Influence of different environmental challenges on the expression of productive traits in Holstein cattle in the southern region of Brazil.

The genotype-environment interaction (GEI) can lead to variations in gene expression related to traits, affecting the breeding value of animals. Assess the effects of GEI on milk yield (MY), fat yield (FY), and protein yield (PY) at 305 days in milk in first-, second and third-parity cows, employing the reaction norms model for Holstein cattle in Paraná state. The study utilized data from the milk testing service provided by the Paraná Association of Holstein Cattle Breeders (APCBRH) in Curitiba, PR, Brazil. This encompassed records from 378,000 records across one to three lactations from 2012 to 2022, originating from 513 herds in 72 cities within the state of Paraná. The environmental gradient was established by standardizing the contemporary group solutions derived from the animal model, disregarding GEI. Reaction norms were calculated using a Random Regression Model, and genotype classification correlations were determined by Spearman's correlation, comparing the breeding values estimated for the analyzed traits in each environmental gradient. Heritability for MY during the first lactation was moderate (0.28) in the least challenging environmental gradient, but of low magnitude (0.18) in the most challenging one. FY heritability estimates varied from low (0.09) to moderate (0.28) across environmental gradients, whereas PY heritability remained low regardless of lactation number and environmental challenge. The study did not identify the occurrence of GEI effects on fat yield, irrespective of parity. No GEI effect was observed on MY or PY in the first and second lactations. However, in the third lactation, GEI affected significantly the MY and PY in Holstein cattle in the state of Paraná, particularly under extreme environmental gradients. The selection for MY, PY, and FY during the first lactation may be the best strategy to promote genetic progress in these traits, because of the smaller effect of GEI at this stage.