Estimating parameters of growth curves in Nellore cattle raised in confinement

Objective. To compare the Brody, Gompertz, logistic and von Bertalanffy nonlinear regression models in their ability to describe the growth in Nellore cattle raised in confinement, receiving a feed supplement and subjected to experimental manipulation. Materials and methods. The parameters for each model were estimated using the procedure for nonlinear models PROC NLIN of SAS, based on weight data collected from 352 to 833 days of age. Results. Mean square error, coefficient of determination and Durbin-Watson statistic allowed to concluding that all functions had similar ability to adjust the weight-age data observed; however, the normality test for the residuals showed that the logistic model was the one that did not meet this assumption (p<0.05), limiting its predictive ability under current experimental conditions. Conclusions. The normality condition in the residuals of Brody, Gompertz and von Bertalanffy models (p>0.05) ensured that the inference about the parameters was adequate, regardless of weight changes resulting from the implementation of various feeding levels. For the biological interpretation of the parameters, the Brody model was the one that best described the recorded growth in Nellore males. The highest mature weight and the lowest mature index obtained indicate that animals reached asymptotic weight at older age because of their slower growth, as consequence of their manipulation during the experiment.