Variabilidade genética e endogamia na população Guzerá sob seleção para produção de leite

Abstract

Animals of the Guzerat breed have strongly fitted to tropical conditions of Brazil, and have been used for milk and beef production. The probability of inbreeding and genetic drift increases due to reduction in the effective population size. In 1994, a selection program for milk production traits was initiated in some purebred herds, using progeny test and MOET nucleus. Its success, otherwise, can be endangered by increase in inbreeding coefficient (F) and loss of genetic diversity, since animals with high genetic value have more chance to reproduce. This study was undertaken to evaluate the evolution and the current genetic status of the Guzerat cattle under milk selection, aiming to monitor the genetic variability and guide matings. Genealogical data of 10,051 animals born until 2007 were used to estimate population parameters. The average F for all inbred animals included in this data set was 0.025 and the increase of inbreeding by unbalancing of founders contribution was 0.16%. The average relationship coefficient was 1.06% and the generation interval was 7.48 years. The mean effective population size by generation was 77, the effective number of founders and effective number of ancestors for the reference population were, respectively, 318 and 101. The bottleneck effect was 3.15. Of 2,106 ancestors, 47 contributed to 50% of the reference population. Total milk production, milk production at 305 days of lactation, total fat production, total protein production and age at first calving was negatively affected (p<0.05) by the increase in F. Average F values and regression coefficients are still low, but the reduced effective population parameters indicates risk of inbreeding, genetic drift and, consequently, loss of variability in the Guzerat breed under milk selection in Brazil.