Estimativas de componentes de (co)variância e parâmetros genéticos para medidas ultrassonográficas de bezerros da raça Nelore

Abstract

The use of carcass ultrasound measures and the simultaneous evaluation of genetic, maternal, and environmental effects are essential to improve selection accuracy and increase the productive efficiency of Nelore herds. The objective of this study was to develop the most appropriate statistical model and to estimate genetic parameters for body weight and carcass traits assessed by real-time ultrasound at weaning in Nelore beef cattle. The study was conducted with 2.001 Nelore calves born between 2015 and 2025 at Nelore Birigui Farm, Bela Vista, Mato Grosso do Sul, Brazil. The traits analyzed included weaning weight adjusted to 210 days (W210), ribeye area (REA), marbling score (MARB), and back fat thickness (BF), measured by ultrasonography in animals aged 7 to 8 months. To identify non-genetic factors influencing the studied traits, analysis of variance was performed using the GLM procedure, considering as fixed effects of sex, recipient breed, month and year of birth, and calf age and dam parity order as linear and quadratic covariates. As contemporary groups and covariates were defined, three models were used for univariate and multivariate genetic analyses: (1) including only the direct additive genetic effect; (2) adding the maternal additive genetic effect; and (3) incorporating, in addition to the previous effects, the maternal permanent environmental effect. Variance and covariance components, as well as genetic parameters and genetic and phenotypic correlations, were estimated using Restricted Maximum Likelihood Estimation. All traits (W210, REA, MARB, and BF) were influenced by direct, maternal, and environmental genetic effects. The univariate and multivariate analyses indicated moderate direct heritabilities for REA (0.31–0.34), MARB (0.24–0.25), and W210 (0.17–0.22), and low heritability for BF (0.08–0.11). Maternal heritabilities ranged from 0.08 to 0.25, representing up to 95% of the corresponding direct heritabilities. Direct additive genetic correlations among traits were positive and of low to moderate magnitude (0.22 to 0.70), suggesting favorable associations. The inclusion of maternal genetic and permanent environmental effects improved model fit and reduced residual variance. The multivariate analysis increased the precision of estimates and demonstrated that selection for REA and BF may result in correlated genetic gains in MARB and W210. Maternal genetic and permanent environmental effects should be included in genetic evaluations of Nelore cattle for weaning weight and carcass traits. The multivariate analyses proved to be appropriate, and the moderate genetic correlations indicate that carcass ultrasonography has potential as a selection criterion at weaning in Nelore cattle.