PS9-8. Residual Feed Intake Divergence Affects Carcass Traits, Meat Quality, and Muscle Multi-omics Profiles in Feedlot-finished Nellore Bulls.

Abstract Improving feed efficiency (FE) is a major goal for sustainable beef production because feed costs represent the largest economic input in cattle systems and contribute substantially to the environmental footprint of livestock production. Residual feed intake (RFI) is widely used as an indicator of FE because it measures variation in feed intake independent of growth and body size. However, the biological mechanisms underlying RFI variation remain incompletely understood, particularly in Bos indicus cattle such as Nellore. Integrating phenotypic measurements with molecular profiling may help clarify the metabolic pathways associated with improved efficiency. This study evaluated the effects of RFI divergence on carcass traits, meat quality, and skeletal muscle multi-omics profiles in feedlot-finished Nellore bulls. Ninety-six animals (initial BW 362.5 ± 25.6 kg) were evaluated in a feedlot test with individual feed intake monitoring and classified as low RFI (efficient) or high RFI (inefficient). Efficient animals consumed less feed (9.14 vs. 10.42 kg/d) while maintaining similar growth performance. Carcasses from low-RFI cattle were leaner, with greater dressing percentage (54.44 vs. 53.23%) and reduced backfat thickness (4.75 vs. 5.61 mm), with a tendency toward larger ribeye area. Meat from efficient animals exhibited lower intramuscular fat (1.43 vs. 1.75%) and reduced myofibrillar fragmentation index, whereas pH, color, cooking loss, shear force, and lipid oxidation were not affected. Consumer sensory panels indicated slightly lower juiciness and overall liking scores for beef from efficient animals. To investigate the molecular basis of these phenotypic differences, Longissimus thoracis et lumborum samples were analyzed using label-free LC–MS/MS proteomics and proton nuclear magnetic resonance (1H-NMR). Proteomic profiling identified more than 2,200 proteins per group, and multivariate analysis clearly separated RFI classes. Differential protein abundance highlighted pathways related to mitochondrial energy metabolism, glycolysis, cytoskeletal organization, and vesicle trafficking. Key discriminant proteins included spectrin alpha chain (SPTAN1), nebulin-related anchoring protein (NRAP), clathrin heavy chain (CLTC), and protein S100-A10 (S100A10). Metabolomic enrichment analysis based on variable importance in projection (VIP > 1) revealed several metabolites discriminating RFI groups. The most influential metabolites included glycerol, hypoxanthine, glycerate, alanine, malonate, lactate, nicotinurate, AMP, carnitine, glycine, tyrosine, and glucose, most of which were more abundant in high-RFI animals. These metabolites are associated with glycolysis, purine metabolism, amino acid turnover, and mitochondrial energy metabolism, suggesting greater metabolic turnover and reduced energetic efficiency in inefficient cattle. In contrast, acetate and choline were more abundant in low-RFI animals, indicating potential differences in lipid metabolism and membrane phospholipid turnover. Overall, the integration of proteomic and metabolomic data indicates that improved feed efficiency in Nellore cattle is associated with coordinated changes in muscle structural organization and energy metabolism, providing new insights into the biological mechanisms underlying RFI variation and identifying potential molecular markers for improving feed efficiency in beef cattle.