摘要：Heat stress is one of the major concerns which affect the production potential of dairy cattle almost in every part of world. Elevated temperature and humidity negatively affects feed intake leading to negatively affecting the reproductive potential which ultimately decrease milk production. High yielding cows more susceptible to heat stress than the low yielders. Heat stress can increase body temperature which may affect the fat synthesis in mammary gland. Apart from reducing the milk production, heat stress can also reduce the quality of milk. Internal metabolic heat production during lactation can further reduce the resistance of cattle to high ambient temperature, resulting in altered milk composition and reduction in milk yield. Heat stress can affect the various components of milk such as fat (%), solid-non-fat, protein, casein and lactose content. Heat stress can increase the somatic cell count indicating the reduction in quality of milk produced. Further, heat stress can also cause endocrine disbalance such as altering the levels of prolactin, thyroid hormones, glucocorticoid, growth hormone, estrogen, progesterone and oxytocin which ultimately affects the milk production. Heat stress through higher udder temperature may also cause mastitis in dairy cows. In addition, heat stress during dry period in particular might trigger mammary gland involution accompanied with apoptosis and autophagy, decreased amount of mammary epithelial cells can ultimately cause decline in milk yield. It may be concluded from this review that heat stress is considered to be adversely impacting both quantity as well as quality of milk produced. Heat stress brings about these impacts through reduced feed intake, altered hormone concentration and pathological changes in udder during mastitis.
