Genetic evaluation for scrotal circumference in Guzerat cattle through different models

The objectives were to obtain estimates of genetic parameters and the genetic trends and to evaluate the direct responses and those correlated to the selection for scrotal circumference (SC) measured at different ages through random regression models (RRM) and multi-trait models (MM). A total of 14,200 information of SC from 4065 Guzerat individuals, with age ranging from 200 to 550 days, were used in the analyses. Both RRM and MM were utilized considering contemporary groups, as the fixed effect, and age of the mother at calving, as the covariate. For RRM, the direct additive genetic and the permanent environment effect of the animals were modeled by Legendre polynomials, with an order of fit ranging from 2 to 5. The residual variances were modeled considering variance homogeneity, and heterogeneity assuming 2, 3, and 4 classes. The results obtained by the RRM fitted better when compared with those obtained by the MM. The model that considered two classes of residual variances and polynomials of third order for the additive genetic and the permanent environment effects was the one that best fitted the scrotal circumference data according to the age based on the Bayesian and the Akaike Information criteria. Scrotal circumference heritability using RRM ranged from 0.42 (200 days of age) to 0.60 (364 days of age) and presented a low reduction from 365 until 550 (0.55) days of age. Correlation estimates for SC at the distinct ages were high (ranging from 0.66 to 0.98) and presented a reduction as the age intervals increased (0.66 +/- 0.05 for SC measured at 205 days of age vs. SC measured at 550 days of age). Genetic trends of scrotal circumference were positive (ranging from 0.048 [SC measured at 205 days of age] to 0.101 [SC measured at 550 days of age]) and significant (P < 0.0001). Using the RRM, an increase in selection response were obtained compared to MM. Based on the results, we conclude that genetic evaluation of scrotal circumference can be performed at 365 days of age utilizing the RRM.