PSII-15 Validation of polymorphisms as molecular markers for milk production and thermo-tolerance in Holstein cows managed in a heat-stressed environment.

Dairy farms located in semi-arid regions face significant challenges in maintaining milk production under a heat-stressed environment. Despite these adverse conditions, some cows are able to sustain adequate milk production, most likely due to genetic factors. Then, objective of this study was to validate three single nucleotide polymorphisms (SNPs) as predictors of milk production and thermo-tolerance in heat-stressed Holstein cows. The study was conducted during summer (June-August) at a dairy farm located in the Yaqui Valley, Mexico. A total of 148 lactating Holstein cows were included, housed in open pens, and provided a ration to meet their nutritional requirements. Milk production was measured twice daily (05:00 an 15:00 hours) through an electronic system, and used to determine the average daily milk yield (DMY). Ambient temperature and relative humidity were recorded with a Kestrel weather meter (DROP D2AG), and used to calculate the temperature-humidity index (THI). Physiological traits, including rectal temperature (RT) and respiratory rate, were measured biweekly (06:00 and 16:00 hours). A blood sample was collected from each cow for genotyping of the SNPs rs8193046, rs43410971, and rs382039214, located within the genes TLR4, GRM8, and SMAD3, respectively. The TaqMan method and real-time PCR were used for genotyping, whereas mRNA expression of these genes was measured through quantitative PCR. Additionally, a blood sample was drawn to analyze serum levels of cortisol and heat shock protein-70 (HSP-70). The THI increased from 76 to 83 units, indicating that heat stress intensified from moderate to severe during the study period. The SNPs in the genes TLR4 and GRM8 were found to be predictors of DMY and RT. Their favorable genotypes (AA and GG, respectively) showed the higher DMY (22.67 ± 1.02 and 23.21 ± 0.89 liters; P< 0.05)) and the lower RT (37.82 °C and 37.65 °C; P< 0.05). The mRNA expression of the genes TLR4 and GRM8 were higher (P< 0.01) in cows with favorable genotype (6.44 and 4.98, respectively) compared to cows without favorable genotype (3.25 and 2.21, respectively). Serum levels of cortisol and HSP-70 were lower (P< 0.05) in cows with favorable genotype (19.75 ± 4.06 ng/mL and 38.65 ± 7.95 pg/mL) compared to cows with non-favorable genotypes (28.64 ± 4.73 ng/mL and 46.38 ± 8.12 pg/mL). Gene expression was strongly correlated (r2=-0.63 to -0.78; P< 0.01) with serum levels of cortisol and HSP-70, which are physiological markers of heat stress. In conclusion, our findings suggest that the genes TLR4 and GRM8 play a critical role in regulating physiological mechanisms associated with thermo-tolerance that contribute to sustain milk production in cows affected by environmental heat stress. Thus, SNPs in these genes may serve as reliable predictors of milk production and thermo-tolerance in Holstein cows managed in warm semi-arid regions.