Modelos de avaliação genética para resistência ao carrapato em uma população multirracial angus-nelore

Abstract

Tick combat, inadequate sanitary management and high prices for acaricides are one of the main problems faced in Brazilian cattle system. The low perspective of new products to combat this ectoparasite, more efficient and inexpensive to be launched in the market has stimulated the accomplishment of researches with the objective to identify accurately and to select genetically resistant animals. The purpose of this work was to evaluate ome animal model that best describes the genetic and residual variation for the characteristic counting of ticks at post weaning, in order to produce more accurate genetic predictions. The database used consisted of 6,951 animals, sired by 383 bulls and 6,199 cows from a multiracial Angus-Nelore population raised on farms in Brazil. The database was provided by Gensys Consultores Associados S / C Ltda and Natura Genetica Sul Americana. Genetic values were calculated from the Bayesian inference methodology and the models tested were: Traditional Animal Model, Multibreed Animal Model with and without segregation, both with two models of residual variance (homocedastic or heterocedastic). The criteria of choice were the Number of Parameters, the Deviance Information and the Predictive Order, which pointed to the Traditional and Multibreed Animal Models with Segregation, both with residual heteroscedastic Gaussian variance, such as those that presented lower number of parameters and better distribution of the additive and non-additive genetic variances and, therefore, provided the best fit. The mean values of the fixed genetic effects were positive and similar, that is, animals with a higher proportion of the Angus breed, suffered a greater infestation of ticks, suggesting that the Nellore breed has an important role for resistance to ticks. It was verified that the genetic variance estimated by the heterocedastic Gaussian Animal Model for the Nellore breed was 4.54 times higher than that estimated for the Angus breed. Estimates of heritability in the different genetic groups ranged from 0.12 to 0.15 and 0.01 to 0.35 in the Traditional Animal Model and in the Heterocedastic Gaussian Multiracial Model, respectively. The Spearman ranking correlation among predicted genetic values considering all sires in the population was 0.94. However, when considering only the better bulls, TOP 10%, 20% and 30%, differences in relation to the ranking were more evident (0.28 to 0.67). The Multiracial Animal Model with Segregation with heterogeneous residual variances is most appropriate to be used in the genetic evaluation of the characteristic tick count of animal products of the Angus-Nellore cross.