Reaction norm analysis reveals environmental sensitivity and genomic architecture of postweaning daily weight gain in Angus–Brangus cattle

Genotype-by-environment interaction ( G×E ) plays a central role in livestock systems characterized by environmental heterogeneity, particularly under (sub)tropical conditions. In such contexts, the assumption of homogeneous genetic expression underlying conventional evaluations may not hold, limiting selection efficiency. Despite the widespread use of Angus–Brangus cattle in Brazil, the characterization of environmental sensitivity for growth traits and its genomic basis remains limited. Therefore, this study aimed to evaluate environmental sensitivity for postweaning average daily gain ( ADG ) at both animal and genomic levels using a reaction norm ( RNM ) approach, quantify genetic variation in responsiveness, and assess the predictive performance of RNM relative to the conventional animal model ( AM ). Data from Angus–Brangus cattle raised under pasture-based systems in Brazil were analyzed using a linear RNM based on a continuous environmental gradient derived from contemporary group solutions. Additive genetic variance for ADG ranged from 6.80 (SD = 0.55) in intermediate environments to 169.29 (SD = 5.75) in favorable conditions, while heritability increased from 0.08 (SD = 0.01) to 0.66 (SD = 0.01). Genetic correlations declined with increasing environmental distance, reaching -0.73 (SD = 0.02) between extreme conditions, indicating substantial re-ranking of sires across environments. Reaction norms revealed both plastic and robust genotypes, with 76% of sire pairs exhibiting crossing points of their reaction norms, predominantly in unfavorable environments. In linear regression validation, RNM outperformed AM in restrictive conditions, whereas AM performed better in intermediate and favorable environments. Genome-wide association analyses identified 12 significant single nucleotide polymorphisms for intercept and 8 for slope, with a major locus on BTA20 (∼5 Mb) harboring STC2, and additional candidate genes including SLC25A24, FCHO2, and TNPO1. Marker effects varied across the gradient, with distinct patterns for intercept and slope. ADG in Angus–Brangus cattle shows strong environmental dependence, with substantial G×E affecting both genetic parameters and genomic architecture. Reaction norm models improve prediction under unfavorable conditions and enable identification of genotypes with differential environmental sensitivity. The detection of shared and parameter-specific loci indicates partial genetic decoupling between baseline performance and adaptability. These findings support incorporating environmental sensitivity into genetic evaluation and selection strategies under heterogeneous (sub)tropical conditions.