Determination of digestible valine requirements for optimal growth of Japanese quail during the starter phase (1–21 days)

1. This study determined the requirement of digestible valine (dVal) and the optimal digestible valine-to-digestible lysine (dVal:dLys) ratio for Japanese quails during the starter phase (1–21 d).2. A total of 1000 one day old unsexed Japanese quail chicks (Coturnix japonica) were randomly assigned to five experimental diets: dVal (7.5, 8.5, 9.5, 10.5 or 11.5 g/kg diet), as five pen replicates each holding 40 birds.3. The optimal dietary dVal level for weight gain (WG), feed intake (FI), and feed conversion ratio (FCR) during d 1–21 were estimated using linear broken-line (LBL), quadratic broken-line (QBL), quadratic and logistic regression models to determine the optimal responses. Model evaluation was based on the coefficient of determination (R2), Akaike information criterion (AIC) and Bayesian information criterion (BIC).4. Chicks receiving a diet containing 0.95% dVal exhibited significantly higher WG (p = 0.0003) and improved FCR (p = 0.0330) compared to other treatments. The FI was unaffected by dVal levels over the 1–21 d period.5. Dose-response analysis using regression models revealed that the quadratic model provided the most accurate estimation of dVal requirements. The optimal dVal requirement for maximising WG and minimising FCR was determined to be 0.95% of the diet. Higher dVal levels (>9.5 g dVal/kg) were associated with increased FCR (p < 0.05). The estimated ideal dVal:dLys ratio to optimise performance in starter phase for Japanese quail was 0.79.6. The data showed that precise Val supplementation at recommended levels in low-protein diets can enhance growth in Japanese quail.