Effect of heat stress on milk yield and somatic cell counts in dairy cows

Heat stress (HS) is a major constraint in dairy production, disrupting metabolic regulation, mammary function, and milk synthesis, thereby limiting the productivity, health, and sustainability of lactating cows. This narrative review synthesizes current knowledge on HS effects, including reduced dry matter intake (DMI), endocrine and metabolic imbalances, and altered nutrient utilization. It highlights impacts on mammary function, milk yield, immune response and somatic cell count (SCC), with emphasis on carryover effects from the dry period into subsequent lactation. The narrative review indicates that HS reduce DMI and milk yield, milk composition, while SCC may increase under HS depending on breed, climate, and management. Heat stress disrupts hormonal signaling, including growth hormone, insulin and prolactin, redirecting nutrients from milk synthesis toward thermoregulation. It also induces oxidative stress and epithelial damage in the mammary gland. Exposure during the dry period further impairs mammary development, shortens gestation length, reduces fetal growth and negatively affects calf performance, ultimately lowering milk yield in the next lactation. The threshold temperature-humidity index (THI) values associated with these effects typically range from 59 to 78, with more severe impacts observed above 80. By integrating physiological, metabolic, and production perspectives, this narrative review identifies key knowledge gaps related to metabolism, mammary function, immune responses, SCC, and dry-period carryover effects under HS. It also outlines evidence-based mitigation strategies, including targeted nutrition, precision cooling, and breed-specific approaches, to enhance thermotolerance and sustain dairy productivity and guide future research under rising global heat stress.