Effects of Reduced Crude Protein Diets with Graded Methionine Supplementation on Growth Performance, Nitrogen Utilization, and Serum Metabolomic Profiles in Growing–Finishing Pigs

This study evaluated reduced dietary crude protein (CP) with graded methionine (Met) supplementation in growing and finishing pigs. In each phase, 48 crossbred barrows were assigned to four treatments with four replicate pens per treatment and three pigs per pen. The growing and finishing trials lasted 43 and 50 d, respectively. Growing pigs received CON (16% CP, 0.37% Met) or low-CP diets (15% CP) with 0.37%, 0.41%, or 0.44% Met; finishing pigs received CON (14% CP, 0.27% Met) or low-CP diets (13% CP) with 0.27%, 0.30%, or 0.32% Met. Nitrogen excretion and apparent total tract digestibility of nitrogen were assessed through metabolism trials. Serum biochemical parameters were analyzed at the end of each phase, and untargeted 1H-NMR-based serum metabolomic analysis was performed in finishing pigs. Most growth performance variables, nitrogen excretion, and nitrogen digestibility were not significantly affected by treatment, whereas selected blood urea nitrogen concentrations were reduced in low-CP diets with Met supplementation. Serum metabolomic profiling suggested exploratory shifts in energy-related metabolites. Overall, these findings suggest that a one-percentage-point reduction in dietary CP with Met supplementation can maintain overall pig performance while inducing selected nitrogen-related biochemical and metabolic responses under the conditions of this study.