Influence of Dietary Rumen-Protected Choline and Nicotinic Acid Supplementation on Growth Outcomes and Meat Tenderness, Rumen Microbial Composition, and Serum Metabolomics of Late-Fattening Beef Cattle

High-concentrate feeding during late fattening can impair metabolic health and meat quality in beef cattle, yet effective nutritional strategies remain limited. Choline and nicotinic acid are involved in lipid and energy metabolism, but information on their combined or sequential supplementation is scarce. This study evaluated rumen-protected choline given alone or together with nicotinic acid on growth performance, meat quality, serum biochemical indices, serum metabolomics, and free amino acid profiles in late-fattening beef cattle. Twenty-four black cattle were randomly allocated to three groups: CON (basal diet), CC (basal diet + 10 g/d RPC for 90 d), and CC-CNA (basal diet + 10 g/d RPC for 90 d + 10 g/d protected NA during the final 30 d). Shear force was significantly lower in both CC and CC-CNA than in CON (p < 0.05), indicating improved tenderness. Exploratory serum metabolomics identified treatment-associated metabolites and pathways related to amino acid, purine, sphingolipid, and lipid metabolism. Taste-related free amino acids were also altered by supplementation (p < 0.05). RPC alone or with short-term NA may improve tenderness and modulate systemic metabolic responses; larger studies with direct sensory evaluation are needed to confirm growth and flavor-related effects.