Optimizing dietary tryptophan in quail chicks: Implications for NRC (1994) recommendation.

This study evaluated the effects of graded dietary tryptophan (Trp) supplementation on growth performance, carcass traits, hepatic enzyme activities, and immune response in growing Japanese quails. A total of 525 seven-day-old quail chicks were distributed in a completely randomized design with seven dietary Trp levels (0.21 %, 0.22 %, 0.23 %, 0.24 %, 0.25 %, 0.26 %, and 0.27 %) and five replicates of 15 birds each. Growth performance traits, including weight gain (G) and feed conversion ratio (FCR), responded significantly (P < 0.05) to increasing dietary Trp. The rational regression model estimated optimal Trp levels of 0.237 % for G and 0.238 % for FCR, corresponding to 7.9 % and 8.3 % higher requirements than NRC (1994) recommendations, respectively. Hepatic enzyme activities exhibited pronounced quadratic responses, with minimum alkaline phosphatase (ALP) and aspartate aminotransferase (AST) activities estimated at 0.242 % and 0.232 % Trp, representing 10.2 % and 5.6 % higher requirements than NRC (1994), respectively. The antibody titer against sheep red blood cells peaked at 0.247 % Trp (12.2 % higher than NRC, 1994), indicating enhanced immune responsiveness. Carcass traits, including thigh and breast meat yields, were optimized at 0.249 % and 0.251 % Trp, exceeding NRC recommendations by 13.0 % and 14.1 %, respectively. Overall, the estimated optimal Trp requirements for maximizing growth, immunity, and carcass yield ranged from 5.6 % to 14.1 % above NRC (1994) values, underscoring the elevated amino acid demands of modern quail genotypes. These findings highlight the need to update current nutritional guidelines to accommodate contemporary genetic potential, improved metabolic efficiency, and functional amino acid utilization, thereby supporting optimal growth performance, liver health, and immune competence.