Heat stress on dairy cattle

Climate change significantly impacts the production, health, welfare, and survival of farm animals. The prolonged heat stress is a major concern in tropical and subtropical regions. Heat tolerance enables livestock to maintain thermal balance under extreme climatic conditions, thus ensuring productivity and animal welfare. There is a feasibility of genetic selection for heat tolerance to improve production and reproduction traits. This genetic selection may provide enduring advantages in maintaining productive and reproductive performances as heat stress is a heritable trait. To identify and integrate traits associated with thermal tolerance into breeding programs, it is imperative to have a robust animal performance records and meteorological data. Temperature humidity index (THI) is a widely used biomarker for assessing heat stress in dairy cows, as it combines air temperature and relative humidity. In regions with sparse weather station coverage, NASA POWER satellite data is a viable alternative for obtaining meteorological information to calculate THI and evaluate heat stress impacts. There are multiple models for calculating THI and those focusing on humidity being more effective in humid climates. Genetic models, particularly reaction norm models based on random regressions, were introduced for estimating genetic parameters for heat tolerance in cattle. These models help to understand genotype-environment interactions and non-linear relationship between THI and milk yield reduction. The continued genetic selection for milk production without considering heat tolerance traits increases susceptibility to heat stress, emphasizing a need for comprehensive selection objectives. This review underscores the importance of incorporating heat tolerance traits into genetic selection for dairy cattle and highlights various biomarkers and genetic models used to estimate heat tolerance. Incorporating these traits into breeding goals is essential to improve thermotolerance while maintaining productivity, bolstering resilience of cattle to climate change.