Multiphase diets may improve feed efficiency in fattening crossbreed Holstein bulls: a retrospective simulation of the economic and environmental impact.

Beef industry needs alternative feeding strategies to enhance both economic and environmental sustainability. Among these strategies, adjusting the diet dynamically according to the change of nutritional requirements (multiphase diet) has demonstrated its economic and environmental benefits in pig production systems. Therefore, this retrospective study aims to assess, through simulation, the theoretical economic and environmental benefits of introducing a multiphase diet for crossbreed bulls feeding (one or more diet changes). For this, individual data of BW, BW gain, and daily intake were recorded from 342 bulls during the last fattening period (112 days). These data were used to estimate individual trajectory of energy and protein requirements, which were subsequently divided by individual intake to calculate the required dietary energy and protein concentrations. The area between two functions (i.e., ƒ1: constant protein concentration in the original diet during fattening and ƒ2: estimated protein concentration requirements) was minimised to identify the optimal moments to adjust the dietary concentration of energy and protein. The results indicated that both energy and protein intake exceeded requirements on average (+16% and +28% respectively, P < 0.001), justifying the adoption of a multiphase diet. Modelling the individual trajectories of required metabolisable protein (MP, g/kg DM) during the fattening period resulted in exponential decay model in relation to BW [32120 × exp(-0.026 × BW) + 59.9], while the dietary net energy concentration followed a slightly quadratic model [2.26-0.0026 × BW + 0.000003 × BW2]. Minimisation of the area between curves showed two optimal moments to adjust the diet: at 312 kg and 385 kg of BW, indicating three diet phases: (a) <312 kg, (b) 312-385 kg, and (c) 385-600 kg. For the second and third phases, the dietary energy and protein concentration should be 70 g MP/kg DM and 1.70 Mcal/kg DM and 61 g MP/kg DM and 1.65 Mcal/kg DM, respectively. These diet adjustments might improve economic profitability by 29 €/animal, reduce estimated nitrogen excretions by 16% (P < 0.001), and maintain similar weight gain (P > 0.16) compared to the commercial diet. However, the decrease in dietary energy concentration led to increased fibre concentration, which in turn increased the estimated CH4 emissions of animals with the multiphase diet (+44%, P < 0.001). Hence, multiphase diet could theoretically reduce feeding cost and nitrogen excretion from fattening cattle. Further in vivo studies should confirm these results and find optimal nutritional strategies to improve economic profitability and environmental impact.

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.

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.

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.