Genotype-environment interaction for growth curve parameters in Nellore cattle using reaction norm models

Growth curve parameters in cattle are influenced by genetics and environmental factors, as well as their interaction. Therefore, assessing genotype-environmental interaction (GxE) becomes essential to ensure the effectiveness of breeding programs. The objective of this study was to evaluate the GxE effects on the asymptotic weight (A) and maturity rate (K) parameters from the growth curve of Nellore cattle. Data from 240,800k pasture-raised animals was provided by the Brazilian Association of Zebu Breeders for the estimation of the parameters. Five models were used: standard animal model (AM), which disregards GxE, and four hierarchical reaction norm models (HOHRNM_1S, HOHRNM_2S, HEHRNM_1S and HEHRNM_2S). The HOHRNM_1S model showed the best fit for both parameters. Heritability estimates of 0.45 and 0.47 for A and K, respectively, were obtained with the AM. With the HOHRNM_1S, the heritability increased as the environmental conditions improved, ranging from 0.24 to 0.82 (A) and from 0.34 to 0.87 (K) along the environmental gradient. High correlations (0.95 for A and 0.88 for K) between the intercept and the slope of the reaction norms, suggest that genetically superior animals respond better to environmental improvements. In general, sires were robust to environmental variation, but GxE effects were evident in those with the highest number of progenies, presenting a scale effect on A and a complex effect on K. Thus, including the GxE effects on the genetic evaluations of parameters A and K is recommended. Moreover, using plastic sires can support environmental-specific selection, improving the efficiency of the Nellore breeding programs in Brazil.