PSIV-10 Polymorphisms in the Prolactin and GH-IGF1 Pathways Associated with a Heat Tolerance Indicator in Dairy Cows under Heat Stress in Sonora Mexico.

Heat stress greatly influences lactating Holstein cattle in southern Sonora Mexico, where the temperature-humidity index (THI) easily exceeds the cow thermal comfort threshold (>72 THI units) from May through October. Despite such warm conditions, some dairy cows are able to maintain their milk production. Using a regression model, daily milk production as a function of daily THI was used to estimate a heat tolerance indicator (HTI) value. The objective of this study was to identify molecular markers associated with HTI in Holstein cows milked during summer. The study included 676 cows from three dairies located in the Yaqui Valley of Sonora Mexico. An individual blood sample was collected and spotted onto FTA cards, from which DNA was extracted and used to genotype 179 tag SNP within 43 genes of the Prolactin and GH-IGF1 pathways. The association analyses among SNP genotypes and HTI values were performed using a mixed effects model for continuous traits. The model included HTI as the response variable, genotype and herd as fixed effects, sire as a random effect, and days in milk as a covariate. Two SNP, one within the Prolactin Receptor gene (rs135164815) and one within the Signal Transducer and Activator of Transcription-4 gene (rs110344022) were associated with HTI (P < 0.01). Least square means for the rs135164815 genotypes AA, AG and GG were 0.365 ± 0.06, 0.217 ± 0.04 and 0.201 ± 0.03 liters/day, respectively; while least square means for the rs110344022 genotypes CC, CT and TT were 0.296 ± 0.05, 0.235 ± 0.04 and 0.183 ± 0.04 liters/day, respectively. In conclusion, two SNP within the Prolactin and GH-IGF1 pathways were predictors of HTI in lactating Holstein cows during the months of summer heat stress. Such results provide evidence of a genetic basis associated with favorable lactation performance under increasing THI during summer.