Genetic parameter estimates for milk yield and milk fat content in Gyr cows

Genetic parameters are tools broadly used in animal breeding programs to evaluate population genetic and phenotypic profiles. The aim of this study was to estimate genetic parameters for milk yield and milk fat content (%) in Gyr cattle. The data set was provided by Brazilian Association of Zebu Breeders. The analyses included 36,760 milk yield and 7,781 records of fat content (%) from 23,884 Gyr and Polled Gyr breeds. Restricted maximum likelihood (REML), under an animal model, was used to estimate variance components and predicted transmitting ability (PTA). The model included the additive genetic, permanent environmental and residual random effects, the fixed effects of contemporary group and the age at calving as covariate (linear and quadratic effects) for both traits. Contemporary groups were defined as animals from the same herd, year and season of calving. Genetics trends for milk yield was obtained from the linear regression analysis of PTA average breeding values according to birth year. The mean, standard deviation and coefficient of variation were 3.442 kg, 1.662 kg, 48.3%, respectively, for milk yield and 4.44 %, 0.73%, 16.4% for fat content (%). The heritability and stander errors estimates were 0.17 ? 0.01 and 0.23 ? 0.03 for milk yield and fat content (%), respectively. Genetic trend estimated for milk yield was 0.245 kg/year (P<0.001), resulting in a genetic progress of 473.22 kg from 1979 to 2011. Phenotypic change in the same period was 2,095.77 kg of milk. Heritability estimates for milk yield and fat content (%) in Gyr cattle were similar to those described for other breeds. Selection applied for milk yield in Gyr cattle increased significantly the average genetic merit for this trait over the last years.