Some plasma biomarkers of residual feed intake in beef cattle remain consistent regardless of intake level.

This study investigated whether plasma biomarkers of residual feed intake (RFI), identified under ad libitum feeding conditions in beef cattle, remained consistent during feed restriction. Sixty Charolais crossbred young bulls were divided into two groups for a crossover study. Group A was initially fed ad libitum (first test) and then restricted (second test) on the same diet, while Group B experienced the opposite sequence. Blood samples were collected from the 12 most divergent RFI animals in each group at the end of the first test and again after the second test. 12 plasma variables consistently increased, while three consistently decreased during feed restriction (FDR < 0.05). Only two metabolites, α-aminoadipic acid for Group A and 5-aminovaleric acid for Group B, were associated with RFI independent of feed intake level (FDR < 0.05), demonstrating moderate-to-high repeatability across feeding levels (intraclass correlation coefficient ≥ 0.59). Notably, both metabolites belong to the same metabolic pathway: lysine degradation. These metabolites consistently correlated with RFI, irrespective of fluctuations in feed intake, indicating a connection to individual metabolic processes influencing feed efficiency. These findings suggest that a portion of RFI phenotypic variance is inherent to an individual's metabolic efficiency beyond variations in feed intake.

Relationship between residual feed intake and digestive traits of fattening bulls fed grass silage- or maize silage-based diets.

Several studies tried to identify digestive determinants of individual variation in feed efficiency between fattening bulls, because of their importance for breeding and management strategies. Most studies focused on single traits or single diet. Little is known about diet-dependent differences in digestive determinants and on their relative importance in distinguishing divergent residual feed intake (RFI) bulls. This research aimed (i) to identify digestive traits that differed between bulls diverging in RFI and fed a maize silage- or a grass silage-based diets; (ii) to highlight the relationships between RFI and digestive traits, and (iii) to explore the hierarchy among digestive traits in discriminating RFI divergent bulls. After an initial RFI test of 84 days on 100 Charolais growing bulls fed two different diets based on grass silage (GS), or maize silage (MS), the 32 most RFI divergent bulls were selected (eight efficient RFI- and eight inefficient RFI+ bulls per diet) and measured thereafter for total tract apparent digestibility and transit rate, enteric gas emissions (CH4 and H2), rumen pH, and feeding behaviour. Rumen particle size and visceral organ and reticulo-omasal orifice (ROO) sizes and rumen and ileum histology were measured at slaughter on the 32 selected extreme RFI bulls. Irrespective of the diet, efficient bulls (RFI-) had lower rumen size, CH4 yield (g/kg DM intake; tendency), lower number of cells in the ileal crypts, tended to have longer time of rumen pH below 5.8 and lower proportion of small size particles in rumen content than non-efficient bulls (RFI+). A long-term test for feed efficiency (197 d on average) was performed on the whole experimental period until slaughter for the 100 animals. The long-term RFI value was negatively related to time spent in activity other than ingestion, rumination, and resting, and positively related (tendency) to the duration of ingestion events, to rumen and abomasum size, irrespective of the diet. Diet-dependent effects were noted: with GS, efficient (RFI-) bulls showed a slower transit rate, whereas with MS, efficient (RFI-) bulls tended to have shorter resting events and a smaller ROO than inefficient bulls (RFI+). The transit rate and the ROO size tended to be positively related, while total tract apparent digestibility of nitrogen was negatively related to long-term RFI value, but only in GS. Rumen size appeared as the most discriminating digestive variable between RFI divergent bulls, but this result should be validated on a larger number of animals and diets.

Comparative analysis of signalling pathways in tissue protein metabolism in efficient and non-efficient beef cattle: acute response to an identical single meal size.

Protein turnover has been associated to residual feed intake (RFI) in beef cattle. However, this relationship may be confounded by feeding level and affected by the composition of the diet being fed. Our aim was to assess postmortem the protein metabolism signalling pathways in skeletal muscle and liver of 32 Charolais young bulls with extreme RFI phenotypes. Bulls were fed two contrasting diets during the whole fattening period but were subjected to a similar and single nutritional stimulus, induced by their respective concentrate, just prior to slaughter. The key targets were protein degradation (autophagy and ubiquitin) and synthesis signalling pathways through western-blot analysis, as well as hepatic transaminase activity. To ensure a precise assessment of all animals at the same postprandial time, they were provided with a test meal (2.5 kg of either a high-starch and high-protein concentrate or high-fibre and low-protein concentrate) 3 hours prior to slaughter, irrespective of their RFI grouping. Blood and tissues were sampled at the slaughterhouse (3 h and 3 h30 postprandially, respectively). In response to an identical single meal size, efficient RFI animals showed higher (P < 0.05) postprandial plasma ß-hydroxybutyrate concentrations and insulinemia (only with the high-starch concentrate) than non-efficient animals. Moreover, efficient RFI bulls had lower muscle (P = 0.04) and liver (P = 0.08) ubiquitin protein abundance (degradation pathway) and tended to have lower alanine transaminase activity in the liver (P = 0.06) compared to non-efficient bulls, regardless of diet. A positive correlation between protein degradation potential and amino acid catabolism was identified in this study (r = 0.52, P = 0.004), which was interpreted as being biologically linked to the RFI phenotype. Efficient RFI bulls also had a faster potential for protein synthesis in the muscle, as indicated by their greater ratio of phosphorylated to total form of ribosomal protein S6 kinase (P = 0.05), regardless of diet. Results on protein synthesis pathway in muscle and plasma metabolite concentrations suggested that efficient RFI cattle may have a faster nutrient absorption and insulin responsiveness after feeding than inefficient cattle. We did not find significant differences in hepatic protein synthesis pathways between the two RFI groups (P > 0.05). Our findings suggest that, in response to an identical single meal size, efficient RFI animals exhibited lower activation of tissue protein degradation pathways and faster muscle protein synthesis activation compared to their inefficient counterparts. This pattern was observed regardless of the composition of the tested meals.

Extrusion of lupines with or without addition of reducing sugars: Effects on the formation of Maillard reaction compounds, partition of nitrogen and Nε-carboxymethyl-lysine, and performance of dairy cows.

The extrusion of leguminous seeds induces the formation of Maillard reaction compounds (MRC) as a product of protein advanced glycation and oxidation, which lowers protein degradability in the rumen. However, the quantitative relationship between the parameters of pretreatment (i.e., addition of reducing sugars) and extrusion, and the formation of MRC has not been established yet. Moreover, the fate of the main stable MRC, Nε-carboxymethyl-lysine (CML), in the excretory routes has never been investigated in ruminants. We aimed to test the effects of the temperature of extrusion of white lupines with or without addition of reducing sugars on the formation of MRC, crude protein (CP) degradability in the rumen, N use efficiency for milk production (milk N/N intake), and performance of dairy cows. Two experiments with a replicated 4 × 4 Latin square design were conducted simultaneously with 16 (3 rumen-cannulated) multiparous Holstein cows to measure indicators of ruminal CP degradability (ruminal NH3 concentration, branched-chain volatile fatty acids), metabolizable protein supply (plasma essential AA concentration), N use efficiency (N isotopic discrimination), and dairy performance. In parallel, apparent total-tract digestibility of dry matter, organic matter, neutral detergent fibers, N, total Lys and CML, and partition of N and CML were measured with 4 cows in both experiments. The diets consisted on a DM basis of 20% raw or extruded lupines and 80% basal mixed ration of corn silage, silage and hay from permanent grasslands, pelleted concentrate, and a vitaminized mineral mix. Expected output temperatures of lupine extrusion were 115°C, 135°C, and 150°C, without and with the addition of reducing sugars before extrusion. The extrusion numerically reduced the in vitro ruminal CP degradability of the lupines, and consequently increased the predicted supply of CP to the small intestine. Nitrogen balance and urinary N excretion did not differ among dietary treatments in either experiment. Milk yield and N use efficiency for milk production increased with extrusion of lupines at 150°C without addition of reducing sugars compared with raw lupines. Nitrogen isotopic discrimination between dietary and animal proteins (the difference between δ15N in plasma and δ15N in the diet) were lower with lupines extruded at 150°C without and with addition of reducing sugars. Regardless of sugar addition, milk true protein yield was not affected, but milk urea concentration and fat:protein ratio were lower with lupines extruded at 150°C than with raw lupines. In the CML partition study, we observed that on average 26% of the apparently digested CML was excreted in urine, and a much lower proportion (0.63% on average) of the apparently digested CML was secreted in milk, with no differences among dietary treatments. In conclusion, we showed that the extrusion of white lupines without or with addition of reducing sugars numerically reduced enzymatic CP degradability, with limited effects on N partition, but increased milk yield and N use efficiency at the highest temperature of extrusion without addition of reducing sugars.

Review: Reducing enteric methane emissions improves energy metabolism in livestock: is the tenet right?

The production of enteric methane in the gastrointestinal tract of livestock is considered as an energy loss in the equations for estimating energy metabolism in feeding systems. Therefore, the spared energy resulting from specific inhibition of methane emissions should be re-equilibrated with other factors of the equation. And, it is commonly assumed that net energy from feeds increases, thus benefitting production functions, particularly in ruminants due to the important production of methane in the rumen. Notwithstanding, we confirm in this work that inhibition of emissions in ruminants does not transpose into consistent improvements in production. Theoretical calculations of energy flows using experimental data show that the expected improvement in net energy for production is small and difficult to detect under the prevailing, moderate inhibition of methane production (≈25%) obtained using feed additives inhibiting methanogenesis. Importantly, the calculation of energy partitioning using canonical models might not be adequate when methanogenesis is inhibited. There is a lack of information on various parameters that play a role in energy partitioning and that may be affected under provoked abatement of methane. The formula used to calculate heat production based on respiratory exchanges should be validated when methanogenesis is inhibited. Also, a better understanding is needed of the effects of inhibition on fermentation products, fermentation heat, and microbial biomass. Inhibition induces the accumulation of H2, the main substrate used to produce methane, that has no energetic value for the host, and it is not extensively used by the majority of rumen microbes. Currently, the fate of this excess of H2 and its consequences on the microbiota and the host are not well known. All this additional information will provide a better account of energy transactions in ruminants when enteric methanogenesis is inhibited. Based on the available information, it is concluded that the claim that enteric methane inhibition will translate into more feed-efficient animals is not warranted.

Individual methane emissions (and other gas flows) are repeatable and their relationships with feed efficiency are similar across two contrasting diets in growing bulls.

In the current economic and environmental context, the selection of livestock phenotypes combining high feed efficiency (FE) and low greenhouse gas emissions is interesting. This study aimed to quantify methane (CH4) emissions and other gas flows (carbon dioxide (CO2) and dihydrogen (H2) emissions, oxygen (O2) consumption) in growing bulls fed with two contrasting diets in order to (i) evaluate the persistence of individual variability in gas flows through time, and (ii) assess the inter-individual relationship between gas flows and FE across diets. Charolais bulls were fattened for 6 months during two consecutive years in two independent batches (50-51 per year). In each batch, half of the animals received a total mixed ad libitum ration either based on maize silage (62% dietary DM) or high-starch concentrate (MS-S), and half based on grass silage (59% dietary DM) and high-fibre concentrate (GS-F). The absolute gas flows (g/d) were individually measured with 2 GreenFeed systems during 88 days (group 1) and 64 days (group 2). All gas flows were also expressed in g/kg DM intake (gas yield), in g/kg average daily gain (CH4 intensity) and residual of daily emissions for CH4 (R CH4). Different FE metrics (residual feed intake (RFI), residual gain (RG) and feed conversion efficiency (FCE)) were investigated during the same period. The relationships between gas flows and FE metrics were tested by linear regression with the diet as fixed effect. For both diets, we observed a consistent individual variability over the measurement period for absolutes values (g/d) of CH4, CO2, and O2 (repeatability >0.7 for GS-F and >0.6 for MS-S). Gas flows (g/d) were positively correlated with RFI with both diets: animals that ingested food in excess of their theoretical maintenance and growth requirements emitted more CH4, CO2 and consumed more O2. The positive relationship between absolute CH4 emissions and RFI highlighted the interest for low-CH4 emitters and efficient growing bulls when fed with high-energy diets rich in starch or fibre. For both diets, RCH4, CH4 yield and CH4 intensity were not related to RFI whereas a significant negative relationship was reported between CH4 intensity and RG, and FCE. These data suggest that intake is the main driver of the phenotypic relationships between CH4 traits and RFI. Further studies including larger numbers of animals on highly contrasting energy diets are needed to investigate the underlying biological regulatory mechanisms of the methanogenic potential of an animal in relation to production traits.

Nitrogen isotopic discrimination as a biomarker of between-cow variation in the efficiency of nitrogen utilization for milk production: A meta-analysis.

Estimating the efficiency of N utilization for milk production (MNE) of individual cows at a large scale is difficult, particularly because of the cost of measuring feed intake. Nitrogen isotopic discrimination (Δ15N) between the animal (milk, plasma, or tissues) and its diet has been proposed as a biomarker of the efficiency of N utilization in a range of production systems and ruminant species. The aim of this study was to assess the ability of Δ15N to predict the between-animal variability in MNE in dairy cows using an extensive database. For this, 20 independent experiments conducted as either changeover (n = 14) or continuous (n = 6) trials were available and comprised an initial data set of 1,300 observations. Between-animal variability was defined as the variation observed among cows sharing the same contemporary group (CG; individuals from the same experimental site, sampling period, and dietary treatment). Milk N efficiency was calculated as the ratio between mean milk N (grams of N in milk per day) and mean N intake (grams of N intake per day) obtained from each sampling period, which lasted 9.0 ± 9.9 d (mean ± SD). Samples of milk (n = 604) or plasma (n = 696) and feeds (74 dietary treatments) were analyzed for natural 15N abundance (δ15N), and then the N isotopic discrimination between the animal and the dietary treatment was calculated (Δ15n = δ15Nanimal - δ15Ndiet). Data were analyzed through mixed-effect regression models considering the experiment, sampling period, and dietary treatment as random effects. In addition, repeatability estimates were calculated for each experiment to test the hypothesis of improved predictions when MNE and Δ15N measurements errors were lower. The considerable protein mobilization in early lactation artificially increased both MNE and Δ15N, leading to a positive rather than negative relationship, and this limited the implementation of this biomarker in early lactating cows. When the experimental errors of Δ15N and MNE decreased in a particular experiment (i.e., higher repeatability values), we observed a greater ability of Δ15N to predict MNE at the individual level. The predominant negative and significant correlation between Δ15N and MNE in mid- and late lactation demonstrated that on average Δ15N reflects MNE variations both across dietary treatments and between animals. The root mean squared prediction error as a percentage of average observed value was 6.8%, indicating that the model only allowed differentiation between 2 cows in terms of MNE within a CG if they differed by at least 0.112 g/g of MNE (95% confidence level), and this could represent a limitation in predicting MNE at the individual level. However, the one-way ANOVA performed to test the ability of Δ15N to differentiate within-CG the top 25% from the lowest 25% individuals in terms of MNE was significant, indicating that it is possible to distinguish extreme animals in terms of MNE from their N isotopic signature, which could be useful to group animals for precision feeding.

Spot urine collection: A valid alternative to total urine collection for metabolomic studies in dairy cattle.

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 hydrophilic-interaction 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 metabolome than samples from total collection. When considering nitrogen levels in the diet, the robustness and precision for discriminating high- and low-nitrogen diets was 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.

The effect of sheep genetic merit and feed allowance on nitrogen partitioning and isotopic discrimination.

Animal nitrogen (N) partitioning is a key parameter for profitability and sustainability of ruminant production systems, which may be predicted from N isotopic discrimination or fractionation (Δ15N). Both animal genetics and feeding level may interact and impact on N partitioning. Therefore, this study aimed to assess the interactive effects of genetic merit (G) and feed allowance (F) on N partitioning and Δ15N in sheep. The sheep were drawn from two levels of G (high G vs. low G; based on New Zealand Sheep Improvement Limited (http://www.sil.co.nz/) dual (wool and meat) growth index) and allocated to two levels of F (1.7 (high F) vs. 1.1 (low F) times Metabolisable Energy requirement for maintenance) treatments. Twenty-four Coopworth rams were divided into four equal groups for a N balance study: high G × high F, high G × low F, low G × high F, and low G × low F. The main factors (G and F) and the interaction term were used for 2-way ANOVA and regression analysis. Higher F led to higher N excretions (urinary N (UN); faecal N (FN); manure N), retained N, N use efficiency (NUE), and urinary purine derivatives excretion (P < 0.05). On the other hand, higher UN/N intake, and plasma Δ15N were observed with the lower F (P < 0.05). Higher G led to increased UN, FN, manure N, apparent N digestibility, and urinary purine derivatives excretion (P < 0.05). Higher F only increased UN in high G sheep, with no effect on low G sheep (P < 0.05). Regression analysis results demonstrated potential to use plasma Δ15N to reflect the effects of G and F on NUE and UN/N intake. Further research is urged to study interactive effects of genetic and feeding level on sheep N partitioning.

Review: Markers and proxies to monitor ruminal function and feed efficiency in young ruminants.

Developing the rumen's capacity to utilise recalcitrant and low-value feed resources is important for ruminant production systems. Early-life nutrition and management practices have been shown to influence development of the rumen in young animals with long-term consequences on their performance. Therefore, there has been increasing interest to understand ruminal development and function in young ruminants to improve feed efficiency, health, welfare, and performance of both young and adult ruminants. However, due to the small size, rapid morphological changes and low initial microbial populations of the rumen, it is difficult to study ruminal function in young ruminants without major invasive approaches or slaughter studies. In this review, we discuss the usefulness of a range of proxies and markers to monitor ruminal function and nitrogen use efficiency (a major part of feed efficiency) in young ruminants. Breath sulphide and methane emissions showed the greatest potential as simple markers of a developing microbiota in young ruminants. However, there is only limited evidence for robust indicators of feed efficiency at this stage. The use of nitrogen isotopic discrimination based on plasma samples appeared to be the most promising proxy for feed efficiency in young ruminants. More research is needed to explore and refine potential proxies and markers to indicate ruminal function and feed efficiency in young ruminants, particularly for neonatal ruminants.

Plasma proteins δ15N vs plasma urea as candidate biomarkers of between-animal variations of feed efficiency in beef cattle: Phenotypic and genetic evaluation.

Identifying animals that are superior in terms of feed efficiency may improve the profitability and sustainability of the beef cattle sector. However, measuring feed efficiency is costly and time-consuming. Biomarkers should thus be explored and validated to predict between-animal variation of feed efficiency for both genetic selection and precision feeding. In this work, we aimed to assess and validate two previously identified biomarkers of nitrogen (N) use efficiency in ruminants, plasma urea concentrations and the 15N natural abundance in plasma proteins (plasma δ15N), to predict the between-animal variation in feed efficiency when animals were fed two contrasted diets (high-starch vs high-fibre diets). We used an experimental network design with a total of 588 young bulls tested for feed efficiency through two different traits (feed conversion efficiency [FCE] and residual feed intake [RFI]) during at least 6 months in 12 cohorts (farm × period combination). Animals reared in the same cohort, receiving the same diet and housed in the same pen, were considered as a contemporary group (CG). To analyse between-animal variations and explore relationships between biomarkers and feed efficiency, two statistical approaches, based either on mixed-effect models or regressions from residuals, were conducted to remove the between-CG variability. Between-animal variation of plasma δ15N was significantly correlated with feed efficiency measured through the two criteria traits and regardless of the statistical approach. Conversely, plasma urea was not correlated to FCE and showed only a weak, although significant, correlation with RFI. The response of plasma δ15N to FCE variations was higher when animals were fed a high-starch compared to a high-fibre diet. In addition, we identified two dietary factors, the metabolisable protein to net energy ratio and the rumen protein balance that influenced the relation between plasma δ15N and FCE variations. Concerning the genetic evaluation, and despite the moderate heritability of the two biomarkers (0.28), the size of our experimental setup was insufficient to detect significant genetic correlations between feed efficiency and the biomarkers. However, we validated the potential of plasma δ15N to phenotypically discriminate two animals reared in identical conditions in terms of feed efficiency as long as they differ by at least 0.049 g/g for FCE and 1.67 kg/d for RFI. Altogether, the study showed phenotypic, but non-genetic, relationships between plasma proteins δ15N and feed efficiency that varied according to the efficiency index and the diet utilised.

Methionine-balanced diets improve cattle performance in fattening young bulls fed high-forage diets through changes in nitrogen metabolism.

Ruminants fed high-forage diets usually have a low feed efficiency, and their performances might be limited by methionine (Met) supply. However, the INRA feeding system for growing cattle does not give recommendation for this amino acid (AA). This study aimed to assess the effects of Met-balanced diets on animal performance and N metabolism in young bulls fed high-forage diets formulated at or above protein requirements. Four diets resulting from a factorial arrangement of two protein levels (Normal (13·5 % crude protein) v. High (16·2 % crude protein)) crossed with two Met concentrations (unbalanced (2·0 % of metabolisable protein) v. balanced (2·6 % of metabolisable protein)) were tested on thirty-four fattening Charolais bulls for 7 months before slaughter. Animal growth rate was greater in Met-balanced diets (+8 %; P = 0·02) with a trend for a greater impact in High v. Normal protein diets (P = 0·10). This trend was observed in lower plasma concentrations of branched-chain AA only when Met supplementation was applied to the Normal protein diet (P ≤ 0·06) suggesting another co-limiting AA at Normal protein level. Feed conversion efficiency and N use efficiency were unaffected by Met supplementation (P > 0·05). However, some plasma indicators suggested a better use of AA when High protein diets were balanced v. unbalanced in Met. The proportion of total adipose tissue in carcass increased (+5 percent units; P = 0·03), whereas that of muscle decreased on average 0·8 percent units (P = 0·05) in Met-balanced diets. Our results justify the integration of AA into dietary recommendations for growing cattle.

Effects of hydrolysable tannin-treated grass silage on milk yield and composition, nitrogen partitioning and nitrogen isotopic discrimination in lactating dairy cows.

The objective of this study was to evaluate the effects of oak tannin extract (OTE) added in forage before ensiling on dairy cows fed at 92% of their digestible protein requirements. Six multiparous lactating Holstein cows were used in a crossover design (two treatments × two periods). The control treatment (CON) was based on a diet including 50% of grass silage, whereas the experimental treatment (TAN) included grass silage sprayed with OTE (26 g/kg DM) just before baling. Milk yield (on average 24 kg fat protein corrected milk per day) was not affected, but both milk and rumen fatty acids profiles were impacted by OTE. Nitrogen intake (415 g N per cow per day) and nitrogen use efficiency (NUE; 0.25 on average) were not affected, but a shift from urine (-8% of N intake relatively to control, P = 0.06) to faecal N (+5%; P = 0.004) was observed with the TAN diet (P ≤ 0.05). Nitrogen apparent digestibility was thus reduced for TAN (-3%; P ≤ 0.05). The effect of OTE on ruminal and milk FA profiles suggests an impact on rumen microbiota. Nitrogen isotopic discrimination between animal proteins and diet (Δ15N) was evaluated as a proxy for NUE. While no differences in NUE were observed across diets, a lower Δ15N of plasma proteins was found when comparing TAN v. CON diets. This finding supports the concept that Δ15N would mainly sign the N partitioning at the metabolic level rather than the overall NUE, with the latter also being impacted by digestive processes. Our results agree with a N shift from urine to faeces, and this strategy can thus be adopted to decrease the environmental impact of ruminant protein feeding.

Nitrogen partitioning and isotopic discrimination are affected by age and dietary protein content in growing lambs.

It is difficult to separate an age-dependent fall in nitrogen use efficiency (NUE; N balance/N intake) in growing ruminants from a progressively decrease in animal protein requirements over time. This study examined the effect of dietary protein content on N partitioning, digestibility and N isotopic discrimination between the animal and its diet (Δ15Nanimal-diet) evaluated at two different fattening periods (early v. late). Twenty-four male Romane lambs (age: 19 ± 4.0 days; BW: 8.3 ± 1.39 kg) were equally allocated to three dietary CP treatments (15%, 17% and 20% CP on a DM basis). Lambs were reared with their mothers until weaning, thereafter housed in individual pens until slaughter (45 kg BW). During the post-weaning period, lambs were allocated twice (early fattening (30 days post-weaning) and late fattening (60 days post-weaning)) to metabolic cages for digestibility and N balance study. When diet CP content increased, the average daily gain of lambs increased (P < 0.05) while the age at slaughter decreased (P = 0.01), but no effect was observed on feed efficiency (P > 0.10). Diet CP content had limited effect on lamb carcass traits. Higher fibre digestibility was observed at the early v. late fattening period (P < 0.001). The N intake and the urinary N excretion increased when diet CP content increased (P < 0.001) and when shifting from early to late fattening period (P < 0.001). Faecal N excretion (P = 0.14) and N balance (P > 0.10) were not affected by diet CP content. Nitrogen digestibility increased (P < 0.001) as the diet CP content increased and on average it was greater at late v. early fattening period (P = 0.02). The NUE decreased (P = 0.001) as the diet CP content increased and as the lamb became older (P < 0.001). However, the age-dependent fall in NUE observed was lower at high v. low dietary CP content (CP × age interaction; P = 0.04). The Δ15Nanimal-diet was positively correlated (P < 0.05) with N intake (r = 0.59), excretion of faecal N (r = 0.41), urinary N (r = 0.69) and total manure N (r = 0.64), while negatively correlated with NUE (r = -0.57). Overall, the experiment showed NUE was lower in older lambs and when lambs were fed high diet CP content, and that Δ15Nanimal-diet was a useful indicator not only for NUE but also for urinary N excretion, which is a major environmental pollution factor on farm.

Effects of pretreatment with reducing sugars or an enzymatic cocktail before extrusion of fava bean on nitrogen metabolism and performance of dairy cows.

The aim of this study was to determine the effects of pretreatment with reducing sugars or with an enzymatic cocktail before extrusion of fava bean on intake, milk yield and composition, N partitioning, and plasma and ruminal parameters. The main hypothesis was that these pretreatment conditions would allow better N protection in the rumen compared with classic pretreatment before extrusion, thanks to an increase of sugar proportion, either exogenous or endogenous with enzymatic actions. Sixteen Holstein cows were used in a 4 × 4 Latin square design experiment. Cow were fed a diet with a 56:44 forage to concentrate ratio and containing 16.2% of crude protein (CP; dry matter basis). Concentrate consisted of fava bean:linseed blends (90:10%, raw basis) distributed either raw, extruded after pretreatment without additive, extruded after pretreatment with reducing sugars, or extruded after pretreatment with an enzymatic cocktail. The experimental blends provided 53% of total CP in the diet. Intake, milk yield and composition, ruminal pH, volatile fatty acids and ammonia kinetics, apparent total-tract nutrient digestibilities and N partitioning, Maillard compounds in feed and feces, plasma AA, and 15N natural enrichment were measured. Data were analyzed using analysis of variance according to the MIXED procedure of SAS (SAS Institute Inc., Cary, NC). Extrusion without additive during pretreatment led to higher contents of Maillard compounds in the blend, a lower enzymatic CP degradability, a numerically lower ammonia content in ruminal fluid, and a trend of higher plasma EAA concentration, all suggesting a decrease in degradability of proteins in the rumen, and a subsequent increase in metabolizable protein supply. Compared with pretreatment without additive, adding reducing sugars or an enzymatic cocktail during pretreatment led to an increase in Maillard compound contents in the extruded blends, and to an increase (+50 mg/L) in rumen ammonia content. With reducing sugars, digestibility of Nɛ-carboxymethyl-lysine was numerically lower, and plasma EAA concentration tended to decrease, suggesting an overprotection of proteins in the intestine. With the enzymatic cocktail, plasma EAA concentration was similar than without additive during pretreatment, suggesting a similar metabolizable protein supply with both treatments. Finally, no change in N partitioning between milk, urine, and feces was observed whatever the diet.

Digestibility contributes to between-animal variation in feed efficiency in beef cows.

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 (+ 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.

Effects of replacing soybean meal with raw or extruded blends containing faba bean or lupin seeds on nitrogen metabolism and performance of dairy cows.

The objective was to test the effects of replacing soybean meal in dairy cow diets with either raw or extruded faba bean:linseed or lupin:linseed blends on intake, milk yield and composition, N partitioning, and ruminal and plasma parameters. Our main hypotheses were that N from extruded blends was less degradable in the rumen than N from raw seeds, and that a higher extrusion temperature favored ruminal protection of proteins and milk protein yield, and lowered urinary N excretion. Eight Holstein cows fitted with ruminal cannulas were used in two 4 × 4 Latin square design experiments conducted in parallel. In both experiments, cows were fed diets with a crude protein content of 14.6%, containing 60% of forage (dry matter basis). Treatments differed by the composition of the concentrates: control in both experiments was based on soybean meal, and experimental treatments were based on proteaginous:linseed (90:10%) blends consisting of faba bean blends (first experiment) or lupin blends (second experiment) presented either raw, extruded at 140°C, or extruded at 160°C. Intake, milk yield and composition, ruminal pH, volatile fatty acids and ammonia kinetics, digestibility, N partitioning, Maillard compounds in feed and feces, plasma AA, and 15N natural enrichment were measured. Data were analyzed using ANOVA according to the MIXED procedure of SAS (version 9.4, SAS Institute Inc., Cary, NC). Ammonia content in ruminal fluid did not significantly change when soybean meal was replaced by either raw or extruded faba bean, but tended to be higher with lupin. Milk yield was increased by 2.6 kg with faba bean blend extruded at 140°C compared with faba bean blend extruded at 160°C. Milk fat and milk protein concentrations were decreased by 3.1 and 2.3 g/kg, respectively, with lupin blends compared with soybean meal. Nitrogen partitioning between milk, feces, and urine did not change. Nitrogen apparent digestibility decreased by 3 g/100 g of N between faba bean blend extruded at low and at high temperatures. The content of Maillard compounds in feces was higher with blends extruded at 160°C than with raw or extruded at 140°C blends within both experiments. Total plasma AA tended to be higher with extruded blends than with raw in the faba bean experiment. Both extrusion temperatures appeared to protect dietary proteins from ruminal degradability, but proteins seemed to be overprotected at 160°C.

Review: Biological determinants of between-animal variation in feed efficiency of growing beef cattle.

Animal's feed efficiency in growing cattle (i.e. the animal ability to reach a market or adult BW with the least amount of feed intake), is a key factor in the beef cattle industry. Feeding systems have made huge progress to understand dietary factors influencing the average animal feed efficiency. However, there exists a considerable amount of animal-to-animal variation around the average feed efficiency observed in beef cattle reared in similar conditions, which is still far from being understood. This review aims to identify biological determinants and molecular pathways involved in the between-animal variation in feed efficiency with particular reference to growing beef cattle phenotyped for residual feed intake (RFI). Moreover, the review attempts to distinguish true potential determinants from those revealed through simple associations or indirectly linked to RFI through their association with feed intake. Most representative and studied biological processes which seem to be connected to feed efficiency were reviewed, such as feeding behaviour, digestion and methane production, rumen microbiome structure and functioning, energy metabolism at the whole body and cellular levels, protein turnover, hormone regulation and body composition. In addition, an overall molecular network analysis was conducted for unravelling networks and their linked functions involved in between-animal variation in feed efficiency. The results from this review suggest that feeding and digestive-related mechanisms could be associated with RFI mainly because they co-vary with feed intake. Although much more research is warranted, especially with high-forage diets, the role of feeding and digestive related mechanisms as true determinants of animal variability in feed efficiency could be minor. Concerning the metabolic-related mechanisms, despite the scarcity of studies using reference methods it seems that feed efficient animals have a significantly lower energy metabolic rate independent of the associated intake reduction. This lower heat production in feed efficient animals may result from a decreased protein turnover and a higher efficiency of ATP production in mitochondria, both mechanisms also identified in the molecular network analysis. In contrast, hormones and body composition could not be conclusively related to animal-to-animal variation in feed efficiency. The analysis of potential biological networks underlying RFI variations highlighted other significant pathways such as lipid metabolism and immunity and stress response. Finally, emerging knowledge suggests that metabolic functions underlying genetic variation in feed efficiency could be associated with other important traits in animal production. This emphasizes the relevance of understanding the biological basis of relevant animal traits to better define future balanced breeding programmes.

Nitrogen isotopic fractionation as a biomarker for nitrogen use efficiency in ruminants: a meta-analysis.

Animal proteins are naturally 15N enriched relative to the diet and the extent of this difference (Δ15Nanimal-diet or N isotopic fractionation) has been correlated to N use efficiency (NUE; N gain or milk N yield/N intake) in some recent ruminant studies. The present study used meta-analysis to investigate whether Δ15Nanimal-diet can be used as a predictor of NUE across a range of dietary conditions, particularly at the level of between-animal variation. An additional objective was to identify variables related to N partitioning explaining the link between NUE and Δ15Nanimal-diet. Individual values from eight publications reporting both NUE and Δ15Nanimal-diet for domestic ruminants were used to create a database comprising 11 experimental studies, 41 treatments and individual animal values for NUE (n=226) and Δ15Nanimal-diet (n=291). Data were analyzed by mixed-effect regression analysis taking into account experimental factors as random effects on both the intercept and slope of the model. Diets were characterized according to the INRA feeding system in terms of N utilization at the rumen, digestive and metabolic levels. These variables were used in a partial least squares regression analysis to predict separately NUE and Δ15Nanimal-diet variation, with the objective of identifying common variables linking NUE and Δ15Nanimal-diet. For individuals reared under similar conditions (within-study) and at the same time (within-period), the variance of NUE and Δ15Nanimal-diet not explained by dietary treatments (i.e. between-animal variation plus experimental error) was 35% and 55%, respectively. Mixed-effect regression analysis conducted with treatment means showed that Δ15Nanimal-diet was significantly and negatively correlated to NUE variation across diets (NUE=0.415 -0.055×Δ15Nanimal-diet). When using individual values and taking into account the random effects of study, period and diet, the relationship was also significant (NUE=0.358 -0.035×Δ15Nanimal-diet). However, there may be a biased prediction for animals close to zero, or in negative, N balance. When using a novel statistical approach, attempting to regress between-animal variation in NUE on between-animal variation in Δ15Nanimal-diet (without the influence of experimental factors), the negative relationship was still significant, highlighting the ability of Δ15Nanimal-diet to capture individual variability. Among the studied variables related to N utilization, those concerning N efficiency use at the metabolic level contributed most to predict both Δ15Nanimal-diet and NUE variation, with rumen fermentation and digestion contributing to a lesser extent. This study confirmed that on average Δ15Nanimal-diet can predict NUE variation across diets and across individuals reared under similar conditions.

An innovative approach combining Animal Performances, nutritional value and sensory quality of meat.

This work sets out a methodological approach to assess how to simultaneously control together Animal Performances, nutritional value, sensory quality of meat. Seventy-one young bulls were characterized by 97 variables. Variables of each element were arranged into either 5 homogeneous Intermediate Scores (IS) or 2 Global Indices (GI) via a clustering of variables and analysed together by Principal Component Analysis (PCA). These 3 pools of 5 IS (or 2 GI) were analysed together by PCA to established the links existing among the triptych. Classification on IS showed no opposition between Animal Performances and nutritional value of meat, as it seemed possible to identify animals with a high butcher value and intramuscular fat relatively rich in polyunsaturated fatty acids. Concerning GI, the classification indicated that Animal Performances were negatively correlated with sensory quality. This method appeared to be a useful contribution to the management of animal breeding for an optimal trade-off between the three elements of the triptych.