Relationship of residual feed intake (Metabolic Efficiency) to post-ruminal metabolism in beef cattle

Abstract Text: Residual feed intake (RFI) is a proposed energetic efficiency measurement for cattle not confounded by mature size or growth rate. Post-ruminal metabolism and cellular functions appear to contribute to observed animal RFI phenotypes based on data from our group and others. To date, one research emphasis of our group has been on energy metabolism in feedlot cattle with divergent RFI phenotypes. Mitochondria function, responsible for most cellular energy, was correlated with RFI phenotype where efficient calves exhibited faster oxygen uptake, greater complex I:III, and greater complex I subunit concentration.  No differences in electron leak or ADP:O₂ ratio were measured among RFI phenotypes.  Differences in ATP synthesis rate have accounted for most of the 1.4-fold DMI range among animals, suggesting phosphorylation ratio influences satiety. Variation in DMI led us to hypothesize diet nutrient density should be altered to most efficiently meet nutrient requirements of animals based on known RFI. We altered diet absorbable AA relative to energy concentration to test growth performance of calves with different RFI phenotypes.  Efficiency of BW gain responded to absorbable AA supply and was dependent on RFI phenotype. In other research areas within our group, carcass fat depot differences may be explained by variable DMI among RFI phenotypes. Additionally, differences in the size and function of the gastrointestinal tract, specifically the small intestine, may influence RFI. For example, more efficient steers had less small intestinal mass but denser intestinal mucosa. High and low efficiency steers had similar mRNA expression of several nutrient transporters, despite differences in DMI. Recent data suggest metabolic efficiency influences milk production with daily milk production increasing as metabolic efficiency improves, implying efficient females have more nutrients available to partition to milk.  Lastly, multi-generation RFI selection appears to be able to challenge the limits of biological efficiency. A current example are calves requiring 68% of the predicted maintenance and growth energy after several generations of selection for improved efficiency. As cellular energetic efficiency in the beef cattle population increases through selection pressure, diet nutrient density will need to be adjusted to maximize growth performance potential.

Keywords: efficiency, RFI, beef