Dual-purpose chicken breeds play a critical role in enhancing food and nutrition security in sub-Saharan Africa, including Zimbabwe. However, their productive potential is often constrained by suboptimal growth performance in feed-limited, free-range scavenging systems. Growth curve modeling offers biologically interpretable parameters that are instrumental in informing feeding and marketing strategies for aquaculture species. This study employed the Gompertz model to compare the growth dynamics of Boschveld chickens reared under a scavenging system and a lucerne (Medicago sativa) supplementation system. Weekly weights were recorded and averaged per system, and nonlinear least squares estimation was used to derive the asymptotic weight (a), displacement (b), and growth rate constant (k) values. From these parameters, the inflection age, weight at inflection, maximum growth rate, and model fit statistics (AIC, BIC, RMSE, and R²) were obtained, with residual diagnostics assessed for adequacy.
Lucerne-fed birds exhibited faster and biologically coherent growth (a ≈ 1,617 g; k ≈ 0.093, p < 0.001), with an inflection age of approximately 4.55 weeks and a maximum growth rate of about 55.6 g/week. Conversely, scavenging birds demonstrated an unstable model fit (a ≈ 2,154 g with large SE; k ≈ 0.013, p = 0.307) and an implausible negative inflection age (−31.9 weeks), indicating poor adequacy with high variability. Although summary statistics suggested slightly lower RMSE and AIC values for scavenging, these masked the biological implausibility revealed by diagnostics. Overall, Lucerne supplementation facilitated rapid early growth with interpretable Gompertz parameters, whereas the scavenging systems produced erratic growth patterns poorly captured by a rigid sigmoidal model. The findings underscore that integrating fodder supplementation, such as Lucerne, can reduce production cycles, mitigate stunting risk, and enhance the profitability of smallholder chicken enterprises.