177 Awardee Talk: Residual feed intake in beef cattle: Biological basis and effect on response to dietary supplementation of rumen bypass protein

Abstract Using ruminal and plasma metabolomics, as well as 16S rRNA gene sequencing, we aimed to identify differences in host metabolome, rumen metabolome, and microbial community in beef steers with high (positive) or low (negative) residual feed intake (RFI) phenotypes. Our microbiome results revealed a greater relative abundance of Weissella, and Muribaculaceae in low-RFI beef steers, while Methanobrevibacter and Clostridium were reduced. Additionally, pathway enrichment analysis of the metabolome data revealed the enrichment of histidine metabolism, and arginine biosynthesis in the plasma of low-RFI beef steers. Overall, our results showed that selection for divergent RFI phenotype is associated with differences in rumen microbiome and overall animal metabolism, notably amino acid metabolism-related pathways, and therefore may be an important animal-related factor that can affect the response of beef cattle to nutritional interventions such as rumen bypass protein supplements. Feeding rumen-bypass protein to cattle has demonstrated positive impacts on weight gain and feed efficiency, especially in rapidly growing young calves. However, the increasing costs of feed protein and the challenges of excessive nitrogen discharge into the environment underscore the need for sustainable protein supplementation in ruminant diets. Although past research has aimed at enhancing amino acid utilization efficiency and/or reducing nitrogen excretion in ruminants by minimizing dietary protein content, there is a gap; no study has assessed how factors intrinsic to the animal, like feed efficiency status, might influence the response of ruminants to dietary protein supplements. In our follow-up study, we investigated the effects of rumen-bypass protein (RBP) supplements on growth performance, plasma and urinary nitrogen concentration, hepatic mitochondrial protein complexes, and mRNA expression of immune genes in crossbred beef steers with divergent negative or positive RFI phenotypes. We assessed the impact of RBP supplementation on feed efficiency in beef steers. Steers [n = 40; body weight (BW) = 492 ± 36 kg) underwent a 42-d trial in a 2 × 2 factorial arrangement, considering RFI classification and RBP supplementation. Liver tissue samples collected on d 42 were analyzed for mRNA expression of immune genes and mitochondrial protein complexes I to V. Regardless of RFI status, RBP increased blood urea nitrogen (BUN; P = 0.01), with decreased BUN in low-RFI beef steers. Interactions of RBP and RFI were observed (P ≤ 0.05) for mitochondrial activities of complexes IV, V, and mRNA expressions of immune genes (TLR2, TLR3, IL23A). While RBP did not impact growth performance, its effects on immune gene expression and mitochondrial protein complexes varied with RFI status, highlighting the importance of considering RFI status in future studies investigating the effects of dietary protein supplements effects in beef cattle.