Genetic associations between growth traits and age at first calving in Guzera cattle using random regression models

Abstract

The aim of this study was to estimate genetic associations between body weight (BW), scrotal circumference (SC), and age at first calving (AFC) in Guzera cattle using random regression models. The analyses used 159,366 and 23,780 records of BW and SC obtained between 335 and 724 days of age and divided into classes at intervals of 10 days, as well as 63,596 records of AFC. A multitrait animal model via Gibbs sampling was adopted that included random direct additive genetic, permanent environmental (only for BW and SC), and residual effects. The fixed effects considered were the contemporary group, age of cow at calving (linear and quadratic effect), and the fixed curve to model the average trend of the population on the age classes of the animals. For BW and SC, four models were compared using the deviance information criterion, with Legendre polynomials ranging from one to four coefficients (intercept, linear, quadratic, and cubic) for additive genetic and permanent environmental effects. The model with three coefficients (quadratic) was considered the most appropriate to describe the (co)variance structure of the traits analyzed. The mean heritability estimates ranged from 0.17 to 0.31 for BW and from 0.19 to 0.28 for SC across age classes. The mean heritability for AFC was 0.07. The genetic correlations between BW and SC across age classes were always positive and favorable, ranging from 0.33 to 0.39. These findings indicate that selection for higher BW should result in animals with greater SC. The genetic correlations between BW at different ages and AFC ranged from -0.56 to -0.38, while those between SC at different ages and AFC ranged from -0.55 to 0.08. Except for some genetic correlations close to zero and positive between SC close to 365 days of age and AFC, in general the results showed that the selection for either higher BW or greater SC should result in a reduction of AFC in Guzera cattle. Calculation of the relative selection efficiency indicated a greater correlated response in AFC by selection for SC, especially after 634 days of age, compared to direct selection for AFC.