Abstract
This study evaluated the dietary lysine (Lys) requirement for growing Japanese quails (Coturnix coturnix japonica) using serum uric acid (UA) concentration as a metabolic indicator. A total of 375 seven-day-old quail chicks were randomly assigned to 25 pens in a completely randomized design with five dietary treatments (0.94 %, 1.09 %, 1.24 %, 1.39 %, and 1.54 % Lys) and five replicates of 15 birds each. Birds were fed experimental diets from 7 to 21 days of age, with Lys levels adjusted by replacing cornstarch in a wheat-soybean meal-corn gluten meal basal diet. Sinusoidal, Truncated Fourier Series (TFS), and Morgan-Mercer-Flodin (MMF) models were fitted using nonlinear regression to find the best dietary Lys to minimize serum UA. The fitted models revealed a U-shaped dose-response pattern, with the TFS model providing the best fit (R² = 0.993; RMSE = 0.178; AIC = 5.62) and estimating the Lys requirement for minimum serum UA at 1.42 %, corresponding to a predicted UA of 4.71 mg/dL. The Sinusoidal model yielded a similar estimate (1.40 % Lys), while the MMF model predicted a higher requirement (1.54 %). These results demonstrate that nonlinear curve fitting, particularly the TFS model, can accurately define dietary Lys needs in growing Japanese quails based on serum UA minimization.