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Session 1. Protein and energy metabolism in ruminantsAbstract N.O15; International audience

International audience; Urine is a highly suitable biological matrix for metabolomics studies. Total collection for 24-h periods is the gold standard as it ensures the presence of all metabolites excreted throughout the day. However, in animal studies, it presents limitations related to animal welfare and also due to alterations of the metabolome originating from the use of acid for preventing microbial growth or microbial contamination. In this study, we investigated whether spot urine collection is a practical alternative to total collection for metabolomic studies in lactating cows. For this purpose, we collected urine samples from 4 lactating Holstein cows fed 4 diets in a 4 × 4 Latin square design. Urine was collected for 24 h using a collecting device (i.e., total collection) or collected once per day 4 h after the morning feeding (i.e., spot urine collection). Dietary treatments differed by the amount of nitrogen content (high vs. low) and by the nature of the energy (starch vs. fiber). Urine metabolome was analyzed by 2 untargeted complementary methods, nuclear magnetic resonance and hydrophilicinteraction liquid chromatography (HILIC) coupled to a time-of-flight mass spectrometer, and by 1 targeted method, HILIC–tandem mass spectrometry. Although sampling technique had an effect on the abundance of metabolites detected, spot urine samples were equally capable of showing differences in urine metabolomethan samples from total collection. When considering nitrogen levels in the diet, the robustness and precision for discriminating high- and low-nitrogen dietswas equally achieved with both sampling techniques. A total of 22 discriminant metabolites associated with the N level of diets were identified from untargeted HILIC coupled to a time-of-flight mass spectrometer (n = 9) and nuclear magnetic resonance (n = 11), and 2 from targeted HILIC–tandem mass spectrometry. Alternatively, starch or fiber in the diet induced less changes in the metabolome that were not clearly discriminated independently of the sampling technique. We concluded that spot urine collection can successfully reveal differences in the urine metabolome elicited by dietary N levels and be used as a substitute of total urinary 24-h collection for metabolomic studies.

L'estimation de l'efficience d'utilisation de l'azote et des pertes vers l'environnement est souvent basée sur des mesures de bilan azoté (entrées – sorties) au niveau de l'animal. En théorie ces bilans, devraient être nuls ; or, ils sont la plupart du temps positifs dans la littérature, soit du fait d’erreurs de mesure, soit à cause de la non prise en compte de certains flux d’azote. Cet azote non comptabilisé pourrait conduire à la sous-estimation des émissions de polluants du système (par exemple NH3, N2O). L'urée est le principal déchet métabolique de l'azote et est principalement excrétée dans l'urine. L'urée diffuse passivement et devrait donc être présente dans tous les compartiments aqueux du corps. Cutullic et al. (2013) ont suggéré que la quantité d’N non prise en compte dans les bilans pourrait être positivement corrélée au bilan hydrique de l'animal (évapotranspiration) au-dessus de la thermoneutralité. Nous avons donc testé l'hypothèse selon laquelle la sueur contient de l'azote sous forme d'urée et que la prise en compte de ce flux peut contribuer à rendre plus fiables les bilan N, en lien avec le bilan d’eau modulé par la température ambiante. Dans un premier temps, nous avons mis au point un dispositif de récupération de la sueur à la surface de la peau de l'animal, à l’aide d'une compresse acidifiée placée sous une capsule plastique, maintenue sur le plateau de la hanche de la vache (une capsule par côté) pendant 24h. Puis, une expérience a été réalisée en conditions contrôlées, selon un schéma en inversion avec 2 groupes de 3 vaches Prim'Holstein, pendant 4 périodes de 3 semaines. Nous avons comparé 2 niveaux de température ambiante (T- : 18°C pendant toute la journée ; T+ : pic diurne de 28°C pendant 7 heures, 20°C pendant la nuit). La teneur en matières azotées totales de la ration était de 150 g/kgMS. La première semaine a été utilisée comme période d'adaptation. Le facteur température (T- ou T+) a été appliqué pendant les deuxième et troisième semaines. Des mesures complètes du bilan azote ont été effectuées, y compris les pertes d'azote par la sueur utilisant le dispositif développé. La teneur en urée moyenne de la sueur a été de 160 ± 113 mg/l, valeur cohérente avec celles couramment mesurées dans le sang sur des rations similaires. Cependant, les variations inter et intra-vaches ont été très importantes. La teneur en azote total de la sueur s’est élevée à 0,85 ± 0,12 gN/l. La part d’azote uréique a donc représenté moins de 10 % de la teneur en azote total, suggérant des pertes par volatilisation durant la collecte ou l’échantillonnage, et/ou une part importante de composés azotés non uréiques. Malheureusement, les dérivés puriques (azote non uréique) n’ont pas pu être dosés pour cause de difficultés techniques au laboratoire. A partir du bilan eau (sorties-entrées) à l’échelle de la vache, la quantité journalière de sueur (90 % de l’évapotranspiration, Maia et al., 2005) associée à sa teneur en azote a permis d’évaluer des pertes potentielles à hauteur de 12 à 28 gN/vache/j selon les vaches et les traitements. L’effet température n’est par contre pas ressorti comme significatif. Il semble donc que cette perte soit loin d’être négligeable et pourrait expliquer une part du défaut de bilan N qui s’élève en moyenne à 40 gN/vache/j dans la littérature (mais pouvant varier entre -50 et +200 gN/vache/j ; Spanghero & Kowalski, 1997). Ces résultats méritent d’être confirmés sur un plus grand nombre de vaches selon des situations alimentaires contrastées.

International audience; Abstract Animals with superior feed efficiency (FE) may also have an improved nitrogen use efficiency (NUE), which would be beneficial to economic profitability while reducing environmental impacts. When genetically selecting animals on FE, it is preferable to use residual traits [e.g. residual feed intake (RFI) or residual body gain (RG)] rather than ratios because of their predictable genetic outcomes. We studied the relationship of RFI and RG with NUE, estimated from the validated 15N abundance in plasma, across two contrasted diets based on corn or grass silages. We evaluated FE of 588 (half by diet) Charolais bulls (545 ± 57 kg BW) from 12 experimental cohorts (different farms and periods) over 200 days. Before the end of the FE test, plasma was sampled and analyzed for δ 15N. NUE was related to FE through simple-linear models with variables previously corrected for the cohort and diet effects. The models’ slopes were standardized according to FE deviation in order to compare the response of NUE to FE between indices. Higher NUE was related to higher FE (P < 0.001), showing positive correlations with RG (r=-0.40) and negative with RFI (r=0.29). However, the standardized slope of NUE to RG was significantly higher (+28%; P < 0.05) than that of NUE to RFI. This stronger NUE relation to RG compared to RFI could reflect a higher potential of RG animals to deposit N as compared to a more conservational N metabolism in RFI individuals. Regarding diets, and despite the correction of NUE and FE for this effect, the slopes of NUE to FE were numerically (P > 0.05) higher (-16% and +36%; for RG and RFI) in corn-based diets, which agrees with superior NUE observed in corn-vs-grass diets. Results suggested that superior RG animals may present proportionally higher NUE than superior RFI animals, with even better results in corn-vs-grass diets.

International audience; Abstract Protein turnover (PT), the continual synthesis and degradation of body proteins not leading to protein gain, is an essential high energy-demanding process. We assumed that PT might explain the between-animal variations of residual feed intake (RFI). The objective was to measure PT in extreme RFI cattle fed two contrasted diets (grass or corn-based). We conducted a RFI test for 84 days with 100 Charolais bulls and we selected the 32 most extreme (8 per diet and RFI group) for PT measurements using 1) the urinary 3-methyl-histidine to creatinine ratio, as a biomarker of the fractional protein degradation rate (FDR) of skeletal-muscle and 2) the isotopic N turnover rate measured in urine and plasma, as a proxy, respectively, of the whole-body FDR and the fractional protein synthesis rates (FSR) of plasma proteins. The 3-methyl-histidine and creatinine were determined from 10 d total urine collection. Isotopic N turnover in urine and plasma was evaluated by modelling the 15N depletion rate over 112 d following an isotopic N dietary change. Higher plasma FSR and higher skeletal-muscle and whole-body FDR were observed with corn-vs-grass diets (≥11%; P ≤ 0.03), in line with higher metabolizable protein and net energy intakes (≥10%, P = 0.001). Differences between extreme RFI animals were noted with the corn diets only, where efficient animals presented significant lower plasma FSR (-10%; P = 0.04) and numerically lower skeletal-muscle and whole-body FDR (-13% and - 8.9%; P > 0.16 respectively) than non-efficient. Non-significant differences were probably due to an insufficient size of our experimental setup. Plasma FSR is related to the PS of hepatic exportation, hence the lower plasma FSR observed in efficient RFI animals fed corn diets may reflect a lower organs to carcass ratio. Altogether results suggests that efficient RFI bulls fed corn diets had a lower hepatic PT with no-significant changes of whole-body and skeletal muscle PT.

International audience; Improving the feed efficiency of cattle contributes to the environmental and economic challenges facing ruminant production. Within this context, a study with 99 fattening Charolais bulls was conducted over two consecutive years (n=49 the first year and n=50 the second). Each year, bulls were allocated to two groups fed ad libitum, corn silage, or a grass silage diet. Bulls were tested for residual feed intake (RFI) during 84 days, and extreme RFI animals were identified thereafter (n = 32; balanced for RFI group, diet and year). Total rumen contents from the eight most extreme RFI+ and RFI- bulls on each diet were sampled at the abattoir (4 bulls/diet/year, n=32). All 49 animals from the first year were also sampled for rumen fluid by stomach-tubing. Total genomic DNA was extracted and subjected to PCR amplification using universal primers targeting the 16S rRNA gene prior to Ilumina sequencing. Raw reads were analyzed using the QIIME2 pipeline with default parameters, and genus-level tables for rumen fluid and total contents were separated and used to assess alpha- and beta-diversity in R using vegan package. Effects of diet, RFI group and the interaction diet×RFI were analyzed using the Kruskal-Walis test. For rumen fluid, Simpson and Shannon diversity indexes were higher in corn-fed bulls, but this effect was not observed for total contents sampled at the abattoir. For both groups of samples, permanova analysis highlighted the effect of diet and diet×RFI on rumen microbial community structure, while the effect of RFI was non-significant. Permanova analysis within each dietary group did not show major changes in microbial community structure. Research from this study indicates that there are no shifts in the overall prokaryotic community structure related to RFI. Further analysis will explore the impact of individual microbial taxa on animal’s feed efficiency.

Protein turnover (PT), the continual synthesis and degradation of body proteins not leading to protein gain, is an essential high energy-demanding process. We assumed that PT might explain the between-animal variations of residual feed intake (RFI). The objective was to measure PT in extreme RFI cattle fed two contrasted diets (grass or corn-based). We conducted a RFI test for 84 days with 100 Charolais bulls and we selected the 32 most extreme (8 per diet and RFI group) for PT measurements using 1) the urinary 3-methyl-histidine to creatinine ratio, as a biomarker of the fractional protein degradation rate (FDR) of skeletal-muscle and 2) the isotopic N turnover rate measured in urine and plasma, as a proxy, respectively, of the whole-body FDR and the fractional protein synthesis rates (FSR) of plasma proteins. The 3-methyl-histidine and creatinine were determined from 10 d total urine collection. Isotopic N turnover in urine and plasma was evaluated by modelling the 15N depletion rate over 112 d following an isotopic N dietary change. Higher plasma FSR and higher skeletal-muscle and whole-body FDR were observed with corn-vs-grass diets (≥11%; P ≤ 0.03), in line with higher metabolizable protein and net energy intakes (≥10%, P = 0.001). Differences between extreme RFI animals were noted with the corn diets only, where efficient animals presented significant lower plasma FSR (-10%; P = 0.04) and numerically lower skeletal-muscle and whole-body FDR (-13% and - 8.9%; P > 0.16 respectively) than non-efficient. Non-significant differences were probably due to an insufficient size of our experimental setup. Plasma FSR is related to the PS of hepatic exportation, hence the lower plasma FSR observed in efficient RFI animals fed corn diets may reflect a lower organs to carcass ratio. Altogether results suggests that efficient RFI bulls fed corn diets had a lower hepatic PT with no-significant changes of whole-body and skeletal muscle PT.

International audience

International audience; We propose a non-invasive proxy based on milk natural abundance of stable isotopes to assess body mobilization in dairy cow, and test whether this proxy can reflect between-animal variation in feed efficiency.

Diet N use efficiency (NUE) is affected by multiple dietary and animal factors that determine the supply of essential amino acids (EAA) relative to requirements and associated faecal and urinary N losses. NUE in ruminants is low relative to nonruminants, largely due to effects of rumen fermentation, with 75% or more of diet N excreted in manure. Increasing concerns over the impacts of ammonia, nitrates, and N20 highlight the need to develop sustainable approaches for milk and meat production by cattle that minimize losses of diet N to the environment. Variation in NUE between animals is substantial and related to feed efficiency. Recent studies demonstrate the improvements in NUE that can be achieved by feeding diets with lower total protein concentration, as long as variation in diet composition does not lead to long-term undersupply of EAA or an imbalance of energy and protein. In addition, dietary starch can also increase NUE through both rumen and metabolic effects. Historically, NUE and body N balance of individual animals has been measured using digestion trials with total faecal and urine (and milk) collection to determine total intake and excretion of N, with body tissue N retention or loss calculated by difference. Meta-analysis of individual measurements of N balance obtained at multiple locations undertaken in the SmartCow project (H2020 N°730924) highlights the variation attributable to methodologies, including volatile N losses, and the need for standardizedapproaches to reduce experimental error and increase precision and comparability of results. Moreover, measurement of N balance of individual animals is time consuming, laborious, and restricts animal movements. The SmartCow project confirms faecal NIRS and natural 15N abundance of animal proteins as biomarkers of digestion and NUE, respectively, highlighting their potential for replacing digestion trials and time consuming phenotyping.

Animals with superior feed efficiency (FE) may also have an improved nitrogen use efficiency (NUE), which would be beneficial to economic profitability while reducing environmental impacts. When genetically selecting animals on FE, it is preferable to use residual traits [e.g. residual feed intake (RFI) or residual body gain (RG)] rather than ratios because of their predictable genetic outcomes. We studied the relationship of RFI and RG with NUE, estimated from the validated 15N abundance in plasma, across two contrasted diets based on corn or grass silages. We evaluated FE of 588 (half by diet) Charolais bulls (545 ± 57 kg BW) from 12 experimental cohorts (different farms and periods) over 200 days. Before the end of the FE test, plasma was sampled and analyzed for δ 15N. NUE was related to FE through simple-linear models with variables previously corrected for the cohort and diet effects. The models’ slopes were standardized according to FE deviation in order to compare the response of NUE to FE between indices. Higher NUE was related to higher FE (P < 0.001), showing positive correlations with RG (r=-0.40) and negative with RFI (r=0.29). However, the standardized slope of NUE to RG was significantly higher (+28%; P < 0.05) than that of NUE to RFI. This stronger NUE relation to RG compared to RFI could reflect a higher potential of RG animals to deposit N as compared to a more conservational N metabolism in RFI individuals. Regarding diets, and despite the correction of NUE and FE for this effect, the slopes of NUE to FE were numerically (P > 0.05) higher (-16% and +36%; for RG and RFI) in corn-based diets, which agrees with superior NUE observed in corn-vs-grass diets. Results suggested that superior RG animals may present proportionally higher NUE than superior RFI animals, with even better results in corn-vs-grass diets.

Improving feed efficiency is an important issue for beef producers. To study its feasibility, 588 Charolaisyoung bulls were fattened in four experimental stations, from 2015 to 2019, with either corn silage or grasssilage. Feed conversion efficiency (FCE) was higher for animals fattened with corn silage (0,164 kg/kgMS vs 0,146 kg/kg MS). Efficient animals better used diet nitrogen and had a lower natural abundance of15N in plasma. Genetic study showed that feed efficiency is heritable, but some correlations wereunfavorable with performance traits. A genetic selection of beef cattle is possible with a selection indexthat combined both feed efficiency and production.; Sélectionner sur l’efficience alimentaire est un enjeu important pour les éleveurs bovins allaitants. Pourétudier sa faisabilité, 588 jeunes bovins ont été engraissés dans quatre stations expérimentales, de 2015à 2019, avec de l’ensilage de maïs ou d’herbe ad libitum. L’efficience de conversion des aliments (EA)était supérieure chez les animaux engraissés avec l’ensilage de maïs (0,164 kg/kg MS contre0,146 kg/kg MS) lié à des croissances élevées. Les animaux efficients valorisaient mieux l’azotealimentaire et avaient une abondance naturelle en 15N dans le plasma moindre. Les analyses génétiquesont montré que l’efficience alimentaire est héritable, mais certaines corrélations sont non favorables avecles performances. Une amélioration génétique du troupeau allaitant est possible via un index de sélectioncombinant efficience alimentaire et production.

International audience; Genetic selection on feed efficiency could increase the profitability of French beef farms. The BEEFALIM 2020project aims to study the genetic determinism of this trait measured from 2016 to 2019 on 588 young Charolais bullsin four experimental farms. After weaning in commercial farms, the young bulls entered the fattening barns at 303(±26) days of age and were fed ad libitum either with grass or corn silage. During the 29 (±3) weeks of the test period,dry matter intake (DMI) was recorded daily and animals were weighted every 14 days. Average daily gain (ADG)and final body weight (FW) were calculated using regression of body weight on time. Three feed efficiency criteriawere used: residual feed intake (RFI), residual gain (RG) and feed conversion efficiency (FCE). Genetic parameterswere estimated using WOMBAT software with an animal model containing the contemporary group as fixed effect,age at the start of the test as covariate and farm origins × year, genetic and residual as random effects. During thetest period, DMI averaged 9.65 (±1.13) kg/day, ADG 1.51 (±0.29) kg/day and FW 698 (±74) kg. The three traitswere closely correlated phenotypically: 0.67 between FI and ADG, 0.77 between FW and ADG and 0.82 betweenFI and FW. Feed intake and ADG were more heritable (0.34) than FW (0.20). The three feed efficiency traits weremoderately heritable (0.22 for RFI and RG; 0.18 for FCE). No genetic correlation was found between RFI and RGor FCE while a strong genetic correlation of 0.99 was estimated between RG and FCE. Residual feed intake wasgenetically correlated with DMI (0.82), ADG (0.52) and FW (0.49). Both RG and FCE were closely correlated withADG (0.79 and 0.75, respectively) and moderately with DMI (0.31 and 0.23, respectively) or FW (0.33 and 0.25,respectively). Even if RFI and RG were phenotypically independent with traits used in both regressions, geneticrelationships were found between them. Genetic improvement is possible through a selection index combining bothfeed efficiency and production traits.

International audience; Phenotyping animal feed efficiency, the animal’s ability to transform feed into food, is challenging because it is timeand labor consuming and not always feasible in livestock systems based on pasture. Thus, biomarkers should bedeveloped and validated for a high-throughput phenotyping of feed efficiency in practical conditions. The natural 15Nenrichment of animal proteins over the consumed diet (Δ15N = δ15Nanimal- δ15Ndiet) has recently been proposed as abiomarker of animal feed efficiency. This study aimed to confirm by meta-analysis the potential of Δ15N to capturethe between-animal variation in feed conversion efficiency (FCE; body weight gain/ dry matter intake) in young beefcattle reared in different European conditions. For this, individual data of Δ15N measured in plasma and FCE of 468growing-fattening bulls of different pure and cross continental breeds, from 25 different diets, 8 experiments and 3countries (France, UK and Switzerland) were evaluated by regression analysis. All animals were tested for at least 60days for FCE and their blood (7 experiments) or muscle tissue (1 experiment) sampled at the end of the test period.Diets were sampled throughout the feed efficiency test. Diets and animal proteins (plasma or muscle) were analysedfor their natural 15N abundance (δ15N) and the Δ15N was calculated for each animal. Two models were used to assessthe relationship between Δ15N and FCE at the individual level. First, a mixed regression model of FCE on Δ15N withthe experiment and diet as random effects, allowing these two effects to be excluded from the explored relationship.For the second model, residuals were first obtained for FCE and Δ15N after correcting for the experiment and dieteffects and then regressed on each other by simple linear regression. For the first approach, the mixed model confirmedthat Δ15N is significantly (P

International audience; Ruminant feed utilisation efficiency is determined in part by organic matter digestibility (OMD). However, OMDis expensive and time consuming to measure it in routine. Faecal near infrared (faecal-NIR) spectroscopy has beendeveloped for predicting OMD in cattle across, but its potential for estimating the between-animal variability ofOMD has not been reported. The objective of this study was to evaluate the use of of faecal-NIR for assessing thebetween-animal variability of OMD across different dietary and experimental conditions in cattle. Faecal samplesfrom individual beef and dairy cows associated (n=454) to individual measured OMD values were used. Valuesranged from 55.1 and 85.0% (mean±SD; 70.6±4.20%) for 9 experiments at 4 EU sites using 50 different diets. ANIR calibration model for predicting OMD was obtained and validated by cross-validation. Relationship betweencross-validated OMD predictions and measured OMD values were analysed by regression analysis with faecal-NIRcross-validated predictions as fixed factor and diet within experiment as random factor. The faecal-NIR predictionwas positive and correlated (P

International audience; Beef cattle industry could benefit from the identification of individuals with superior feed efficiency. Thus, biomarkersshould be developed and validated to contribute to this objective. This study aimed to explore plasma biomarkersof residual feed intake (RFI) in fattening beef cattle through a targeted metabolomics approach. For this, 167 youngCharolais bulls were tested for RFI in 3 independent cohorts during at least 6 months. Animals within each cohortwere evenly assigned to either a high-starch corn silage diet or a high-fibre grass silage diet. Blood was sampled fromeach animal one month before the end of the test. Plasma from the 48 most extreme animals in terms of RFI (24 lowvs 24 high RFI; balanced within cohorts and diets) were analysed for 630 metabolites by FIA- or LC- tandem massspectrometry. Metabolites were analysed by ANOVA separately for each diet including the fixed effect of cohort, RFIgroup and their interaction. In the corn silage diet, plasma concentration of the 3 branched-chain amino acids (BCAA;Leu, Ile and Val) was lower (-15%; P≤0.01) in low-compared to high-RFI, likely reflecting their higher metabolicuse for protein synthesis and muscle accretion. In contrast, creatinine and hydroxyl-proline, markers of muscle andcollagen body mass respectively, showed higher plasma concentration (+18-20%; P≤0.01) in low- compared tohigh-RFI animals. In the grass silage diet, most of the RFI discriminant metabolites belonged to different classes oftriglycerides and their plasma concentrations were always higher (+50% on average; P=0.003) in low- comparedto high RFI animals, indicating either their lower incorporation into fat cells or their lower use as energy substrateby the muscle. Only creatinine was a common RFI discriminant metabolite in the two diets and Val, though alsodiscriminant, showed opposite trends. Our results suggest that discriminant metabolites of RFI are not the same acrosstwo contrasting diets. However, differences in muscle body mass revealed by plasma creatinine concentration seemsto be a determinant of RFI regardless the diet.

International audience; Nitrogen (N) isotopic discrimination (i.e. the difference in natural N-15 abundance between the animal proteins and the diet; Delta N-15) is known to correlate with N use efficiency (NUE) and feed conversion efficiency (FCE) in ruminants. However, results from the literature are not always consistent across studies, likely due to isotopic discrimination pathways that may differ with the nature of diets. The objective of the present study was to assess at which level, from rumen to tissues, Delta N-15 originates and becomes related to NUE and FCE in fattening yearling bulls when they are fed two contrasted diets. Twenty-four Charolais yearling bulls were randomly divided into two groups and fed during 8 months, from weaning to slaughter, either 1) a high starch diet based on corn silage supplying a balanced N to energy ratio at the rumen level (starch) or 2) a high fiber diet based on grass silage supplying an excess of rumen degradable N (fiber). All animals were slaughtered and samples of different digestive pools (ruminal, duodenal, ileal and fecal contents), animal tissues (duodenum, liver and muscle), blood and urine were collected for each animal. Ruminal content was further used to isolate liquid-associated bacteria (LAB), protozoa and free ammonia, while plasma proteins were obtained from blood. All samples along with feed were analyzed for their N isotopic composition. For both diets, the digestive contribution (i.e. the N isotopic discrimination occurring before absorption) to the Delta N-15 observed in animal tissues accounted for 65 +/- 11%, leaving only one third to the contribution of post-absorptive metabolism. Concerning the Delta N-15 in digestive pools, the majority of these changes occurred in the rumen (av. Delta N-15 = 2.12 +/- 0.66 parts per thousand), with only minor N-15 enrichments thereafter (av. Delta N-15 = 2.24 +/- 0.41 parts per thousand), highlighting the key role of the rumen on N isotopic discrimination. A strong, significant overall relationship (n = 24) between Delta N-15 and FCE or NUE was found when using any post-absorptive metabolic pool (duodenum, liver, or muscle tissues, or plasma proteins; 0.52 < r < 0.73; P

Feed efficiency is an increasingly important driver for livestock systems, as it contributes not only to farm economy but also to sustainability. Feed efficiency can be studied at different levels, in this review we focus on the animal level and on variation between individuals. The review starts by considering the different definitions of efficiency with two broad classes of definition, those based on ratios of products to intake and those based on identifying residuals relative to average performance. Among the latter, the residual feed intake method is the most widely used and is accordingly described. Subsequent sections deal with the underlying biological mechanisms. With respect to digestive efficiency, retention time in the rumen and the microbial population are key. With respect to metabolic efficiency, the mechanisms involved in heat production, mitochondrial respiration, protein turnover, etc., as well as the influence of body reserves, are highlighted. The challenges and opportunities for generating sufficient phenotypic records to allow genetic selection for feed efficiency are discussed in the context of the opportunities provided by the advent of on-farm technologies. Finally, the links between efficiency and resilience are explored, as there are differences when comparing short- and long-term evaluations, with the latter being especially important for sustainability., ’efficience alimentaire est un caractère de première importance pour l’élevage, tant du point de vue économique que comme élément de sa durabilité. Si l’efficience alimentaire peut s’étudier à différentes échelles, le choix est fait de situer cette synthèse au niveau de l’animal et des variations individuelles. Dans un premier temps, les différentes définitions de l’efficience et des critères utilisés pour l’estimer sont abordées, en comparant les possibilités offertes par les critères de type ratio et ceux de type résidu. L’ingéré résiduel (ou Residual Feed Intake : RFI), critère le plus plébiscité aujourd’hui dans les études sur les ruminants, est présenté. Dans un deuxième temps, les mécanismes biologiques impliqués dans l’efficience alimentaire sont passés en revue, ainsi que les moyens de les étudier. On retrouve tout d’abord ce qui a trait à l’efficience digestive, avec notamment l’impact du temps de séjour ou de la population microbienne. Puis, les déterminants de l’efficience métabolique tels que la production de chaleur, le taux de renouvellement protéique, la respiration mitochondriale, ou encore l’influence de la composition corporelle sont également détaillés. Dans un troisième temps, nous évoquons les défis à relever afin de mettre en place une potentielle sélection sur le caractère, la difficulté majeure résidant dans le phénotypage d’un nombre suffisant d’individus. Enfin, les liens entre efficience et résilience sont explorés, afin de prendre en compte la performance de l’animal sur un pas de temps plus long et d’inclure l’importance de la durabilité.

Improving the ability of animals to convert feed resources into food for humans is needed for more sustainable livestock systems. Genetic selection for animals eating less while maintaining their performance (i.e. low residual feed intake) appears a smart strategy but its effectiveness relies on high-throughput animal phenotyping. Here, we explored plasma nitrogen (N) isotope ratios in an attempt to identify easily superior young bulls in terms of residual feed intake (RFI). For this, 48 Charolais young bulls fed two contrasting diets (corn vs grass silage diets) were selected from a larger population as extreme RFI animals (24 low-RFI vs 24 high-RFI) and their plasma analyzed for natural 15N abundance (δ15N) in the whole protein (bulk protein) and in the individual protein-bound amino acids (PbAA). For the first time, we showed that the δ15N in plasma bulk protein differed (P = 0.007) between efficient (Low-RFI) and inefficient (High-RFI) cattle regardless of diet. Furthermore, most analyzed PbAA followed the same trend as the bulk protein, with lower (P < 0.05) δ15N values in more efficient (Low-RFI) compared with less efficient (High-RFI) cattle, again regardless of diet. The only three exceptions were Phe, Met and Lys (P > 0.05) for which the first metabolic reaction before being catabolized does not involve transamination, a pathway known naturally to enrich AAs in 15N. The contrasted isotopic signatures across RFI groups only in those PbAA undergoing transamination are interpreted as differences in transamination rates and N-use efficiency between Low- and High- RFI phenotypes. Natural isotopic N signatures in bulk proteins and specific PbAA can be proposed as biomarkers of RFI in growing beef cattle fed different diets. However, the current study cannot delineate whether this effect only occurs post-absorption or to some extent also in the rumen. Our data support the conclusion that most efficient cattle in terms of RFI upregulate N conservation mechanisms compared with less efficient cattle, and justify future research on this topic.

Additional file 5: Fig. S2. Network interaction between GO terms, KEGG pathways and genes from the AWM analysis with RG as key phenotype. Fig. S3. Network interaction between GO terms, KEGG pathways and genes from the AWM analysis with FE as key phenotype.

Additional file 1. List of breeds included in the reference population panel for whole-genome imputation.

Additional file 2: Fig. S1. Manhattan plots of residual feed intake (a), residual gain (b) and feed efficiency ratio (c).

Residual feed intake (RFI) is an alternative measure of feed efficiency (FE) and is calculated as the difference between actual and expected feed intake. The biological mechanisms underlying animal-to-animal variation in FE are not well understood. The aim of this study was to investigate the digestive ability of beef cows selected for RFI divergence as heifers, using two contrasted diets. Fifteen 4-year-old beef cows were selected from a total of 69 heifers based on their RFI following the feedlot test. The selected heifers were ranked into high-RFI (þ 1.02 ± 0.28, n = 8) and low-RFI (−0.73 ± 0.28, n = 7), and a digestibility trial was performed after their first lactation. Both RFI groups were offered two different diets: 100% hay or a fattening diet which consisted of a DM basis of 67% whole-plant maize silage and 33% high starch concentrates over four experimental periods (two per diet). A diet effect was observed on feed intake and apparent digestibility, whereas no diet × RFI interaction was detected (P > 0.05). Intake and apparent digestibility were higher in cows fed the fattening diet than in those fed the hay diet (P < 0.0001). DM intake (DMI) and organic matter apparent digestibility (OMd) were repeatable and positively correlated between the two subsequent periods of measurements. For the hay and fattening diets, the repeatability between periods was r = 0.71 and r = 0.73 for DMI and r = 0.87 and r = 0.48 for OMd, respectively. Moreover, both intake (r = 0.55) and OMd (r = 0.54) were positively correlated (P < 0.05) between the hay and fattening diets. Significant differences between beef cows selected for divergence in RFI as heifers were observed for digestive traits (P < 0.05), DM and organic matter (OM) apparent digestibility being higher for low-RFI cows. Overall, this study showed that apparent digestibility contributes to between-animal variation in FE in beef cows.

Cheap and rapid methods to predict between-animal variability of feed efficiency (FE) are needed. We evaluated, by using a large dataset on beef cattle (n = 355) fed contrasting diets (grass vs corn based diets), the potential of plasma natural 15N abundance (δ15N) as a biomarker of between-animal variation in FE, measured either as feed conversion efficiency (FCE) or residual feed intake (RFI). Data were analyzed by mixed models, with diet and δ15N as a fixed effect and experimental farm, batch and pen as random effects. A negative and significant correlation was found between FCE and δ15N within each contemporary group, with a lower response (slope) however in grass vs corn diets (δ15N × diet interaction; P = 0.002). Likewise, we found a significant but positive relationship between δ15N and RFI, but without the influence of diet or any tested random effect. Our results indicate that δ15N is a promising biomarker of between-animal variation of feed efficiency (FCE and RFI) in growing beef cattle.

Knowledge on the flux of nitrogen (N) excreted through urines is essential in animal nutritionsince it may provide information on animal N metabolism and the extent of environmental Npollution. Both the 15N natural enrichment of animal proteins over the diet (Δ15Nanimal-diet) and themilk urea N (MUN) concentration have been previously linked to N partitioning. The objective ofthis meta-analysis was to test the effect of combination of Δ15Nanimal-diet in plasma and MUN forpredicting the urinary N excretion (UNE) in dairy cows. Individual data on Δ15Nanimal-diet in plasma,MUN, and UNE of 177 cow/period observations were evaluated by meta-analysis. Data wereanalyzed by mixed-model regression analysis. When model was fitted with either Δ15Nanimal-diet orMUN, both biomarkers were capable to independently predict UNE. However, when Δ15Nanimal-dietand MUN were combined in one model, both biomarkers remained significant and the error ofthe model decreased. In conclusion, both Δ15Nanimal-diet and MUN can predict UNE and theircombination can strengthen the prediction capacity of the model.

Session 45 - Théâtre 14; Diet digestibility and intake are animal traits difficult to measure in practice. Thus, several indirect methods including faecal visible/near infrared (VIS/NIR) spectroscopy, have been developed for predicting these traits. The development of this technology requires great number of samples, which are not always obtained in standardized conditions among experiments. Consequently, the lack of standardization could be associated to a loss of precision of VIS/NIR models. In order to test this hypothesis, we used 88 VIS/NIR spectra of faeces sampled on cattle beef and dairy cows from 3 different experiments (De la Torre et al., 2019; Herremans et al., 2019; Fanchone et al., 2013). Experiments included measurements of the individual organic matter digestibility (OMD) and daily dry matter intake (DI) of animals. Two calibration strategies were compared for predicting these measurements: (1) specific experiment calibration and (2) a global calibration procedure. Models were assessed by standard error of cross-validation (SECV) and coefficient of determination of cross-validation (R2CV). The SECV was separated into bias and SECV corrected by bias SECV(C). Bias and SECV(C) for both calibration strategies were compared using the procedure of Fearn (1996). Data sets gave value ranges of 0.60-0.78 for OMD and 53-181 g/kg body weight (BW)0.75 for DI. For OMD, the SECV and R2CV were 0.02 and 0.86 for both specific experiment calibration and global calibration procedure. For DI, the SECV and R2CV were 6.90 g/kg BW0.75 and 0.96 respectively, for the specific experiment calibration and 7.62 g/kg BW0.75 and 0.95, respectively, for the global calibration procedure. For both, OMD and DI, no significant differences between calibration strategies were found for bias and SECV(C). From the results of this preliminary study, we conclude that mixing data from experiments conducted in very different conditions does not degrade the precision of the obtained VIS/NIRS models to predict OMD and DI in cattle.

National audience

National audience

National audience