Digestive and metabolic mechanisms associated with the between-animal variability of feed efficiency in beef cattle

Understanding the mechanisms underlying desirable phenotypes in animal production is crucial for animal genetic selection and nutrition. Under the BEEFALIM 2020 program, aimed at improving the feed efficiency of suckling beef cattle, several experiments were conducted to study the digestive and metabolic mechanisms associated with the between-animal variability of feed efficiency in young Charolais cattle, heifers, and cows. To evaluate the digestive and metabolic mechanisms involved, direct measurement using reference methods were coupled with indirect measurements using indicators (proxies and biomarkers). Feed efficiency was characterized by residual feed intake, defined as the portion of measured dry matter intake not explained by the observed animal performance (live weight, body weight gain and composition). The results presented confirm that certain metabolic mechanisms, as well as the body composition of animals, would be more strongly associated with individual variations in feed efficiency than digestive mechanisms, especially when diets are rich in starch. However, the experimental setup does not exclude the possibility that appetite or feeding capacity contributes to differences in feed efficiency between individuals. Indeed, the identified mechanisms could be associated with the efficient phenotype solely because they covary with the level of feeding. Based on our results, and those of the literature, we suggest, however, that protein metabolism and turnover would be associated with between-animal variation of feed efficiency regardless of the feeding level and could be proposed as a candidate determinant of RFI variation. Further studies are needed to discern the true causality of the identified mechanisms.