The impacts of thermal stress on dairy cattle physiology, metabolism, health, and performance: a comprehensive review

Climate change is becoming a global issue, with important implications for dairy cow performance and well-being. It is distinguished by a gradual rise in universal temperature and the risk of extreme weather occurrences. Studies have shown that heat stress (HS) impacts many biological processes that can have significant economic issues. Due to their elevated metabolic rate, cows are mostly liable to HS, which negatively affects immune function, particularly cell-mediated immune response, and subsequent reduced production performance and inferior immunity, which leads to elevated susceptibility to disease, increased incidence of intramammary infections, and an elevated somatic cell count, as well as calf mortality, particularly during the summer season. Furthermore, dry cows subjected to HS had reduced immunoglobulin levels after vaccination, although this impact fades with cooling after parturition. On the other hand, cows subjected to HS while dry demonstrate carryover impacts on the innate arm of the immunity in early lactation, resulting in losses. Heat mitigation technologies are cost-effective and necessary for sustaining milk production and the dairy farm's profitability. Furthermore, a check of present HS mitigation measures is required to understand better and identify acceptable abatement plans for future stress management.