Interactive Effects of Dietary Crude Protein and Methionine Levels on Growth Performance, Carcass Quality and Blood Biochemistry of Japanese Quail During Starter and Grower Phases

BACKGROUND: This study evaluated the interactive effects of dietary crude protein (CP) and methionine (Met) on growth performance, carcass traits, blood biochemistry, thyroid hormones and liver enzymes in Japanese quail during the starter (0-14 days) and grower (14-35 days) phases. METHODS: A total of 1,350 mixed-sex chicks were used in a completely randomised 3 × 5 factorial design, testing three CP levels (22%, 24%, 26% of diet starter; 21%, 23%, 25% of diet grower) and five Met levels (0.30%-0.70% of diet). Performance traits, carcass characteristics, blood indices, thyroid hormones (T3 and T4) and liver enzymes (AST and ALT) were measured, and nonlinear regression models were used to estimate optimal Met requirements. RESULTS: Raising starter-phase CP to 26% significantly improved weight gain (+4.1%), feed conversion ratio (-4.1%), and carcass yield (+1.5%). Methionine supplementation up to 1.30% further enhanced these traits, with optimal Met levels ranging from 0.50%-0.70% depending on CP concentration. In the grower phase, the combination of 25% CP and 1.30% Met produced the highest male weight gain (161.84 g) and breast yield (29.88%), while females showed the highest breast yield (30.90%) at the same levels, indicating sex-specific responses. Increasing Met reduced blood glucose, triglycerides, and cholesterol while raising HDL, with most blood parameters showing significant CP × Met interactions. Thyroid hormones rose with higher CP, though T3 declined with Met supplementation; both factors also influenced liver enzyme activity. CONCLUSIONS: These findings confirm a synergistic relationship between dietary protein and methionine, supporting 24%-26% CP with 0.50%-0.70% Met in the starter phase and 23%-25% CP with appropriate Met levels in the grower phase for modern quail strains-offering updated recommendations beyond the older Nutrient Requirement Council (NRC) guidelines for commercial production.