Heterogeneidade de variâncias na avaliação genética sobre pesos corporais de bovinos da raça Nelore

Abstract

Aiming to evaluate the existence of variance heterogeneity and its influence on the estimation of variance components for body weights and their impact on the genetic evaluation of Nellore breeders. Data from 43,331, 40,306, 35,024 and 31,697 standardized body weight records were used at 120 (P120), 210 (P210), 365 (P365) and 450 (P450) days of age, respectively, in animals participating in the Nellore Breeding Program of the National Association of Breeders and Researchers (ANCP) from herds located in the states of Mato Grosso, Mato Grosso do Sul and Goiás. The standard deviation classes (SD) were formed by the standardization of averages of herd-year classes, with positive values composing the high DP class and values equal to and less than zero composing the low DP class. The mixed linear model used to obtain variance component included the fixed effects of contemporaneous group, age of cow at calving as covariate, genetic additive effects, maternal genetic, maternal permanent environment. For models of post-weaning weights (P365 and P450), the maternal genetic effect was removed. Variance components were estimated by the restricted maximum likelihood method and also by Bayesian inference using the Gibbs sampler to obtain a posteriori means. In general, estimates of body weights mean in the situation where the variance heterogeneity was disregarded (general analysis), were intermediate to the estimates observed among the DP classes. The estimates of direct heritability obtained in analyzes without considering variance heterogeneity (general analysis) were equal to 0.17, 0.22, 0.27 and 0.28 for P120, P210, P365 and P450, respectively, indicating a better response to selection in the post-weaning period. Estimates of maternal heritability of 0.09 and 0.07 for P120 and P210, respectively, indicate low accuracy and efficiency in selection for improved maternal ability in the Nellore breed of the Midwest. In general, for body weights in pre and post weaning, ignoring the presence of variance heterogeneity, the ordering of genetic values of the breeders under higher selection intensity, mainly in the pre-weaning phase is more influenced by the class of high DP, which in turn has greater additive genetic variability. When considering all breeders it is not expected changes in the order of them.