Supplementation of Rumen-Protected Methionine to Heat-Stressed Transition Cows Impacts Mammary Gland Cell Dynamics and Gene Expression

Abstract

Simple Summary Dairy cows undergo physiological changes around calving as they shift from non-lactating and pregnant to lactating. Heat stress during this time intensifies these challenges by reducing intakes, weakening the immune system, lowering fertility, and decreasing milk production. As global temperatures rise, managing heat stress is challenging for producers. This study explored whether methionine supplementation, an amino acid involved in cell growth and stress response, could help protect the mammary gland during heat stress. A total of 53 cows were either fed a control diet or one supplemented with rumen-protected methionine. To induce heat stress, half the control cows and all methionine-fed cows were fitted with an electric heat blanket; the remaining cows were kept at thermoneutrality. After calving, milk samples and tissue biopsies were collected to examine mammary gland function. Heat-stressed cows showed disrupted mammary tissue, with fewer but enlarged alveoli, increased leakage of metabolites from blood into the milk, suggesting compromised epithelial remodeling and weakened tight junctions, ultimately reducing the capacity for milk synthesis. Methionine supplementation partially attenuated these heat-induced effects and was associated with altered gene expression related to growth, nutrient utilization, and one-carbon pathways. Overall, heat stress harms mammary development, but methionine may help cows cope during this critical period.