4a	33.4b	3.2	G×P*
	ADG, g/d	913	933	983	886	183
	ADFI, g/d	1947	1938	2043	1896	152
	FCR	1.81	1.79	1.73	1.79	0.12
	Period 2
	Initial BW, kg 41.6ab	43.6a	43.2a	39.6b	3.7	G×P*
	Final BW, kg	104.8	102.4	98.1	92.9	7.0	G**
	ADG, g/d	1124	1051	985	942	99	G**
	ADFI, g/d	2615	2694	2257	2250	90	G***
	FCR	2.33	2.35	2.28	2.31	0.08
	1 ADFI = average daily feed intake (adjusted for a DM of 88.0%); ADG = average daily gain; BW= body weight; FCR = feed conversion ratio
	(adjusted for a DM of 88.0%).

Scientific communication no. 2 from Exp. 2

  However, the carcass composition was similar (P>0.05) in the GTN and GHS groups and in the PTN and the PHS groups. When data were adjusted for a same slaughter BW, carcass lean percentage was numerically lower (P=0.109) and backfat thickness was higher (P=0.012) in the GHS-PHS group. The results of the present study confirm the negative effects of postnatal HS on ADFI and ADG but show that prenatal HS did not impair to impact growth performance. Changes in lean and fat tissue accretion were observed only when pigs were exposed to both the prenatal and postnatal HS. Serviento, A.M.,[START_REF] Serviento | Effect of heat stress and of feed provision modification on growth and thermoregulation in pigs[END_REF]

	The FCR did not change in HS conditions (2.31 kg/kg on average; P=0.529). At slaughter, hot carcass weight and length were significantly lower in postnatal HS conditions (95.2 vs 105.1 kg and 94.3 vs 96.7 cm, respectively; P<0.05). Annex 2.

Effect of heat stress and of feed provision modification on growth and thermoregulation in pigs.

  In: Book of Abstracts of the 71st Annual Meeting of the European Federation of Animal Science. Presented at the EAAP 2020 Virtual meeting,December 1 -4, 2020. (Oral presentation)

	Abstract
	With the increasing global mean temperature and more frequent summer heat waves,
	there is a need to find techniques to mitigate heat stress (HS) consequences on pig production.
	The aim of the study was to evaluate the effect of HS and of increasing feed provision frequency
	on performance and thermoregulation responses in finishing pigs. A total of 48 pigs (66.1±1.7
	kg) were allocated to 4 experimental groups in 3 replicates. After being housed at
	thermoneutrality (TN; 22°C) during 7 days (P1), pigs were subjected to either TN or HS (32°C)
	conditions during 20 days (P2). In each environment, the diet (14.9%CP, 9.64 MJ NE/kg) was
	provided either ad libitum (AL; 2 distributions/day) or pair-fed (PF8; 8 distributions/day) with

Growth and feeder presence of the eight experimental groups in Exp. 2

  Annex table 1. Effect of heat stress, live yeast, and feeding management on growth performance of finishing pigs in Experiment 2.1

			TN				HS
	Items	N-AL	N-PF8	Y-AL	Y-PF2	N-AL	N-PF8	Y-AL	Y-PF2 RSD 2	Statistics 3
		Trt 1	Trt 2	Trt 3	Trt 4	Trt 5	Trt 6	Trt 7	Trt 8
	Animals, n
	Live BW, kg d -7 d 0 d 20	66.1 74.5 97.6 a	66.0 73.8 91.7 bc	66.3 74.2 96.2 ab	66.1 73.6 91.7 bc	65.8 73.5 91.6 bc	67.0 74.6 90.9bc	65.7 74.2 92.5 bc	66.3 73.9 89.3 c	1.4	R**, D**, G×D**, R×G×D**
	ADFI, g•d -1										G**, P**,
	Period 1 Period 2	2603 2927 a	2514 2402 b	2648 2939 a	2581 2421 b	2643 2376 b	2669 2364 b	2568 2417 b	2600 2293 b	132	R×G T , G×P**, R×G×P**
	ADFI per BW 0.60 , g•kg BW -0.60 •d -1								R**, G**, P**,
	Period 1 Period 2	203 202 a	197 170 b	206 204 a	202 171 b	208 169 b	207 167 b	202 170 b	204 164 b	9.6	R×G*, G×P**, R×G×P T
	ADG, g•d -1										R**, G**, P**,
	Period 1 Period 2	1198 ab 1156 a	1115 abc 1124 abc 897 bc 1101 a	1067 c 905 b	1105 abc 905 b	1092 bc 817 bc	1222 aa 915 b	1079 bc 773 c	133	R×G T , G×P**
	FCR
	Period 1 Period 2	2.19 ab 2.55 a	2.28 abc 2.70 b	2.42 bc 2.67 ab	2.44 bc 2.73 b	2.45 bc 2.63 a	2.48 bc 2.91 b	2.12 a 2.64 a	2.49 c	0.30	G**, P**, R×G**

1) before and after a chronic HS challenge on d 78 and 136 d of age, respectively. Annex 4.

Nicotinamide adenine dinucleotide (NADH), nicotinamide adenine dinucleotide phosphate (NADPH), and flavin adenine dinucleotide (FADH2)

The pentose phosphate pathway branches out from glucose 6-phosphate (G6P) to produce sugars for nucleic acid synthesis and NADPH that provides reducing power for the synthesis of fatty acids, sterols, nucleotides, and nonessential amino acids. In contrast to glycolysis, it does not require ATP[START_REF] Ge | The role of the pentose phosphate pathway in diabetes and cancer[END_REF].

BW 0.60 : metabolic body weight; ME: metabolizable energy; FHP: fasting heat production; TEF: thermic effect of feeding; AHP: activity heat production; RE: retained energy; FL: feeding level; SC: surgically castrated; IC: immunocastrated males

68-75 kg male growing pigs were fed at 60, 80, or 100% of their estimated ad libitum feed intake level (at 1.56, 2.08, and 2.60 MJ ME kg BW -0.60 •d-1 , respectively (de Lange et al., 2006).

114-120 kg male growing pigs were kept as entire males, were surgically castrated, or were immunocastrated(Labussière et al., 2013).

[START_REF] Cunha | Hypoglycemic effect and hepato protective role of Saccharomyces boulardii THT 500101 strain in a murine model of streptozotocin-induced diabetes[END_REF][START_REF] Salgado | Insulin sensitivity as a modulator of body composition with implications for lipogenesis and gene expression in finishing pigs[END_REF][START_REF] Marsland | The gut-lung axis in respiratory disease[END_REF][START_REF] Kayser | Effects of live yeast supplementation on complete blood cell count and febrile responses in heifers after viral-bacterial respiratory challenge[END_REF][START_REF] Durmaz | Probiotic Saccharomyces boulardii alleviates lung injury by reduction of oxidative stress and cytokine response induced by supraceliac aortic ischemia-reperfusion injury in rats[END_REF][61] kg male growing pigs were fed a basal diet limiting in lysine supply at 1.7 MJ ME kg BW -0.60 •d -1 (Diet 1) or one of the four other diets at 2.55 MJ ME kg BW -0.60 •d -1 : the basal diet and a dietary supplement that consisted of starch (Diet 2), of starch and corn gluten meal (Diet 3), of starch and casein (Diet 4), or of starch and lipid (Diet

5)[START_REF] Van Milgen | Energetic efficiency of starch, protein and lipid utilization in growing pigs[END_REF].4 25-26 kg castrated male pigs were reared at 23°C and fed ad libitum, at 33°C and fed ad libitum,and at 23°C and pair fed to the level of 33-AL pigs.

Metabolizable energy (ME) utilization efficiency is a function of the net energy (NE) which is the difference between the ME and the heat increment. ME utilization efficiency is calculated as ME/NE.

International Commission of Agricultural and Biosystems Engineering (CIGR).

Sum of all conductive, convective, and radiation heat exchanges.

This can pertain to different variables such as body temperature, blood glucose, salt and water balance, blood pressure, pH concentration, among others.

Central thermoreceptors are found in the hypothalamus, spinal cord, viscera, and great veins while peripheral receptors are found in the skin.

Maximum protein deposition (PDmax) is the genetic upper limit to whole body protein deposition wherein excess energy intake after PDmax has been reached will be deposited as lipid[START_REF] Moughan | A genetic upper limit to whole-body protein deposition in a strain of growing pigs[END_REF].

Stearoyl-CoA-desaturase catalyzes biosynthesis of monosaturated fatty acids from saturated fatty acids.

Very low lipoproteins are produced in the liver and carry triglycerides in the bloodstream to supply body tissues.

Residual feed intake (RFI) is a genetic measure of net feed efficiency and represents the difference between the total feed intake and part of the feed intake unexplained by maintenance and production energy cost. Thus, selection for a low RFI is a selection for improved feed efficiency without compromising production[START_REF] Gilbert | Genetic parameters for residual feed intake in growing pigs, with emphasis on genetic relationships with carcass and meat quality traits[END_REF].

TN= thermoneutral; HS=heat stress; LY=live yeast; PF2: pair-fed with

feed provisions; PF8: pair-fed with 8 feeding provisions.

NEFA: Non-esterified fatty acids; BAP: biological antioxidant potential; SOD: Superoxide dismutase.

Postptrandial plasma kinetics measurement except for the thyroid hormones which was measured once at the beginning of the meal.

Recall that in estimating sensible and latent HL, latent HL is calculated from the evaporated water in the respiratory chamber (Text box 2) and sensible HL is calculated by difference between total HP and latent HL.

Acknowledgements ii Acknowledgements Acknowledgements iii Acknowledgements Acknowledgements ........................................................................................................... ii Abstract............................................................................................................................

Acknowledgments

The authors want to thank G. Poupeau, F. Guerin, J. Fortin, J. Georges, M. Genissel, A. Chauvin, and F. Le-Gouevec for animal care, J.F. Rouaud and S. Duteil for diet preparation, and C. Perrier, G. Le Roy, C. Mustière, A. Marchais, X. Arondel-Piekarz, P. Ganier, and R.

Comte for lab analyses.

Acknowledgements

This project was funded by INRA (PHASE Divison).

Financial support

This study and the PhD grant of A.M. Serviento were funded by Lallemand Animal Nutrition (Blagnac, France) and French National Association for Research and Technology with the contract number N°2018/1731 under the CIFRE PhD program (Conventions Industrielles de Formation par la Recherche). M.C. is a permanent employee and A.M.S. is a contractual employee of Lallemand SAS.

List of tables

Results

Discussions

Table 2. Effect of the prenatal and postnatal (growing) climatic environments on the carcass and organ traits of growing pigs.

Conclusion Abstract

Results

Discussion

Table 4. Effect of HS and feeding management on the blood gas parameters of finishing pigs. /

Conflict of interest

None.

Authorship

A.M.S., E. L., D.R., and M.C. designed the study. A.M.S. with direction from E. L. and D. R. was responsible for data collection and statistical analysis. E. L. assisted with study execution. A.M.S. wrote the manuscript. E. L., D. R., and M. C. reviewed the manuscript. All authors read and approved the final manuscript. E.L. is the corresponding author.

Annex 3. Scientific communication no. 3 from Exp. 1

Serviento, A.M., [START_REF] Serviento | Effect of heat stress on fecal microbiota composition of pregnant sows and of their female offspring[END_REF]. Effect of heat stress on fecal microbiota composition of pregnant sows and of their female offspring. In: Book of Abstracts of the 72nd Annual Meeting of the European Federation of Animal Science. Presented at the EAAP 2021 Davos, Switzerland, August 30 -September 3, 2021. (Oral presentation)

Abstract

The objective of this study was to evaluate the impact of chronic heat stress (HS) on microbiota composition of gestating sows and of their female offspring. There were two environmental conditions: thermoneutral (TN; cyclic 18 to 24 °C) and HS (cyclic 28 to 34 °C). temperatures. Firmicutes and Bacteroidetes were the most dominant phyla (~98%) in the samples collected pre-challenge regardless of physiological stage but sows had higher percentage of Firmicutes (+20.4%; P<0.01) and lower percentage of Bacteroidetes (-20.2%; P<0.01) than the growing pigs. Using a permutational multivariate analysis of variance on the environment effect on microbiota, chronic HS exposure did not significantly affect sow microbiota at d 105 (P=0.120). Microbiota from the pigs born from PHS sows was resistant to a further heat stress during the finishing (P=0.167), while microbiota of pigs born from the PTN sows were strongly affected (P=0.027). Therefore, the prenatal environment of the growing pigs affect how their microbial composition change upon exposure to a thermal stress during their postnatal life. The difference in the effect of heat exposure on gut microbiota of sows and of growing pigs may be related to their microbial composition or to the duration of their thermal stress exposure.

Annex 5. Continuation…

Supplementary table 1. Parameter estimates describing the postprandial kinetics of plasma insulin plasma metabolites of entire male finishing pigs exposed to high ambient temperature. 1 A total of 27 pigs were allocated to 6 experimental groups in 9 replicates with 2 blocks per replicate. All pigs were housed at 22°C am b ient temperature from d -5 to -1 (Period 1 and then at 28°C from d 0 to 4 (Period 2). Serial blood sampling was done on d -2 (P1) and on d 1 (P2). Plasma kinetic profile per variable were then analyzed using non-linear regression using a modified erlang model that was parameterized to estimate Cinitial, λ, AUC and Cfinal (Supplementary figure 1). 2 The predicted values for each parameter per pig and per period from the non-linear regression were analyzed using PROC MIXED model with feeding window (FW), diet (D), period (P), and their interactions as fixed effects, and with pig as a repeated unit per period. **P < 0.01, * P < 0.05, T P < 0.10.