Breeding for improved heat tolerance in dairy cattle: Methods, challenges, and progress.

Heat tolerance in dairy cattle has likely deteriorated over time due to unfavorable genetic correlations between milk production level and heat tolerance, with stronger deterioration in later parities. The dairy cattle industry has responded to the challenge of heat stress by implementing management modifications. Methodology exists to calculate genetic prediction of breeding values for high production under heat stress conditions, with high reliabilities of breeding values obtained when incorporating genomic information. However, cows that maintain production during the heat stress peak have an increased likelihood of death. One remedy would be selection for better fertility and survival under heat stress, but with a low volume of data and low heritabilities, corresponding reliabilities may be too low for an efficient selection. In environments where intensive management is too expensive, an ideal cow would maintain production in a favorable climate, would briefly reduce production during heat stress, and would restore production after the heat stress conditions are over. As there are many biological mechanisms involved in heat stress response, in addition to deriving heat tolerance indicators based on variability in performance traits under heat stress conditions, novel traits that capture physiological, behavioral, and anatomical traits related to heat stress response, less dependent on production level, could also contribute to breeding for improved heat tolerance.