Alternative animal models to estimate (co) variance components and genetic and phenotypic parameters for birth weight in Nellore Cattle

The objective of this work was to study the genetic-quantitative aspects of birth weight (BW), in animals of Nellore breed, in the herd from the Nellore Cattle Genetic Improvement Program, conducted by the Department of Genetics of the University of Sao Paulo. Only the weights of sound calves, born alive and from births, obtained by artificial insemination were considered. The (co) variance components were estimated under four different animal models where direct and maternal genetic and permanent environmental were taken into account by including the appropriate random effects, as well as contemporary groups and age of cow at calving effects, in the statistical model. This procedure allowed the quantification of the contribution of different random effects, especially maternal effects. The number of records was 6511; the overall means 31 kg, and the standard deviation 4,1 kg. The direct, maternal and total estimated heritability ranged from 0.20 to 0.37, 0.11 to 0.13 and 0.22 to 0.37, respectively. The correlation's between genetic direct and maternal effects were negative (-0,14 to -0,16). The BW is a trait greatly determined by direct additive genic action, insignificantly affected by maternal phenotype, and with antagonism between the genetic effects. Response to selection for BW would be maximized if based on the parameters obtained by the analysis fitting animal models with direct and maternal effects.