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Genetic groups on selection efficiency for composite beef cattle (Bos taurus x Bos indicus) The inclusion of genetic groups in sire evaluation has been widely used to represent genetic differences among animals not accounted by the absence of parentage information. However, the definition of these groups is still arbitrary, and research assessing the effects of genetic grouping strategies on the selection efficiency is rare. The aim of this study was to compare genetic grouping strategies in breeding values prediction, determining the appropriate structure for the genetic evaluation. Data on birth weight, weaning weight, post-weaning weight gain, scrotal circumference and muscling score of Montana Tropical composite beef cattle population were used. Grouping strategies involving birth season of the animal, sex of the unknown parent, birth farm of the animal, path selection and breed composition were evaluated. For each approach, simulating a loss of genealogy information, analyses were performed to predict breeding values. Thereafter, these breeding values were compared to those obtained in an analysis involving the complete relationship matrix, in order to estimate the selection efficiency and the correlations between breeding values and animal rankings. The analysis model included the fixed effects of contemporary groups and class of the dam age, the covariables related with the direct and maternal additive and non-additive genetic effects and age, and the additive random effect of animal. A second model included, additionally, the fixed effect of genetic groups. The use of genetic groups resulted in means of selection efficiency and correlation of 66.0% to 94.9% and 0.53 to 0.94 for birth weight, 70.4% to 97.1% and 0.51 to 0.94 for weaning weight, 73.5% to 97.8% and 0.60 to 0.96 for post-weaning weight gain, 85.8% to 98.8% and 0.82 to 0.98 for scrotal circumference, and 85.1% to 98.6% and 0.74 to 0.97 for muscling score, respectively. The best results were associated with the strategies of birth season and breed composition. High selection losses were observed when complex grouping criteria were adopted, possibly due to the formation of many groups with few animals, since this could lead to some confounding with other fixed effects and hamper the estimation effects of genetic groups. It is conclude that the definition of grouping strategy must consider the decisions regarding the selection and the number of genetic groups formed, so that genetic groups represent the genetic differences in population and allow an adequate prediction of breeding values.

Grupos genéticos na eficiência de seleção de bovinos de corte compostos (Bos taurus x Bos indicus)