Genetic parameters for age at first calving estimated by different models for Nellore breed herds

With the objective of estimate genetic parameters and study the utilization of different effects on the genetic evolution for the age of the first calving (AFC) by different models, they were used AFC records front Nellore breed animals, born from 1990 to 2005. They were considered the following models (M): M1, including the fixed effect of the contemporary group (CG), CG1 (grouping the animals that were born at the same farm and year), besides the co variable, weight at 365 days of age (linear and quadratic effects), totalizing 24,263 records of AFC; M2, considering the effects of CG1, year and season of the calving, totalizing 59,792 records of AFC and M3, including effects CG2 (grouping the animals born at the same farm, year and submitted at the same reproductive management), year and season of calving totalizing 59,792 records of AFC. The components of variance and Genetic Value (GV) were obtained by Restricted Maximun Likelihood Method, with the inclusion for the relationship matrix. The Differences Expected on the Progeny (DEPs) were obtained by dividing the GV by two, after this they were estimated the correlations between GV and ranks of the reproductor's DEPs for AFC, utilizing the procedure CORR (SAS, 2003). While considering yea; and season of calving in the analysis models (M2 and M3), it was produced a bigger R indicating that those models could explain, in a larger scale, the existing differences between the animals for AFC. The heritabilities estimated were of low magnitude (0, 14 and 0, 15). The correlations between the GV obtained by the different models were .73 (M1 x M2); .91 (M2 x M3) and .66 (M1 x M3).