Non-linear mixed models in the study of growth of naturalized chickens

ABSTRACT This study was conducted to examine the inclusion of random effects in non-linear models, identify the most suitable models, and describe the growth of naturalized chickens. Live-weight records of 166 birds of the Graúna Dourada, Nordestina, and Teresina ecotypes were estimated. The asymptotic weight (A), integration constant, related to animal initial weight (B), and the maturing rate (k) parameters of the non-linear Gompertz, Logistic, and von Bertalanffy models were estimated and adjusted using the Gauss-Newton method. Residual variance decreased by more than 50% when random effects were added to the model. The best fits in the estimate of the growth curve of females were obtained by associating the random effects with the three parameters of the Gompertz and Logistic models. The association of random effects with two parameters (asymptotic weight and maturing rate) and with the three parameters of the Logistic model provided the best fits for the males. The Teresina ecotype has the highest adult weight in both sexes, despite its slower growth. The opposite is true for the Graúna Dourada ecotype, formed by lighter and earlier-growing animals. The inclusion of random effects in models provides greater accuracy in the estimate of the growth curve.