The critical point of temperature humidity index and Holstein fraction level affected on milk production and genetic parameters during heat stress conditions

Abstract

The investigating of critical point of heat stress affecting milk production and genetic parametersin crossbred Thai-Holstein dairy cattle using a total of 218,083 first-parity milk yield test-day records of 24,825Thai-Holstein crossbred. The results showed that the impact of heat stress began at a temperature humidity index(THI) of 75. The rate of decline of milk production was found when Holstein fraction level was more than 87.5%,and highest when Holstein fraction level was more than 98%. Cow in the North East region had highest average milkproduction loss (91.05 kg/head/year) compared to another region in Thailand. The heritability for test-day milk ata THI of 75 was 0.17. Heritability for milk yield exhibited a slight increase from THI of 75 to 78 that is the highestheritability (0.19) and tends to decline as THI increases after this point. Genetic correlation between regular and heatstress effect was -0.21. This result indicated that selecting cows for high milk yield leads to an increase in heatstressed cows. However, the genetic correlation from this study was low, therefore combine selection for milkproduction and heat tolerance is possible in the breeding program.