domain: shs sdv info

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J Guyader

M Doreau

D P Morgavi

C Gérard

C Loncke

C Martin

Long-term effect of linseed plus nitrate fed to dairy cows on enteric methane emission and nitrate and nitrite residuals in milk

Keywords: linseed, methane, milk product, nitrate, ruminant Implications

A previous study showed the additive methane (CH 4 )-mitigating effect of nitrate and linseed fed to non-lactating cows. Before practical application, the use of this new strategy in dairy cows requires further investigation in terms of persistency of methanogenesis reduction and absence of residuals in milk products. The objective of this experiment was to study the long-term effect of linseed plus nitrate on enteric CH 4 emission and performance in dairy cows. We also assessed the effect of this feeding strategy on the presence of nitrate residuals in milk products, total tract digestibility, nitrogen (N) balance and rumen fermentation. A total of 16 lactating Holstein cows were allocated to two groups in a randomised design conducted in parallel for 17 weeks. Diets were on a dry matter (DM) basis: (1) control (54% maize silage, 6% hay and 40% concentrate; CON) or (2) control plus 3.5% added fat from linseed and 1.8% nitrate (LIN + NIT). Diets were equivalent in terms of CP (16%), starch (28%) and NDF (33%), and were offered twice daily. Cows were fed ad libitum, except during weeks 5, 16 and 17 in which feed was restricted to 95% of dry matter intake (DMI) to ensure complete consumption of meals during measurement periods. Milk production and DMI were measured weekly. Nitrate and nitrite concentrations in milk and milk products were determined monthly. Daily CH 4 emission was quantified in open circuit respiration chambers (weeks 5 and 16). Total tract apparent digestibility, N balance and rumen fermentation parameters were determined in week 17. Daily DMI tended to be lower with LIN + NIT from week 4 to 16 (-5.1 kg/day on average). The LIN + NIT diet decreased milk production during 6 non-consecutive weeks (-2.5 kg/day on average). Nitrate or nitrite residuals were not detected in milk and associated products. The LIN + NIT diet reduced CH 4 emission to a similar extent at the beginning and end of the trial (-47%, g/day; -30%, g/kg DMI; -33%, g/kg fat-and protein-corrected milk, on average). Diets did not affect N efficiency and nutrients digestibility. In the rumen, LIN + NIT did not affect protozoa number but reduced total volatile fatty acid (-12%) and propionate (-31%) concentrations. We concluded that linseed plus nitrate may have a long-term CH 4 -mitigating effect in dairy cows and that consuming milk products from cows fed nitrate may be safe in terms of nitrate and nitrite residuals. Further work is required to optimise the doses of linseed plus nitrate to avoid reduced cows performance.

Introduction

Linseed and nitrate are both proven dietary treatments for reducing enteric methane (CH 4 ) emission in ruminants (Doreau et al., 2014) and their combination as a CH 4 -mitigating strategy appears promising. In a short-term experiment on non-lactating cows, linseed oil (4% of dry matter; DM) plus nitrate (2.25% of DM) reduced methanogenesis by 32% without affecting apparent diet digestibility [START_REF] Guyader | Additive methane-mitigating effect between linseed oil and nitrate fed to cattle[END_REF]. Compared to linseed oil and nitrate fed individually, the effect of this combination on CH 4 production was additive because these two strategies share different modes of action in the rumen: polyunsaturated lipids from linseed are thought to act as inhibitors of H 2 producers such as protozoa, whereas nitrate is thought to act as a H 2 sink, competing with methanogenesis. Nitrate and nitrite are also toxic to methanogens [START_REF] Guyader | Dietary nitrates decrease methane emission by inhibiting rumen methanogenic archaea without influencing nitrate reducing bacteria[END_REF].

Before practical application at the farm scale, the long-term effect of dietary linseed plus nitrate on CH 4 emission in dairy cows requires further investigation. Another issue to evaluate is the potential adverse effects of nitrate supplementation on human and animal health. To our knowledge, the effect of dietary nitrate on milk quality, including the absence of nitrate and nitrite residuals in milk, has never been tested, whereas excess nitrite from nitrate reduction in the mouth may promote gastric irritation in humans [START_REF] Weitzberg | Novel aspects of dietary nitrate and human health[END_REF]. Nitrate may also alter animal health by increasing the concentration of blood methemoglobin (metHb; [START_REF] Lewis | The metabolism of nitrate and nitrite in the sheep. 1 The reduction of nitrate in the rumen of the sheep[END_REF]. Without adaptation, nitrite from nitrate reduction can accumulate in the rumen, passing through the blood and leading to subclinical methemoglobinemia (30% to 40% of metHb; [START_REF] Lee | A meta-analysis of effects of feeding nitrate on toxicity, production, and enteric methane emissions in ruminants[END_REF].

The main objective of this experiment was to investigate the long-term effect of linseed plus nitrate on CH 4 emission and lactating performance in dairy cows. As a secondary objective, nitrate metabolism was assessed by measuring metHb levels in blood and nitrate and nitrite levels in milk and processed milk products. We also evaluated the effect of linseed plus nitrate on total tract apparent digestibility, nitrogen (N) balance and rumen fermentation parameters.

Material and methods

The experiment was conducted at the dairy cows' experimental facilities at the INRA's Saint-Genès-Champanelle research centre in France from January to May 2014. All procedures involving animals were performed in accordance with French Ministry of Agriculture and European guidelines and regulations for animal research and experimentation.

Animals, diets and feeding A total of 16 lactating (including seven primiparous) Holstein cows accustomed to handling were used. At the start of the experiment, cows were 61 ± 23 days in milk, with an average milk yield of 33.4 ± 7.1 kg/day and BW of 706 ± 67 kg. The experiment was conducted for 17 weeks as a randomised block design where cows were separated into two groups balanced for calving date and milk production. Cows were housed in a free stall barn in which they had an individual trough accessed via an electronic tag. Cows were adapted for 4 days to a tie stall barn before the start of the two measurement periods (week 5 and weeks 16 to 17) that required tying cows.

The first group of cows (n = 8 of which four primiparous) was fed the control diet (CON), and the second group of cows (n = 8 of which three primiparous) was fed CON with 9.8% extruded linseed and 2.4% calcium ammonium nitrate (75% nitrate in DM) on a DM basis (LIN + NIT). The doses of extruded linseed and nitrate were estimated to reduce CH 4 emission by 10% to 15% when fed alone [START_REF] Doreau | Mitigation of enteric methane for French cattle: potential extent and cost of selected actions[END_REF] and by 20% to 30% when fed together. Diets were formulated to meet the requirements of lactating dairy cows (30 kg daily milk production without BW change) and to be equivalent in terms of CP, gross energy (GE) and starch proportions (INRA, 2010; Table 1). On a DM basis, diets were composed of 54% maize silage, 6% natural grassland hay and 40% concentrate given as pellets (InVivo NSA, Longué Jumelles, France).

Two weeks before starting the experiment, all cows were fed CON diet ad libitum. Then, animals fed LIN + NIT were progressively adapted by replacing CON concentrate with LIN + NIT concentrate over a 2-week period to achieve the dose of 2.4% calcium ammonium nitrate at the beginning of week 3. Hay was offered once daily (0800 h) and before other feeds to ensure ingestion of fibre and prevent ruminal acidosis. Maize silage mixed with concentrates was offered twice daily (66% at 0930 h and 34% at 1600 h). All cows were fed ad libitum except during measurement weeks in which feed offered was restricted to 95% of individual voluntary feed intake to ensure complete consumption of the diet. Forage-to-concentrate ratio was kept as close as possible to the target ratio by adjusting the amounts of feed offered every week based on quantity and composition of the refusals of the previous week. Cows had free access to water throughout the experiment.

Measurements and analyses

Liveweight and blood metHb. Animals were weighed the week before starting the experiment (week 0) then in weeks 5, 10, 14 and 20. Blood metHb levels were measured 3.5 h after morning feeding on cows fed LIN + NIT and compared with levels of control samples taken on these same animals in week 0. Blood was then sampled twice a week from week 1 to 3 (adaptation to nitrate) and once a week from week 4 to the end of the experiment (week 17). Blood (10 ml) was sampled from the tail vein into K2-EDTA collection tubes and stored on ice (Venosafe; Terumo, Guyancourt, France) until measurement of metHb concentrations by spectrophotometry (UV-160; Shimadzu, Marne-La-Vallée, France; [START_REF] Kaplan | Méthode de mesure rapide du taux de la methémoglobine dans les globules rouges[END_REF] within 1 h (CHU Gabriel Montpied, Clermont-Ferrand, France).

Intake. Offered feed and refusals were weighed and recorded daily throughout the experiment. During the two measurement periods (week 5 and weeks 16 to 17), samples (200 g) of hay and concentrates were taken once a week, and samples (200 g) of maize silage were taken twice a week. For each feed and refusals sample, one aliquot was used to determine DM content (103°C for 24 h) and the other aliquot was stored at 4°C (hay and concentrates) or -20°C (maize silage) until further analyses. Chemical composition analyses (ash, N, NDF, ADF, starch, GE, ether extract and fatty acid) were carried out on fresh (hay, concentrates) or freeze-dried (maize silage) samples after grinding (1 mm) (InVivo Labs, Chierry, France) and as previously described [START_REF] Guyader | Additive methane-mitigating effect between linseed oil and nitrate fed to cattle[END_REF]. Juice from fresh maize silage was obtained by maceration to analyse pH, ammonia (N-NH 3 ; Kjeldahl method 2001.11;[START_REF] Aoac | Official methods of analysis[END_REF], acetic and lactic acid (gas chromatography with a flame ionization detector) concentrations (InVivo Labs).

Methane emission. Daily enteric CH 4 emission of each animal was continuously measured for 2 consecutive days using open circuit respiration chambers (one animal per chamber) after 5 and 16 weeks of distribution of dietary treatments (Guyader et al., 2015). During measurement periods, chamber rear doors were opened in the morning for milking and to remove faeces and urine, and in the afternoon for milking. Chamber front doors were opened three times a day for feeding. In total, the doors of each chamber were opened for 30 min/24 h. Data collected while doors were open were deleted. To recover 24-h CH 4 emission, missing data were estimated as being similar to the last measurement data before chamber disturbance. Real-time gas emissions in a chamber were calculated by the difference between chamber and ambient gas concentrations multiplied by the airflow corrected for temperature, relative humidity and pressure (Pinares-Patiño et al., 2012).

Diet apparent digestibility and N balance. Total tract apparent digestibility and N balance were determined from total and separate collection of faeces and urine for 5 days during week 17 [START_REF] Guyader | Additive methane-mitigating effect between linseed oil and nitrate fed to cattle[END_REF]. At the end of week 16, cows were moved from CH 4 chambers to individual digestibility crates to give animals a 3-day adaptation period to new housing conditions before the first collection. To separate urine from faeces, cows were fitted with a urine collection device connected by a flexible tube to a 30-l flask containing 500 ml of 3 M sulphuric acid to keep a urine pH lower than 3 and thereby avoid N volatilisation. Faeces and urine were removed once daily. Every day, after weighing and mixing of faeces, a 1% fresh aliquot was used to determine DM (103°C for 24 h), and another 1% fresh aliquot was pooled across days for each animal and frozen (-20°C). At the end of the experiment, pooled samples were thawed, freeze-dried and ground (1 mm) to determine organic matter (OM), N, NDF and ADF content as for feed (InVivo Labs). For urine, every day after weighing, a 1% fresh aliquot was pooled across days for each animal and frozen (-20°C). At the end of the experiment, after thawing, the N concentration of faeces and urine was, respectively, determined by the Dumas (method 968.06; AOAC, 2005) and the Kjeldahl (method 2001.11;[START_REF] Aoac | Official methods of analysis[END_REF] methods (InVivo Labs), as the Dumas analyser was not adapted to handle liquid samples. The Kjeldahl method does not take into account N-nitrate, but it was assumed that the influence of N-nitrate in urine on the overall N balance was minimal: [START_REF] Lee | Effects of encapsulated nitrate on enteric methane production and nitrogen and energy utilization in beef heifers[END_REF] reported that beef heifers fed 2% nitrate lost 0.17 g/day of N-nitrate in urine (0.39% of total N excretion in urine). Therefore, the use of the Kjeldahl method did not mask the potential effect of treatments on N balance.

Milk yield and composition. Throughout the experiment, milk yield was determined daily. For determination of milk composition (fat, protein, lactose and urea concentration), individual milk samples (30 ml) mixed with potassium bichromate (Merck, Fontenay-Sous-Bois, France) were taken and stored at 4°C before analysis within 2 days (Galilait, Theix, France). Samples were taken at morning and afternoon milking 2 days/week when animals were in the CH 4 chambers (weeks 5 and 16). Milk fat, protein and lactose concentrations were analysed by IR spectrometry with a 3-channel spectrophotometer (MilkoScan;Foss Electric, Hillerod, Denmark;method 972.16;[START_REF] Aoac | Official methods of analysis[END_REF]. Milk urea concentration was analysed by the dimethylamino-4-benzaldehyde colorimetric method [START_REF] Potts | Colorimetric determination of urea in feeds (report of AOAC Committee)[END_REF].

Linseed and nitrate reduce long-term methane emission

From data collected in weeks 5 and 16, milk production was converted to fat-and protein-corrected milk (FPCM, kg/day) with 4.0% fat and 3.3% protein [START_REF] Gerber | Productivity gains and greenhouse gas emissions intensity in dairy systems[END_REF] and feed efficiency was calculated as the ratio between FPCM and dry matter intake (DMI).

For analysis of nitrate and nitrite residuals in individual milk, samples (300 ml) from the morning milking were taken once a week in weeks 5, 9, 13 and 17. For analysis of nitrate and nitrite residuals in pooled milk and milk products, the morning milk of all animals was pooled by diet in weeks 9 and 17. Pooled milk was sampled (100 ml) and local farmhousestyle products were made (yoghurts, whey, curd and 6-week ripened Saint-Nectaire cheese). All samples were stored at 4°C before analysis within 2 days (Eurofins Analytics, Nantes, France). Nitrate and nitrite residuals in individual milk samples were analysed by ion chromatography (method 993.30; AOAC, 2005) with a limit of quantification of 10 mg/kg for nitrate and 5 mg/kg for nitrite. In pooled milk samples and processed milk products, nitrate and nitrite residuals were analysed by spectrometry after nitrate reduction with cadmium (ISO 14673; ISO, 2004) with a limit of quantification of 5 mg/kg for nitrate and 0.5 mg/kg for nitrite.

Rumen fermentation parameters. On the last day of week 17, rumen samples were collected 3.5 h after the morning feeding by stomach tubing [START_REF] Shen | Insertion depth of oral stomach tubes may affect the fermentation parameters of ruminal fluid collected in dairy cows[END_REF]. Samples were strained through a polyester monofilament fabric (250 μm pore size) and the filtrate was subsampled for volatile fatty acids (VFA; 0.8 ml of filtrate in 0.5 ml of a 0.5 M HCl solution containing 2% (w/v) metaphosphoric acid and 0.4% (w/v) crotonic acid) and NH 3 (1 ml of filtrate in 0.1 ml of 5% orthophosphoric acid) concentration analyses. These samples were stored at -20°C until analysis. For protozoa counting, 2 ml of filtrate was mixed with 2 ml of methyl green-formalin saline solution, and stored at room temperature in the dark until counting.

Concentrations of VFA and NH 3 were analysed by gas chromatography with a flame ionization detector and colorimetry, respectively [START_REF] Morgavi | Changes in methane emission and rumen fermentation parameters induced by refaunation in sheep[END_REF]. Protozoa were counted by microscopy and data were log 10transformed before statistical analyses.

Statistical analyses

Data were analysed using the MIXED procedure of SAS (Version 9.4; SAS Institute, 2009). All statistical models included the animal nested within diet as random effect. Data collected throughout the experiment (intake, milk production and composition) or on two occasions (CH 4 emission) were averaged per individual cow and per week as there was no statistical difference between days within a week. The statistical model included diet (n = 2), week (n = 17 for intake and milk and n = 2 for CH 4 ) and diet × week interaction as fixed effects. Week was treated as a repeated measurement. For intake, milk production and composition (except for urea), data collected in week 0 were used as covariates, as these parameters were different between groups at the start of the experiment. For continuous measures of CH 4 emission, the model included diet (n = 2), week (n = 2), hour (n = 24), diet × week and diet × hour interactions as fixed effects. Hour was treated as a repeated measurement. As the interaction diet × week was not statistically significant, averaged data of the 2 weeks are presented in the last figure. For the repeated measurements, several covariance structures were tested (variance component, autoregressive, compound symmetry, unstructured and toeplitz) and structure with the lowest Akaike's information criteria was chosen. Then, variance component was always used as covariance structure, except for daily CH 4 emission where compound symmetry was used. Data collected at the end of the experiment (apparent digestibility, N balance, rumen fermentation and microbial parameters) were analysed with diet (n = 2) as fixed factor. Differences between diets were considered significant at P ⩽ 0.05, and trends were discussed at 0.05 < P ⩽ 0.1. Least squares means are reported throughout.

Results

Liveweight and blood metHb

During the 17-week experiment, cows fed CON or LIN + NIT lost on average 32 and 22 kg BW to reach a final BW of 697 ± 62 and 662 ± 67 kg, respectively. During the 3-week period of adaptation to nitrate, the maximum metHb level was 13.0% (Figure 1). From week 4 to 17, average metHb level was 1.2%. Maximum metHb level peaked at 30.8% for one cow in week 17, whereas average metHb level for all other cows in that week averaged 4.4%.

Intake and milk yield Daily DMI was similar between diets in weeks 1, 2, 3 and 17 and tended to be lower with LIN + NIT from week 4 to 16 (-5.1 kg/day on average; P ⩽ 0.10; Figure 2). This tendency between diets was also observed for DM and OM intake (P = 0.070 and P = 0.078, respectively) when cows were in Figure 1 Boxplot of blood methemoglobin levels of lactating cows fed 10% extruded linseed plus 1.8% nitrate (n = 8) during 17 weeks. In week 0, animals were fed a control diet. Linseed and nitrate were first incorporated in week 1. Blood was analysed in week 0 and then twice a week during weeks 1, 2 and 3 and once a week from weeks 4 to 17. The box represents the quartiles with the median within the box, and the vertical lines represent the maximum and minimum value, respectively. Arrows indicate measurement weeks.

Guyader, Doreau, Morgavi, Gérard, Loncke and Martin chambers (Table 2). Fibre intakes were lower with LIN + NIT (P = 0.008 for NDF and P = 0.007 for ADF) whereas dietary treatments did not affect GE intake.

We found no difference between diets in milk production over two-thirds of the experiment (11 weeks out of 17), whereas in weeks 4, 5, 7, 9, 10 and 17, milk production was lower with LIN + NIT (-2.5 kg/day on average; P < 0.05; Figure 3). In chambers, cows fed LIN + NIT produced less milk (-2.8 kg milk/day on average, P = 0.078; -4.7 kg FPCM/day on average, P = 0.045; Table 2). Feed efficiency was similar between diets in week 5 and tended to be higher for LIN + NIT in week 16 (diet × week, P = 0.061).

In chambers, milk fat and lactose concentrations were similar between diets, whereas LIN + NIT reduced milk protein (P = 0.045) and urea (P < 0.001) concentrations by 6.8% and 60.6%, respectively. For both diets, nitrate and nitrite concentrations in individual milk samples, pooled milk samples and milk products were lower than the limit of quantification, except for curd from CON in week 17 and cheese from CON and LIN + NIT in week 9 in which low nitrite concentrations were detected (1.5 mg/kg).

Methane emission

Diet LIN + NIT decreased daily CH 4 emission by 47% (g/day; P < 0.001), 30% (g/kg DMI; P = 0.002) and 33% (g/kg FPCM; Linseed and nitrate reduce long-term methane emission P = 0.001) on average for weeks 5 and 16, without significant effect of week or diet × week interaction (Table 2). This shows that CH 4 emissions after 5 and 16 weeks were similar for cows fed CON and also for cows fed LIN + NIT. Except for a 2-h period before morning feeding, LIN + NIT decreased CH 4 emission all along the day (P ⩽ 0.05; Figure 4).

Diet apparent digestibility and N balance

Apparent digestibility of DM, OM and NDF was similar between diets, and averaged 67.5%, 69.4%, and 50.6%, respectively (Table 3). The LIN + NIT diet tended to reduce ADF (-3.8%; P = 0.070) and CP (-2.9%; P = 0.074) apparent digestibility. N intake was 22% lower with LIN + NIT (P = 0.001). Consequently, LIN + NIT led to lower faecal N losses, urinary N losses and N retained in milk (P = 0.016, P < 0.001 and P = 0.003, respectively). However, N distribution was unaffected by diet. On average for both diets, 35.7% (P = 0.074), 24.1% (P = 0.071) and 29.9% (P = 0.937) of N intake was directed towards faeces, urine and milk, respectively. Finally, N balance was positive and similar between diets and averaged 52.6 g/day or 10.5% of N intake.

Rumen fermentation and microbial parameters Concentration of NH 3 in the rumen did not change with diets (Table 4). Diet LIN + NIT reduced total VFA (-12 mM; P = 0.020) and propionate concentrations (-8 mM; P = 0.003) without affecting acetate and butyrate concentrations. These differences in VFA profile induced an increase in C2/C3 and (C2 + C4)/C3 ratios (P = 0.003) with LIN + NIT. Total concentration of protozoa in the rumen tended to increase with LIN + NIT (+53%; P = 0.052).

Discussion

Intake, milk production and N balance Throughout the experiment, intake and milk production tended to be lower for dairy cows supplemented with LIN + NIT. As feed efficiency (kilograms of FPCM per kilograms of feed) was similar between diets, the lower intake may explain the lower milk production. The lower intake with LIN + NIT is difficult to explain because diets had similar net energy content. Individual nitrate supplementation at higher doses than here (1.8%) did not reduce intake of restricted-fed dairy cows (2.1%, Van Zijderveld et al., 2011b; 2.0%, Veneman et al., 2014), but tended to reduce DMI of dairy cows (2.0%, [START_REF] Veneman | Dietary nitrate but not linseed oil decreases methane emissions in two studies with lactating dairy cows[END_REF] and steers (2.3%, [START_REF] Hulshof | Dietary nitrate supplementation reduces methane emission in beef cattle fed sugarcane based diets[END_REF] fed ad libitum. Linseed applied at doses higher than here (3.5% added fat) did not have a negative effect on the intake or milk production of dairy cows (5.1% added fat, [START_REF] Ferlay | Effects of incremental amounts of extruded linseed on the milk fatty acid composition of dairy cows receiving hay or corn silage[END_REF]4% added fat, Veneman et al., 2014) fed ad libitum or restricted. One study reported a lower DMI (-7%) by lactating cows fed a grass silage-based diet supplemented with linseed (3% added fat, [START_REF] Martin | Methane output and rumen microbiota in dairy cows in response to long-term supplementation with linseed or rapeseed of grass silageor pasture-based diets[END_REF]. The only study that simultaneously used linseed plus nitrate (4% added fat plus 2.3% nitrate) on cows did not result in intake changes, but the cows were non-lactating and restricted-fed [START_REF] Guyader | Additive methane-mitigating effect between linseed oil and nitrate fed to cattle[END_REF]. We hypothesise that LIN + NIT fed together ad libitum may have an inhibitory effect on voluntary intake linked to a tendency for lower ADF digestibility. [START_REF] Allen | Physical constraints on voluntary intake of forages by ruminants[END_REF] highlighted the negative correlation between fibre digestibility and voluntary intake through a lower passage rate of particles from the rumen and greater rumen filling.

The LIN + NIT diet had no effect on concentration and production of fat and lactose in milk. This result confirms previous experiments with dairy cows supplemented with nitrate (2.1%, Van Zijderveld et al., 2011b) or extruded linseed (up to 5.1% added fat, [START_REF] Ferlay | Effects of incremental amounts of extruded linseed on the milk fatty acid composition of dairy cows receiving hay or corn silage[END_REF]. The LIN + NIT diet reduced milk protein concentration and production by 7% and 15%, respectively. In dairy cows fed 2.1% nitrate, Van Zijderveld et al. (2011b) also reported reduced milk protein concentrations (-5%) but no effect on milk protein production whereas milk yield was stable. The reduced milk protein concentration may not be linked to linseed supplementation, as it was not affected by 3.5% added fat from extruded linseed in hay-or maize silage-based diets [START_REF] Ferlay | Effects of incremental amounts of extruded linseed on the milk fatty acid composition of dairy cows receiving hay or corn silage[END_REF].

N balance was positive and similar between diets with the same N distribution between milk, faeces and urine. In addition, average N efficiency (N in milk/N intake) was similar between CON and LIN + NIT (30%) and close to the data given in the literature (25%, with a range between 15% and 40%, [START_REF] Calsamiglia | Strategies for optimizing nitrogen use by ruminants[END_REF]. This result shows that dairy cows use nitrate in the same way as they use other N sources. The marked decrease in milk urea from cows fed LIN + NIT was surprising and in contradiction with previous experiments on dairy cows fed extruded linseed (1.1% added fat, [START_REF] Pezzi | Effects of extruded linseed dietary supplementation on milk yield, milk quality and lipid metabolism of dairy cows[END_REF] or nitrate (2.1% nitrate, Van Zijderveld et al., 2011b). We assume that this difference comes from the lower N intake of animals fed LIN + NIT, as N intake is known to correlate positively with milk urea [START_REF] Spek | A review of factors influencing milk urea concentration and its relationship with urinary urea excretion in lactating dairy cattle[END_REF].

The main concern when using nitrate in animal nutrition is its potential negative effect on animal and human health. To avoid increase of blood metHb in animals, progressive adaptation to nitrate is essential [START_REF] Lee | A meta-analysis of effects of feeding nitrate on toxicity, production, and enteric methane emissions in ruminants[END_REF]. In this study, we did not observe rises in metHb levels in animals fed LIN + NIT, similarly to a previous experiment on dairy cows fed 2.1% nitrate (Van Zijderveld et al., 2011b). However, we cannot explain the greater metHb level observed in the last week of the experiment. Analyses were carried out by an external lab and could not be repeated as metHb needs to be analysed quickly after sampling. In terms of human health, nitrate and nitrite are common food additives used for their anti-bacterial properties against lethal pathogens [START_REF] Food | Scientific opinion of the panel on contaminants in the food chain on a request from the European Commission on nitrite as undesirable substances in animal feed[END_REF]. However, excessive consumption of nitrate from several food sources may promote gastric inflammation linked to the production of nitrite from nitrate reduction in the mouth [START_REF] Weitzberg | Novel aspects of dietary nitrate and human health[END_REF]. Regulations have been adopted to keep concentrations of nitrate and nitrite residuals within recommended daily allowances for nitrate and nitrite intake (3.75 and 0.13 mg/kg BW per day, respectively; [START_REF] Food | Scientific opinion of the panel on contaminants in the food chain on a request from the European Commission on nitrite as undesirable substances in animal feed[END_REF], and Europe has limited nitrate concentration in drinking water (50 mg/l, [START_REF] Benjamin | Nitrates in the human diet-good or bad?[END_REF]. Here, nitrate and nitrite residuals in milk products were lower than the limit of quantification of the technique (5 mg/kg for nitrate and 0.5 mg/kg for nitrite), except in cheese from CON and LIN + NIT (1.5 mg/kg nitrite). These novel data confirm previous work on lamb meat (El-Zaiat et al., 2013), and show that animals can metabolise nitrate and nitrite without transferring residuals into animal products. Consequently, long-term supplementation with nitrate (4 months) can be safely proposed in ruminant nutrition without risks for human health and as a source of non-protein N.

Methane emission and associated digestive mechanisms Methane emission (% of GE intake) observed in cows fed CON was low compared to the literature for dairy cows fed similar diets. Nonetheless, methanogenesis was decreased by 30% (g/kg DMI) when dairy cows were supplemented with 1.8% nitrate plus 3.5% added fat from extruded linseed, corresponding to our expected theoretical CH 4 reduction. This confirms our previous results (32%) Linseed and nitrate reduce long-term methane emission obtained on non-lactating cows fed a hay-based diet supplemented with 2.2% nitrate plus 4% added fat from linseed oil [START_REF] Guyader | Additive methane-mitigating effect between linseed oil and nitrate fed to cattle[END_REF] and shows that LIN + NIT can efficiently reduce CH 4 emission under various physiological animal conditions and for different diets. However, LIN + NIT may fail to reduce methanogenesis under some specific conditions. Indeed, nitrate (2.2%) did not reduce CH 4 emissions in finishing beef cattle fed a high concentratebased diet as these animals were already low CH 4 -emitters (Troy et al., 2015). The CH 4 -mitigating effect of linseed was also not observed in studies in which the level of FA in diets was low (1% to 2% of DM; [START_REF] Livingstone | Effects of forage type and extruded linseed supplementation on methane production and milk fatty acid composition of lactating dairy cows[END_REF] or not different from their control diet, which contained a rumen inert fat source (Van Zijderveld et al., 2011a; Veneman et al., 2014). We observed a severe CH 4 -mitigating effect of LIN + NIT just after feeding, which was most probably linked to the effect of quickly metabolised nitrate in the rumen. This result agrees with [START_REF] Van Zijderveld | Nitrate and sulfate: effective alternative hydrogen sinks for mitigation of ruminal methane production in sheep[END_REF] and [START_REF] Guyader | Additive methane-mitigating effect between linseed oil and nitrate fed to cattle[END_REF]. Methane reduction with LIN + NIT corresponds to a saving of 1.8% of GE intake, without positive responses on apparent digestibility, BW gain or condition score (data not shown). The absence of relationship between CH 4 reductions and dairy cows performance has also been reported previously (Van Zijderveld et al., 2011b).

The CH 4 -mitigating effect of LIN + NIT was maintained throughout the 4 months of the experiment, indicating that this dietary strategy could be applied on farms. The longterm CH 4 -mitigating effect of nitrate (2.1%) and extruded linseed (up to 3% added fat) fed individually to dairy cows was also maintained during 3 months (Van Zijderveld et al., 2011b) and 1 year [START_REF] Martin | Methane output and rumen microbiota in dairy cows in response to long-term supplementation with linseed or rapeseed of grass silageor pasture-based diets[END_REF], respectively.

The LIN + NIT diet did not change rumen protozoa concentration as previously observed with non-lactating cows [START_REF] Guyader | Additive methane-mitigating effect between linseed oil and nitrate fed to cattle[END_REF]. Diet LIN + NIT increased the acetate/propionate and (acetate + butyrate)/propionate ratios due to a decrease in ruminal propionate, which is normally a competitive pathway of methanogenesis. This contrasts with our previous work in which LIN + NIT did not change rumen fermentation parameters [START_REF] Guyader | Additive methane-mitigating effect between linseed oil and nitrate fed to cattle[END_REF]. However, in the present work, the relationship between CH 4 production and rumen fermentation and microbial parameters should be interpreted with caution given the large differences in time scale between CH 4 measurement periods and rumen samplings through stomach tubing. Consequently, the CH 4 -mitigating effect of LIN + NIT would not be explained by a reduction in acetate and butyrate synthesis, or by a reduction in number of protozoa, which are important H 2 producers. Instead, both supplements may act as H 2 sinks. Based on stoichiometric calculation and assuming complete reduction of nitrate to nitrite and ammonia, and complete biohydrogenation of polyunsaturated fatty acids, 325.8 g/day of nitrate and 600.9 g/day of fatty acid (23%, 32% and 25% of C18:1, C18:2 and C18:3, respectively) ingested by dairy cows could have reduced CH 4 by 90.1 and 14.9 g/day, respectively. In total, H 2 consumption by LIN + NIT could have reduced CH 4 emission by 105.0 g/day, explaining 54% of the observed CH 4 reduction. The remaining decrease must therefore be explained by non-stoichiometric processes as LIN + NIT may also act on rumen microbiota: nitrate reduced both quantity (2.6% nitrate to sheep, [START_REF] Van Zijderveld | Nitrate and sulfate: effective alternative hydrogen sinks for mitigation of ruminal methane production in sheep[END_REF] and activity (2.3% nitrate to non-lactating cows, [START_REF] Guyader | Dietary nitrates decrease methane emission by inhibiting rumen methanogenic archaea without influencing nitrate reducing bacteria[END_REF] of methanogens. The anti-methanogenic effect of polyunsaturated fatty acids from linseed has also been demonstrated in cattle (2.6% added fat from linseed oil, Guyader et al., 2014; 3.5% added fat from extruded linseed, C. Martin, unpublished results). In addition, H 2 production must have been lowered with LIN + NIT owing to a lower quantity of fermentable substrates in the rumen (lower DMI, quantity of carbohydrates due to lipids substitution and fibre digestibility), which directly reduced CH 4 emission.

Linseed plus nitrate is an efficient feeding strategy to reduce CH 4 emission in the long-term without altering diet apparent digestibility, N efficiency or animal health. However, to make this dietary strategy acceptable by farmers, further work is required to optimise the doses of linseed plus nitrate in an effort to avoid concomitant reduction in intake and milk production. Additional data is needed on changes in rumen microbiota in order to fully understand the CH 4 -mitigating effect of the association of linseed plus nitrate. A life cycle assessment will also be needed to evaluate the environmental benefit and economic cost of this dietary strategy in order to raise the prospects of using this strategy at farm level.

Figure 2

 2 Figure 2 Dry matter intake of lactating cows fed a control diet (CON; n = 8) or CON supplemented with 10% extruded linseed plus 1.8% nitrate (LIN + NIT; n = 8) during 17 weeks (averages of 4 days/week). Error bars indicate SD. Symbols indicate weekly statistical comparison between CON and LIN + NIT ( †P ⩽ 0.10; *P ⩽ 0.05; **P ⩽ 0.01; ***P ⩽ 0.001). Arrows indicate measurement weeks.

4

  Feed efficiency = FPCM/DMI.

Figure 3 Figure 4

 34 Figure 3 Milk yield of lactating cows fed a control diet (CON; n = 8) or CON supplemented with 10% extruded linseed plus 1.8% nitrate (LIN + NIT; n = 8) during 17 weeks (averages of 4 days/week). Error bars indicate SD. Symbols indicate weekly statistical comparison between CON and LIN + NIT ( †P ⩽ 0.10; *P ⩽ 0.05; **P ⩽ 0.01; ***P ⩽ 0.001). Arrows indicate measurement weeks.

Table 1

 1 Ingredients and chemical composition of the experimental diets

	Diet

1 Fermentation characteristics of fresh silage juice: pH = 3.57; acetic acid = 0.74 g/100 g; lactic acid = 3.01 g/100 g; N-NH 3 = 0.02 g/100 g. 2 Extruded linseed (InVivo NSA, Longué Jumelles, France).

3 Calcium ammonium nitrate (5Ca(NO 3 ) 2 .NH 4 NO 3 .10H 2 O; Phytosem, Pont-du-Château, France) contained 75% nitrate on a DM basis.

Table 3

 3 Total tract apparent digestibility and nitrogen balance of lactating cows after 17 weeks feeding a control diet ( n = 8) or a diet supplemented with a combination of linseed and nitrate ( n = 8)

			Diet
	Item 1	CON	LIN + NIT	SEM	P-value
	Total tract apparent digestibility (%)
	DM	67.8	67.2	0.74	0.531
	OM	69.8	69.0	0.73	0.458
	NDF	51.3	49.9	1.11	0.393
	ADF	47.5	43.7	1.35	0.070
	CP	65.8	62.9	1.05	0.074
	Starch	98.5	97.9	0.24	0.109
	N intake (g/day)	548	425	21.6	0.001
	Faecal N losses
	g/day	187	157	7.9	0.016
	% of N intake	34.2	37.1	1.05	0.074
	Urinary N losses
	g/day	138	96	5.7	<0.001
	% of N intake	25.4	22.7	0.98	0.071
	Milk N output
	g/day 2	164	126	7.2	0.003
	% of N intake	29.9	29.8	1.05	0.937
	N balance 3
	g/day	59.1	46.1	9.87	0.365
	% of N intake	10.5	10.5	1.73	0.990

CON = diet control; LIN + NIT = diet CON containing 10% extruded linseed and 1.8% nitrate on a DM basis; DM = dry matter; OM = organic matter; N = nitrogen. 1 Average of 5 days of total tract apparent digestibility and N balance measurement in week 17.

Table 4

 4 Fermentation parameters and protozoa concentration in the rumen of lactating cows after 17 weeks feeding a control diet ( n = 8) or a diet supplemented with a combination of linseed and nitrate ( n = 8)

			Diet
	Item 1	CON	LIN + NIT	SEM	P-value
	NH 3 (mM)	10.1	11.0	1.65	0.736
	VFA concentration (mM)
	Total VFA	104.1	91.7	3.35	0.020
	Acetate (C2)	58.6	56.9	1.95	0.561
	Propionate (C3)	25.6	17.6	1.65	0.003
	Butyrate (C4)	15.2	14.1	1.61	0.635
	Minor VFA 2	4.71	3.08	0.577	0.055
	C2/C3	2.36	3.27	0.170	0.003
	(C2 + C4)/C3	2.99	4.08	0.213	0.003
	Total protozoa (log 10 /ml)	5.03	5.32	0.095	0.052

CON = diet control; LIN + NIT = diet CON containing 10% extruded linseed and 1.8% nitrate on a DM basis; VFA = volatile fatty acid. 1 Data from rumen samples taken the last day of week 17. 2 Minor VFA = sum of isobutyrate, isovalerate, valerate and caproate.

Gusti, Nutriad, Chester, England.

Average chemical composition from samples (n = 3) taken in weeks 5, 16 and 17.Guyader, Doreau, Morgavi, Gérard, Loncke and Martin

Milk N output = (milk yield × milk protein concentration)/average N concentration in milk (6.38 g N/g milk protein).

3N balance = N intakefaecal and urinary N lossesmilk N output.

Acknowledgements

J. Guyader is the recipient of an INRA-Région Auvergne PhD scholarship. The authors thank L. Genestoux for laboratory analysis and transport of blood samples, and L. Mouly, D. Roux, S. Rudel and V. Tate for animal care.


Livio De Luca

Philippe Véron
email: philippe.veron@aix.ensam.frwww.lsis.org

Michel Florenzano

Semantic-based modelling and representation of patrimony buildings

Keywords: architectural heritage, architectural knowledge, feature-based modelling, virtual environments, semantic shape

This article presents a methodological approach for the semantic description of patrimony buildings based both on theoretical reflections and on research experiences. To develop this approach, a first process of extraction and formalisation of architectural knowledge based on the analysis of architectural treaties is proposed. Then, the identified features are used to produce a template shape library dedicated to the buildings surveying. Finally, the problem of the overall model structuration and organization using semantic information is addressed for user handling purposes.

Introduction

Understanding studies of architectural heritage can be today widely improved by the use of 3D reconstruction of the real buildings. This way is also efficient to document historic buildings and sites for their reconstruction or restoration, to create resources for researchers and to analyze their historical evolutions. The spatial data acquisition techniques have known in these last years large improvements due to the introduction of laser scanning technologies [START_REF] Barber | Laser Scanning and Photogrammetry: 21st century metrology[END_REF]. Nevertheless, if the digitizing phase can profit of automatic and high-speed procedures, the 3D point clouds post processing requires many manual actions when the objective is to produce an analytical representation of architectural objects. In fact, if this technical solution theoretically allows having an accurate geometric reproduction of a building with all its details, it remains still the problem of producing a semantically enriched model [START_REF] Trevisan | Proporzioni e vera forma di particolari architettonici rilevati con scanner 3D: caratteristiche di un software specifico[END_REF]. A triangulated-model can be widely appropriate to represent artistic objects such as sculptures or facing relief [START_REF] Curless | A volumetric method for building complex models from range images[END_REF]. But, when the definition of geometrical relations and rules between dimensions are required to describe the proportions of typical objects participating to an architectural project, this type of geometrical representation is not efficient. With regard to the set of classical architectural shapes, relevant information, which must be acquired, recognized and modelled, can be found in a set of relevant profiles describing these shapes on the basis of the conventions of architectural drawings. Whatever the digitizing device used, before starting the treatment of the 3D data sets, it is essential to take into account the representation problems, because the reconstruction process carried out makes a geometrical interpretation of the architectural elements. Indeed, if the actual reconstruction techniques provide good surface reconstructions, they do not necessarily integrate the semantic interpretation of the architectural shapes. In this sense, the efficient proposed way to define an architectural surveying is to consider it as a rebuilding problem [START_REF] Migliari | Per una teoria del rilievo architettonico; primi appunti[END_REF]. Architectural surveying is a reverse process which starts from the real object, rebuilds a digital model and interprets the idea which is upstream of its realization. In an architectural building project two essential points of views can be characterized. The first one is related to the architect who has an ideal and perfect design reference and the second one relates the concrete and imperfect real realization. In the same manner, the architectural surveying can be articulate around two points of views. The first one is related to the crude processing of the 3D data sets which produce an imperfect digital model and the second one is related to the exploitation of semantic information producing an enhanced design model of the real building.

In this way, the extraction of relevant information from the 3D digitized data, according to the architectural drawing conventions (related to the different historic periods), forms an important problem which must be addressed. To this end, semantic-based representations of patrimony buildings can thereby play an important role for the development of information systems in the architecture field. Beyond survey, in fact, the development of qualitative descriptions of architectural objects is a major research area. Nevertheless, if the scientific analysis of crude documentary sources has benefited from the development of various data management techniques, it has most often been done without a concern for graphical visualisation or for a clear reference to the architectural morphology. Architectural surveying and documentation seems to relate to totally independent research areas whereas they have naturally common problematic around the integration of information and knowledge associated to an edifice. Consequently, today it appears as essential to take into account and integrate quantitative information supported by the surveying process and qualitative information or knowledge extracted from documentation sources. The approach proposed, sets a method to formalize the theoretical knowledge of patrimony buildings which is then used to support the different investigation stages of an edifice, from 3D surveying to representation and handling in a 3D real-time environment. The paper is organized as follow. Section 2 discusses the problem of the semantic description of patrimony buildings. Section 3 describes the knowledge-based modelling tools developed, starting from a method for the feature formalisation from architectural treaties. Section 4 presents a result example of semantic-based representation for information interfacing. Finally, section 5 concludes the paper and suggests some future works.

Architectural knowledge system

The study of the drawing convention in the history of the architectural representation has a double finality: the first one leads to the representation and the second one leads to the object surveying. These two analysis points of views of the architectural elements are strictly interdependent [START_REF] Docci | Geometria e Architettura[END_REF]. The knowledge extraction problem consists in identifying the genesis of the element shape to define, both, the appropriate way for its measurement and for its representation. To this end, architectural knowledge rules have to be formalised. An architectural knowledge system can be described as a collection of structured objects, identified through a precise vocabulary. Several studies led to the definition of classification methods for classical architectural elements. These are based on a structure of different level of abstraction of architectural space [START_REF] Tzonis | Classical Architecture -The poetics of Order[END_REF]. These classifications are based on the study of the architecture treaties, which organize the 'art to build' knowledge relatively to the different historical periods. Many treaties develop an identity coding of architectural elements. This identity is normally expressed through a hierarchical description of all the elements, which make a build unit (Fig. 1).

In [START_REF] Palladio | The four books of Architecture[END_REF], by means of a representation convention, each architectural element is expressed by a geometrical description level (i.e. lines, curves), a topological relations level (i.e. parallelism, concentricity, etc) and a spatial relations level (i.e. proportions, harmonic reports/ratios). The problematic of the semantic description of patrimony buildings return to the extraction of these three levels starting from the 3D digitized data of a real building. It starts from the analysis of various sources (Fig. 2 a) of knowledge (including the study of particular cases) to extract drawing rules. These rules are formalised (Fig. 2 b) to produce their appropriate digital translation into a 3D semantic-based description (Fig. 2 d). The semantic description of an architectural element gathers modelling functions, constraints links and hierarchical relationships. To explain the notion of semantic description of patrimony building, we take as example the analysis of a teatrastyle, prostyle temple of Corinthian order (Fig. 3). The cella is preceded by a pronaos composed by four columns in frontage; this pronaos is closed on the sides of the cella from which a side column separates it.

Architectural vocabulary

The temple rises on a stylobate. That finishes each side, and in front of the pronaos by two long pedestals including between them the stairs. The bases of the Corinthian order are composed of a surmounted plinth of a lower torus and a higher torus connected by two scoties separate one of the other by a rod and two l i n t e l s . The architraves are simply profiled. The cornice of the entablature is richly decorated.

Based on the previous description, the first phase carried out a reasoned decomposition of the building using a vocabulary analysis [START_REF] Perouse De Monclos | Architecture vocabulaire. Principes d'une analyse scientifique[END_REF].

The goal is to identify the vocabulary and to describe in a theoretical way an architectural entity (i.e. definition of its model) (Fig. 4). This analysis allows expressing the morphology of the sub-elements that participate to the architectural unit. The goal of the next phase is to identify the composition rules which govern the building arrangement.

Architectural grammar

By looking at the frontage of the temple, one finds the distinction given by Vitruve [START_REF] Pollio | Ten books on architecture[END_REF], which says that the building must be ordered:

• with symmetry ( [START_REF] Pollio | Ten books on architecture[END_REF] book I, course, II), i.e. its various parts respect an aspect report/ratio (the module) which drives a large number of the building dimensions in the same way that for the human body with the report/ratio between the feet and the hands for example; • with the suitable proportion ( [START_REF] Pollio | Ten books on architecture[END_REF] book III, course I), i.e. a relationship between the sub-elements and the whole building, the overall aspect report/ratio.

The relations which connect the sub-elements between them are expressed through constraints between surfaces (i.e. faces or axis coincidences, distances, angles). Four kinds of constraints are used: position, orientation, scale and contact. For example: the eustyle is the rule guiding the "betweencolumns". These intervals must be of 2.25 diameters (Fig. 5). Several rules of this type constitute the geometry layouts which are used for scheduling and dimensional coordination. They constitute the geometricae rationes, which are based on the properties of the Pythagoras triangle about which Vitruvius [START_REF] Pollio | Ten books on architecture[END_REF] speaks without exposing the details: "its proportions are useful in many cases, both for the design of the buildings and for their surveying". For example, the lines resulting from the torus of the base of first column and parallel with the hypotenuse of the Pythagoras triangle cut the axis of the third column in various points which give the height of the architrave, the plank and of the cornice (Fig. 6).

Architectural knowledge-based modelling tool

Making of a 3D building model requires a geometrical representation of the objects that make it up, then the determination of the aspect of its surfaces. The developed tool presented here is an element of a complete approach for 3D surface reconstruction. This approach is founded on a hybrid registration of two types of digitalized sources (point-clouds and digital images) in order to provide a complete support for the 3D modelling process (Fig. 8). It includes three distinct concerns: the first one is the surveying of the object under these metric and photometric points of views. The second one is the construction of its geometrical model and its enrichment by photometrical and dimensional information. The last one is the semantic organization (i.e. structuration) of the building.

For each concern, the approach introduces the architectural knowledge and its exploitation as a support to interpret and reproduce shapes.

Combining range-based and image-based techniques, the surface reconstruction process uses: the 3D point-clouds to extract relevant profiles and to create triangulated-mesh; the digital images to cover shadow areas in the 3D point-clouds, to recover additional coordinates and to extract textures for their future mapping. In the modelling module, 3D point-clouds and digital images constitute respectively the metric and the visual (i.e. graphic) support [START_REF] Blaise | Architectural surveying -from a point cloud to a 3D model. EVA2004 -Electronic Imaging and the Visual Arts[END_REF]. The knowledge-based modelling module presented here allows the surface reconstruction focusing on relevant profiles and using a library of architectural feature shapes in order to produce semantic representations of architectural elements and to organize them into hierarchical structures.

The next section describes the knowledge formalisation process proposed. Section 3.2 presents the architectural feature shape library developed and the section 3.3 illustrates the feature instantiation process using dimensional information extracted from the digitized data of a real object.

Knowledge formalisation process

Based on an interpretative study of documentary sources or on a comparative analysis of different real elements, the formalization of an architectural element starts by the definition of the relevant shape profiles.

A network of B-Splines curves associated with a modelling procedure is defined and constitutes the geometrical description level of the architectural element.

The method proposed to formalize the architectural primitives starts from an analysis of historical sources such as, for example, a simple frontage element (Fig. 9). The (MEL) Maya Embedded Language's core uses a data flow paradigm [START_REF] Gould | Complete Maya Programming: An extensive guide to MEL and C++API[END_REF]. This core is incorporated in the dependency graph (DG). The data and their operations are encapsulated in the DG as nodes. In order to perform some complex modification to some data, a network of simple nodes is created. Each node has one or more properties associated with it. These properties are commonly defined as attributes.

An entire geometric mesh or Non-Uniform Rational B-Splines (NURBS) surface can be stored as an attribute in a node. Thus, the first level of primitives is defined as set of simple nodes: the mouldings in our simple frontage example (Fig. 10). Each node contains the essential attributes according with the entity position, the orientation of its centre point, plus its width and height. These nodes are connected to children nodes defining the geometrical representation of the entities using B-Splines curves of different degrees. Five types of first level primitives are presented below. In the same manner, a set of extrusion paths which drives the surface generation is defined. By connecting simple nodes, a chain of computation can be created to produce the final result [START_REF] Gould | Complete Maya Programming: An extensive guide to MEL and C++API[END_REF].

To set up the concept of hierarchy, a Direct Acyclic Graph (DAG) is used. The DAG describes a hierarchy in which a node can not be both a parent and a child in the same lineage. The second level of primitives represents profiles defined by progressive combination of several first level primitives (Fig. 11). A constraint node links the last curve's control point to the last one of the precedent entity. Thus, in the DAG paths, the relationship between a 3D object local reference (x, y, z) and a 2D (u, v) parametric space is known. This property is used to generate surfaces by creating a connection between a profile node, a modelling function and a path node. Each first level primitive of a profile node is extruded along the parametric space of the associated path node (Fig. 12). In the same manner, but in an upper hierarchical level, the third level of primitives is created using Boolean operations (Fig. 13). The DG is based on a push-pull model. So, when one pull information from a node, it propagates that request through all the nodes that provide input connections to the node (Fig. 14).

Architectural feature library

A MEL command makes it possible to introduce a Web browser inside an interface layout. This browser allows receiving MEL script since a distant site. A simple script can contain the procedure for the surface generation, the parameter setting and the structuration of a formalized entity. One of the fundamental properties of NURBS surfaces allows a deformation process to produce a shape which respect the parameters defined. This property is used to locate the primitive into the 3D space containing the 3D pointclouds and to adapt it precisely to the real shape of the digitized object. Such an instantiation process opens an interesting way around the structuration of a library of architectural entities taking again the treaties of architecture of different periods, whose described elements are translated into parametric modelling terms. The procedure developed to reconstruct architectural elements using an instantiation process based on a feature shape library is divided into three steps:

• Selection and generation of an architectural feature shape since a distant site,

• Fast positioning in the 3D point-cloud,

• Dimensional adaptation of the surface to the 3D point-cloud.

Imported once, the feature template is introduced automatically into the 3D scene and assigned at the geometric centre of a group of cameras with orthogonal projection. Then the user can locate the feature on different distinct windows (also including a calibrated image as show in Fig. 15). The camera planes are located at centre of each relevant profile of the primitive. A device of "deformation tools" allows applying transformations (i.e. translation, orientation and scale factor) to the feature parameters. Transformations can be applied to an entire profile, to a single profile's module or to a simple parametric control point. The constraint set encapsulated in the feature template produces a propagation of the deformations applied, to keep the coherence of the overall entity shape according with the architectural rules involved.

Extraction of dimensional information

One of the most interesting exploitation of the knowledge-based modelling tool consists in the possibility of creating, at the same moment of the positioning of the feature shape, an organized structure of drifting measurement. This is possible extracting the dimensional values associated the parameters of the primitives instanced. Modelling by architectural primitives becomes in this direction a way to structuring an organized abacus of dimensional information (Fig. 16).

We work actually to the management of these information in relational databases. To allow the future handling of the 3D building digital mock-up, the approach proposed ensures the structuration of the model of the architectural object in parts and sub-parts, and the identification of their reciprocal relations to guarantee a correspondence between the topological description model of the architectural object and its 3D geometrical representation. This aspect is very important in the objective of a semantic structuration of the elements which compose the scene for 3D realtime visualization as presented below.

Semantic 3D representation for information interfacing

In the "3D Architectural Information System" (Sia3D) prototype [START_REF] Blaise | Architectural Modelling and Information Interfacing: Learning from Three Case Studies[END_REF] developed to illustrates the approach proposed on the basis of the remains of the Roman Theatre in Arles (in the south of France), the issue of how to better exploit, in terms of readability for architects, the results of a surveying process based on an image-based modelling techniques is addressed. According with the principles established in [START_REF] Dudek | New experimentation of a generic framework for architectural heritage data visualisation[END_REF], we consider it is necessary to identify and organise non-ambiguous morphological elements to which we will attach various information, including raw results of the surveying campaigns. The results of the surveying and modelling process can be contextualized in the following way:

• by locating in a same reference space of both the elements surveyed and the theoretical elements (Fig. 17), • by positioning them with regards to their associated knowledge.

For example, once surveyed, a cornice's remain can be displayed in a 3D model of the whole building with indications concerning:

• its belonging to an object typology (i.e. a stylistic references, a role in the building), • its hypothetical position, marked as such, in the virtual building reconstruction,

To illustrate the previous contextualization concept, the following part of this section focuses on the final output structure of a 3D interpretative model of a Roman Theatre. The surveyed remains are localised in the reference space of a theoretical Roman Theatre defined through the relevant architectural vocabulary previously analysed. The analysis made aims at identifying non-ambiguous concepts that on one hand correspond to physical objects which exist in the building (base, capital, …) and, on the other hand, have a significant role in the edifice's composition.

The vocabulary produced is used to rebuild, level after level, the theoretical model of a Roman Theatre in the Augustinian period (Fig. 18). These levels can be read as rebuilding point of view starting by individual elements such as a capital to whole groups such as colonnade. Thus the levels defined match the concept of architectural scales. The building is thus described by a hierarchical structure that derives form the vocabulary analysis (Fig. 19). The hierarchical structure formalizes as composition links the relations between the elements in a five (for the case of a Roman Theatre) level hierarchy (example: capital / part-of / column / part-of / colonnade / part-of / …). Once the elements are classified as shown above, and once the compositional rules are expressed, the understanding of the edifice is improved. The hierarchical structure also allows establishing bilateral relations between the information sources and the 3D model. Indeed, to each hierarchical level specific information sources are associated. Thus, each element of the hierarchy acts as a filter in the handling of the 3D model since it is represented by its own geometry or by subelements.

Figure 19: the five-level hierarchical structuration of the architectural elements of a Roman Theatre

The hierarchical structure controls transitions between levels, either inside the 3D scenes where geometrical representations of objects are grouped according with the level displayed, and also inside the information sheets. These information sheets gather data pieces on theoretical elements, but also descriptions of the surveyed remains (current localisation, conservation state, materials). The 3D model is finally a digital mock-up of the building with an interface that allows the user:

• to select an element by its name in the hierarchical structure and to display its position in the scene, • to get the information sheets related to this element, • to select an element into the 3D model itself and to visualize its position in the hierarchical structure, • to switch between levels according with the user requirements,

• to show / hide elements or groups of elements, • to interactively control transparency level of selected elements or groups of elements, • to show / hide selected remains, and so one. The interface developed (Fig. 20) is accessible on a standard Web browser with the Virtools plug-in to read the 3D scene. Scene/windows interactions are written in JavaScript, enabling easy updating of the various links implemented. Indeed, a wide variety of information sheets links can be implemented such as surveying results or purely bibliographic sources.

Conclusion

An interesting development prospect relates to the problem of the multi-representation. Indeed, the proposed approach allows introducing the concept of objective or point of view, which governs the choices between various representations of the same object (i.e. multi-representation). These various representations will be able to correspond at various levels of consultation and necessary comprehension according to the user profiles. Moreover, the geometrical model associated with the object must also be able to take into account the temporal dimension of the building (i.e. its evolution in the time). Within this framework, the 3D structured representation of buildings becomes a privileged support for real-time handling and to interact with the associated documentary sources (i.e. search and consultation of documents or information). These information and documents will be of course associated with the geometrical model of the object in a structured way and according to various user points of views (i.e. architects, historians, archaeologists, general public, and so one).

Figure 1 :

 1 Figure 1: extract of the Palladio's treaty of Architecture

Figure 2 :

 2 Figure 2: synthesis of the elements participating to a 3D building reconstruction process: (a) documentary sources, (b) formalized knowledge, (c) real object, (d) semantic description

Figure 3 :

 3 Figure 3: a Roman temple of Corinthian order: the Capitol of Dougga inTunisia

Figure 4 :

 4 Figure 4: semantic description of a Corinthian order using the appropriated vocabulary (basis, capital, entablature)

Figure 5 :

 5 Figure 5: the eustyle rule for example

Figure 6 :

 6 Figure 6: the geometricae rationes based on the Pythagoras triangle propertiesAs example, Figure7illustrates the complete semantic description of the analysed Roman Temple.

Figure 7 :

 7 Figure 7: semantic description of the Capitol of Dougga

Figure 8 :

 8 Figure 8: hybrid registration of point-clouds and digital images

Figure 9 :

 9 Figure 9: extract of an architectural treaty of the five classical orders

Figure 10 :

 10 Figure 10: a set of mouldings

Figure 11 :

 11 Figure 11: a combined profile: the second level of primitives

Figure 12 :

 12 Figure 12: example of an extrusion node

Figure 13 :

 13 Figure 13: Boolean operations applied to two extruded surfaces

Figure 14 :

 14 Figure 14: hierarchical organization of the formalized entities

Figure 15 :

 15 Figure 15: illustration of the feature positioning step using a 3D handler

Figure 16 :

 16 Figure 16: extraction of dimensional information

Figure 17 :

 17 Figure 17: remains relocated in the space of their corresponding theoretical elements

Figure 18 :

 18 Figure 18: the theoretical model of a Roman Theatre in the Augustinian period

Figure 20 :

 20 Figure 20: illustration of the web interface developed in the Sia3D prototype

Acknowledgements

We thank MENSI Company for helpful comments; Prof. Alberto Sdegno of IUAV for the survey of "Convento della Carità" in Venice; the "Musée de l'Arles et de la Provence Antiques" for providing us documentary sources; Francesca De Domenico for providing us images from her implementation work (Fig. 17,18,19,20).