A polymorphism within the prolactin gene is associated with milk production in Holstein dairy cows managed under summer heat stress conditions in northwest Mexico

Abstract Text: Holstein dairy cows managed in northwest Mexico are exposed during summer to extreme ambient temperature and humidity that lead to heat stress. The physiological response from cows exposed to such weather conditions results from the perturbation of a gene network related to heat stress homeostasis. The prolactin signaling pathway has been proposed as an important mediator of this response in cattle. The objective of this study was to assess the association between a SNP polymorphism (rs110494133-A/G within intron 1) in the prolactin gene (PRL) and performance traits such as average and total milk production in Holstein cattle. The SNP allele frequency was 70% for A and 30% for G. No deviations from Hardy-Weinberg equilibrium (X² = 1.00, P > 0.38) were observed in the cow population. DNA was extracted from blood spotted on FTA cards from 118 cows, and genotyped using the sequenom massarray platform. Genotype to phenotype association analyses were conducted using a mixed effects model that included phenotype as the response variable, genotype as a fixed term, sire as a random term, and days in milk as a covariate. Mean values for milk production, serum prolactin, and rectal temperature were 22.1 ± 0.5 kg/d, 32.2 ± 1.2 ng/mL, and 38.3 ± 0.1°C, respectively. In a previous study, we reported that reduced serum levels of prolactin and increased rectal temperature were associated with lower milk performance, which was remedied by spray cooling the cows. In this study, the genotype term was as a significant (P < 0.05) source of variation in predicting milk production. Least square means among genotypes AA, AG, and GG were 24.6 ± 1.3, 21.2 ± 1.5, and 21.2 ± 1.7 kg/d for average milk production, and 7,403 ± 13.8, 7,397 ± 10.5, and 6,241 ± 61.8 kg for total milk production, respectively. The A allele from the SNP in the PRL gene was the most favorable (P < 0.05) and increased average milk production (1.2 ± 0.3 kg/d) and total milk production (325 ± 63.5 kg). When the mixed model included serum prolactin or rectal temperature as the response variable instead of milk traits, the genotype term also resulted as a significant predictor (P < 0.05). We conclude that a SNP within the prolactin gene appears to be a predictor of lactation performance in Holstein dairy cows managed under summer heat stress conditions common to northwest Mexico.

Keywords: heat-stress, prolactin and polymorphism.