comparison of mathematical models for describing the growth of baluchi sheep

the objectives of this study were to identify a suitable mathematical model for describing the growth curve of baluchi sheep based on monthly records of live weight from birth to yearling; and to evaluate the efficacies of nonlinear mixed effect model (nlmm) and the nonlinear fixed effect model (nlm) methodologies. growth models were fitted to a total of 16,650 weight–age data belonging to 2071 lambs. five nonlinear growth functions of von bertalanffy, gompertz, brody, logistic, and richards and two linear polynomial functions were applied. the growth models were compared by using the akaike’s information criterion (aic) and residual mean square (mse). among all nonlinear fixed effect models, the brody function had the smallest aic and mse values, indicating the best fit for both sexes. the brody fixed effect model compared with nlmm including one random effect of asymptotic mature weight. the model evaluation criteria indicated that the brody mixed effect model fitted the data better than the corresponding fixed effect model. it can be concluded that, among the linear models, the polynomial of the third order and, among nonlinear models, brody mixed model were found to best fit the baluchi sheep growth data.