Effect of Heat Stress on the Health and Performance of Dairy Cattle: A Comprehensive Review

Heat stress (HS) is a concerning issue for the health and performance of cattle and is manifested by various behavioral and physiological changes. HS leads to a decrease in dry matter intake, rumen activity, growth performance, and milk yield in animals. Extreme HS for at least three consecutive days increases the risk of death. HS also causes a decline in the ruminal microbiota, pH, and acetate concentration, and an increase in lactate-producing bacteria and the lactate concentration. Reduced feed and water intake lead to a decline in milk production. The milk protein and casein content go down due to insulin resistance and apoptosis of mammary epithelial cells caused by oxidative stress, but urea concentrations become higher. Milk’s metabolomes, glycine, proline, isoleucine, and phosphocreatine have been found to decrease, while citrate, acetone, and β-hydroxybutyric acid concentrations evidently increase. Blood cell count, glucose, protein, and IgG concentrations also fall due to HS. Short estrus, impaired follicular development, poor quality oocytes and spermatozoa, a reduced conception rate, and decreased semen quality have become very common. For all of this, the present study demonstrates the various effects of HS on cattle health, production, and reproduction.