NONLINEAR MODELS FOR FITTING GROWTH CURVES OF NELLORE COWS REARED IN THE AMAZON BIOME

Abstract

Growth curves of Nellore cows were estimated by comparing six nonlinearmodels: Brody, Logistic, two alternatives by Gompertz, Richards and VonBertalanffy. The models were fitted to weight-age data, from birth to750 days of age of 29,221 cows, born between 1976 and 2006 in theBrazilian states of Acre, Amapa, Amazonas, Para, Rondonia, Roraima andTocantins. The models were fitted by the Gauss-Newton method. Thegoodness of fit of the models was evaluated by using mean square error,adjusted coefficient of determination, prediction error and meanabsolute error. Biological interpretation of parameters was accomplishedby plotting estimated weights versus the observed weight means,instantaneous growth rate, absolute maturity rate, relativeinstantaneous growth rate, inflection point and magnitude of theparameters A (asymptotic weight) and K (maturing rate). The Brody andVon Bertalanffy models fitted the weight-age data but the other modelsdid not. The average weight (A) and growth rate (K) were: 384.6 +/- 1.63kg and 0.0022 +/- 0.00002 (Brody) and 313.40 +/- 0.70 kg and 0.0045 +/-0.00002 (Von Bertalanffy). The Brody model provides better goodness offit than the Von Bertalanffy model.