Reaction norms of direct and maternal effects for weight at 205 days in Polled Nellore cattle in north-eastern Brazil

Abstract. This study investigated the presence of genotype-environment interaction (GEI) for body weight adjusted to 205 days of age (W205) in Polled Nellore cattle raised in north-eastern Brazil using reaction norm (RN) models. The reaction norm hierarchical models (RNHM) included the fixed (linear and quadratic) effect of cow’s age, random effect of contemporary group (CG), RN level and slope for additive direct and maternal genetic effects and permanent maternal environmental effect. The one-step reaction norm model with homogeneous residual variance (RNHM_1sHm) yielded the best adjustment compared to the others. Based on this model, the estimates of direct additive and maternal variances and increased with environment improvement (35.34±7.92 kg₂ to 134.42±25.97 kg₂ and 12.76±5.38 kg₂ to 58.22±19.74 kg₂ for low and high-quality environments, respectively). The estimates of heritability direct additive and maternal too increased with environmental improvement (0.08±0.02 to 0.24±0.04 and 0.03±0.01 to 0.10±0.04). The correlation between the intercept and the slope of RNHM indicates that animals with higher genetic values respond more efficiently to environmental improvements, representing a scale effect for W205. These results allowed us to characterize the GEI for W205 and showed that specific evaluation should be performed with low, intermediary and high production levels.