Genotype-By-Environment Interaction for Scrotal Circumference in Young Nellore Bulls Under Pasture and Feedlot Conditions and Its Correlation With Days to Calving.

Scrotal circumference (SC) is a key reproductive indicator in beef cattle and an important predictor of male puberty and correlated female fertility. In this study, we evaluated young Nellore bulls raised under combinations of pasture and feedlot environments and modeled SC using a "double" reaction norm model. Additive genetic variance for SC increased from approximately 1.3 cm2 under the poorest environmental combinations to about 3.5 cm2 under the most favorable conditions, while heritability rose from roughly 0.30-0.33 to 0.60-0.63 across the same gradient. The genetic (co)variance structure indicated that most genetic variation was associated with the intercept, whereas pasture- and feedlot-specific slopes represented smaller yet relevant components of environmental sensitivity. Positive correlations between the intercept and both slopes (0.21 for pasture and 0.50 for feedlot) showed that bulls with higher baseline SC tended to express stronger responses across environments, particularly under feedlot conditions. Conversely, the negative correlation between pasture and feedlot slopes (-0.64) revealed an antagonistic pattern or a trade-off between sensitivities to these two axes. Genetic correlations for SC across environments were predominantly high and positive (≥ 0.80), declining only under the most divergent combinations. Correlations between SC and days to calving were consistently negative, ranging from -0.051 to -0.112 for approximated estimates and from -0.127 to -0.192 for partial estimates. Overall, the "double" reaction norm approach proved effective for dissecting genotype-by-environment interactions and guiding selection strategies aimed at improving reproductive efficiency in tropical beef cattle.