Abstract In this study, 240 day-old guinea fowl (Numida meleagris) keets were utilized. They were divided into four treatment groups each containing 20 chicks and were randomly distributed into 12 mobile coops placed in a 100-m2 grazing area. Guinea fowl chicks were randomly allocated to 4 treatment diets containing 0%, 5%, 10%, and 15% dry oregano leaf (DOL) supplements. Nonlinear Gompertz and logistic growth models were used to estimate the mean age-body weight. The growth curve parameters for these models and the following characteristics for fowl were estimated: β0, the asymptotic weight parameter; β the scaling parameter; β2, the instantaneous per week growth rate; weight at age of inflection point (WIP); maximum weight gain at inflection point (MWG); and age at the inflection point (AIP). The goodness of fit (GF) for the models was assessed using the following variables: coefficients of determination (r2), mean square error (MSE), adjusted determination coefficient (ADR2), Akaike's information criteria (AIC), chi-square test (Chi.Sq2) and residual standard deviation (RSD). The different nonlinear function results of the individual data indicated that supplementation of diets with DOL had no significant effects on growth curve parameters when compared with the control diet. Greater correlation values were estimated among β0, β1, β2, WIP, MWG and AIP in the Gompertz equation, and similar results were estimated in the logistic equation, but there was no significant correlation between β2-β1 and β2-MWG. According to the results obtained from the GF, high r2 and ADR2 were estimated in Gompertz and logistic equations (above 0.96).