N-Carbamylglutamate Improves Production Performance and Muscle Growth by Regulating Protein Digestive Function and Muscle Protein Synthesis in Broiler Chickens

N-carbamylglutamate (NCG) is a structural analog of N-acetylglutamate and has multifunctional roles in animal production. However, few studies have been conducted to evaluate the effects of NCG on protein digestive function and muscle growth in broiler chickens. This study investigated whether NCG could improve muscle growth via protein digestive ability and amino acid metabolism in broiler chickens. A total of 144 one-day-old male broiler chickens were randomly allocated to four treatments with six replicates of 6 broiler chickens each. The treatments were a basal diet and a basal diet with NCG supplementation at three graded levels (150 g/t, 300 g/t, and 450 g/t of NCG). The results showed that NCG supplementation significantly improved the average daily weight gain (ADG) and decreased the feed conversion ratio (FCR) compared to the control group (p < 0.05) and increased the weight of leg muscle and breast muscle. Furthermore, NCG supplementation significantly increased protein digestibility, the activities of amylase, trypsin and lipase, and villus height in the ileum (p < 0.05), which demonstrated that digestibility and absorption were improved by NCG in broiler chickens. Analysis of plasma amino acids, hormone levels and the gene expression of breast muscle revealed that NCG increased the concentration of GABA, glutamate, glutamine, leucine, threonine, valine, branched-chain amino acids and essential amino acids and the levels of testosterone and IGF-1 in plasma (p < 0.05). Additionally, NCG increased the expression of mTOR and P70S6K in breast muscle tissue. Therefore, NCG supplementation could be an important nutritional strategy to improve product performance, muscle growth and development, and meat yield. The 300 g/t supplementation level was found to be the practical optimum dose, as it produced effects comparable to the highest dose (450 g/t) on most parameters while offering better cost-effectiveness. This study provides new insights into the application of NCG for meat production improvement in broiler chickens.