Sustainability in livestock production system is largely affected by climate change. An imbalance between metabolic heat production inside the animal body and its dissipation to the surroundings results to heat stress (HS) under high air temperature and humid climates. The foremost reaction of animals under thermal weather is increases in respiration rate, rectal temperature and heart rate. It directly affect feed intake thereby, reduces growth rate, milk yield, reproductive performance, and even death in extreme cases. Dairy breeds are typically more sensitive to HS than meat breeds, and higher producing animals are, furthermore, susceptible since they generates more metabolic heat. HS suppresses the immune and endocrine system thereby enhances susceptibility of an animal to various diseases. Hence, sustainable dairy farming remains a vast challenge in these changing climatic conditions globally.

Heat shock proteins (HSPs) are expressed in response to heat stress, and the polymorphism in HSP genes at single-nucleotide level has been reported to be associated with heat tolerance and production performance traits in cattle. HSPB8 gene has been mapped on Bos taurus autosome 17 (BTA-17) spanning nearly 13,252 bp and comprising three exons and two introns. The present study was conducted in Sahiwal cows (n = 108) reared in subtropical climate with the objectives to identify SNPs in all three exons and part of intron 1 of HSPB8 gene and to analyze their association with heat tolerance traits in Sahiwal cows. Respiration rate (RR) and rectal temperature (RT) were recorded once during probable extreme hours in different seasons or Temperature-Humidity Index (THI), i.e., winter, spring, and summer. Heat tolerance coefficient (HTC) was also calculated to check the adaptability of the animals during the period of heat stress. The comparative sequence analysis revealed a total two single-nucleotide polymorphisms (SNPs), i.e., g.507G>A in exon 1 and g.881T>C in intron 1 of HSPB8 gene. Out of these two identified SNPs, only one SNP, i.e., g.507G>A, was found to be significantly associated with heat tolerance indicator traits (RR, RT, and HTC) in Sahiwal cows. The perusal of results across different seasons showed the significant (P < 0.01) difference in RR, RT, and HTC between winter, spring, and summer seasons. RR, RT, and HTC were found to be significantly lower (P < 0.01) in GA as compared to GG genotype of g.507G>A SNP of HSPB8 gene. However, in case of another SNP, i.e., g.881T>C, located on intron 1, the RR, RT, and HTC were having non-significant association with the different genotypes, i.e., TT and TC. These findings may partly suggest that GA genotype of SNP g.507G>A of HSPB8 gene has a probable role in heat tolerance in Sahiwal cattle and can therefore be utilized as a marker in propagation of thermo-tolerance cattle in hot tropical and subtropical climate. Nevertheless, the involvement of other regulatory mechanisms cannot be overruled.

Expression of stress genes under thermal stress partially explain the relatively higher thermal adaptability of indigenous breeds compared to exotic breeds of cattle. ATP1A1 gene is one of such gene that encodes for a1 isomer of Na+, K+- ATPase enzyme for Na+- coupled transport of metabolites, nutrients, ions and represents a plausible candidate for heat tolerance traits. Present study was designed to compare SNP variations in ATP1A1 gene and to evaluate their association with respiration rate, rectal temperature and heat tolerance coefficient in Sahiwal (51) and Karan Fries (50) cows maintained at Livestock Research Centre of National Dairy Research Institute, Karnal. Two SNPs (T27008243C and A27008223G) were identified in both the dairy cattle breeds. Association of each SNP genotype was analyzed using Generalized Linear Model procedure in Statistical Analysis System (SAS). Sahiwal cows with TT (15.91±1.89b) at T27008243C locus had lowest RR compared to TC genotypes (18.25±1.77ab) and CC genotype (19.24±1.52a), while in Karan Fries cows RR for AA genotype at A27008223G locus was lower (28.85±1.96b) compared to GG genotype (32.37±2.51a). Thus, the study indicated that the TT genotype at T27008243C locus in Sahiwal and AA genotype at 27008223 locus in Karan Fries cows are desirable genotypes for genetic adaptability under heat stress.