Brody, Gompertz, Logistic, Richards and von Bertalanffy functions were compared as descriptors of growth in 319 coypus (Myocastor coypus) of Greenland and Silver varieties, from birth to 10–19 months of age. Several criteria were considered for the selection of the model: (a) bias in the asymptotic weight; (b) bias in birth weight; (c) error sum of squares; (d) error mean squares weighted by the number of animals converging in all functions; (e) determination coefficient. To adjust the curves NLIN procedure from SAS and iterative method of Marquardt were applied. A mixed model was used to analyse the parameters of the selected growth curve (asymptotic weight, A; constant of integration, B; and maturation rate, k) which included effects of farm, variety, sex, sire within farm and variety, reproductive status within sex, birth season and first order interactions. The effect of sire was considered as random. The von Bertalanffy function was selected and mean values for the parameters of this function were 8016 g and 0·008 g/g per day, for A and k respectively. There were differences in the estimated parameters between sexes; males had larger values of A and k than females.

Weights at fixed ages (birth, weaning, 6–7 months) were analysed, as well as the final recorded weight and average postweaning gain. The model of analysis was similar to the previous one. There were no differences between varieties. Significant differences between sexes for almost all studied traits were detected. With age, weight difference between males and females increased. At 6 months of age, males were 37% heavier than females. Male average daily gain was higher than that of females, sexual dimorphism being evident in all analysed traits.