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We compared five nonlinear models (Brody, von Bertalanffy, Gompertz, logistic, Richards) to describe growth and carcass protein, ash, and fat deposition in Japanese quail (1–36 days). Two hundred and twenty-five females were reared with ad libitum access to feed and water; body weight was recorded every three days, and serial slaughter provided body composition. Models were fitted in SAS (PROC NLIN; Levenberg–Marquardt). Model fit was assessed via coefficient of determination (R²), Akaike (AIC), Bayesian (BIC) information criteria, and Asymptotic Index (AI). All models showed high R² (≥97%). For body weight, Gompertz provided the best information criteria and biologically plausible parameters (inflection 15.7 days). logistic best described protein (lowest AIC/BIC/AI; inflection 17.5 days). For ash, Gompertz balanced fit and parsimony (inflection 15.1 days). The Richards model best captured fat (R² ≈ 0.98; lowest AIC; inflection at 35.6 days), indicating late fat accretion. Protein and ash peaked at 15–18 days of age, followed by rising fat accretion. Gompertz best described body weight and ash, logistic best described protein, and Richards best described fat. These results support a three-phase feeding program (1–18, 19–29, 30–36 days): emphasize amino acids/minerals early; balance diets mid-phase; and moderate energy later.