Novel significant associations between temperature-humidity index and HSP70 g.1117G>A with milk yield traits of Holstein-Friesian cows reared in semi-arid regions

This study aimed to determine the relationships between the HSP70 g.1117G > A gene and some milk production traits such as daily milk yield (DMY), conductivity (Cond), milking time (MT) and activity (Act) in Holstein-Friesian dairy cattle. Blood samples from the coccygeal vein of 203 Holstein-Friesian cows in different lactations were collected. The population, with A and G allele frequencies of 0.2365 and 0.7635 and AA, AG, GG genotype frequencies of 0.089, 0.295, and 0.616, was not in Hardy-Weinberg equilibrium (P < 0.01). The lowest DMY was in the THI<65 group, while the highest was in the 65≤THI<68 and THI≥68 groups. Regarding temperature-humidity index (THI), the lowest MT and Act values were recorded in the THI<65 group, while the highest were in the THI≥68 group. For DMY and Cond, the highest values were in AG and the lowest were in AA genotype cattle (P < 0.01). Regarding MT, the highest values were observed in the AA and AG, with the lowest in GG genotypes (P < 0.01). For Act, AG and GG genotypes showed the highest values, while AA had the lowest (P < 0.01). As a result, the G allele (sensitive to heat stress) could be beneficial for both increasing DMY and reducing MT. However, the threshold values for the THI should be re-evaluated based on the effectiveness of the cooling systems used and the specific conditions of the farm.