Nonlinear growth modeling offers a robust framework for evaluating poultry performance under diverse management systems. This study compared Brody, Gompertz, and Logistic models in describing the body weight–age relationship of Austrolope chickens reared under lucerne supplementation and scavenging systems. Weekly body weights were collected from week 14 to week 62 and analyzed using nonlinear regression. Model performance was evaluated using the coefficient of determination (R²), root mean square error (RMSE), and Akaike Information Criterion (AIC), with residual analyses confirming unbiased predictions. Results revealed distinct system-specific growth patterns: lucerne-fed chickens exhibited rapid growth, an earlier inflection point, and higher asymptotic weights, whereas scavenging birds demonstrated slower, protracted growth trajectories. The Logistic model best described lucerne-fed birds, accurately capturing the sigmoidal growth curve, while the Brody model was most suitable for scavenging birds. These findings underscore the importance of management-specific growth modeling for accurate prediction of mature weight, optimization of feeding strategies, and development of targeted breeding programs. Aligning growth models with production context enhances both biological interpretation and practical utility, providing actionable insights for smallholder and semi-intensive poultry systems.