Effects of crude protein reduction and a moderate increase in metabolizable energy on growth performance, digestibility, and blood metabolites in pigs

Reducing dietary crude protein (CP) with essential amino acid (AA) supplementation can improve nitrogen utilization; however, it may compromise growth performance if nitrogen supply becomes limiting. This study evaluated the effects of dietary CP reduction on growth performance and metabolic responses in growing–finishing pigs. Two 28-day feeding trials were conducted using pigs with initial body weights of 55 and 70 kg. In Experiment 1, pigs were fed diets containing 15.5% or 13.5% CP, and in Experiment 2, CP levels were 13.5% or 11.5%. Reducing dietary CP decreased average daily gain in growing pigs whereas feed intake and gain-to-feed ratio were not affected. Low-CP diets reduced apparent total tract CP digestibility and blood urea nitrogen, and altered circulating proline and serine concentrations, while most essential AA were maintained. In finishing pigs, low-CP feeding downregulated jejunal AA transporter and mammalian target of rapamycin gene expression. However, fecal fermentation characteristics, microbial composition, and carcass traits were not significantly affected by dietary treatments. Overall, dietary CP reduction influenced growth performance and metabolic responses in growing–finishing pigs, suggesting that adequate nitrogen supply remains important even when essential AA requirements are met.