COMPONENTS OF (CO)VARIANCE OF POSTWEANING GROWTH IN SANTA GERTRUDIS HEIFERS USING RANDOM REGRESSION MODELS

The capacity for growth, one of the most important factors in beef cattle is determined by the pre-and post-weaning growth, so that both steps are important in the selection process of sires. The accurate estimation of components of (co)variance and genetic parameters during these periods improves the response in the selection schemes. With this objective, components of (co)variance and genetic parameters of growth were estimated from weaning to 900 d of age in Santa Gertrudis heifers, through a random regression model using Legendre polynomials for the additive effects, maternal and permanent environment. Thus, 5709 weighings were used from 1903 heifers and their weights at weaning, at a year of age and at the beginning the reproductive stage were recorded. The analysis of the results indicated an increase in the additive variances and permanent environment with the age, while the variance due to maternal effects decrease. The trend of heritability for direct effects was declining from weaning (0.08) to 280 d of age (0.04); from here and up to 900 d, the trend increased (0.20). The effects of permanent environment were opposed to heritability for the direct effects. Heritability for the maternal effects decreased from 0.10 (at weaning) up to 0 (at 900 d). Genetic correlations were negative between more distant, but high and positive between consecutive weighings, with values higher than 0.85 since the 400 d, so that selection for body weight at later ages will be more effective.