Impact of heat stress on lactational performance of dairy cows.

Lactating dairy cows exhibit a myriad of responses to heat stress. These responses partially facilitate the thermal balance between heat gain and heat loss, but also account for reduction in productivity. Decreased milk yield is the most recognized impact of heat stress on a dairy cow and results in significant economic loss to dairy producers. The reduced milk yield by heat stress is observed when daily average temperature-humidity index exceeds 68, above which the milk yield of a cow is negatively correlated with temperature-humidity index or dry bulb temperature. Milk yield is also positively correlated with body temperature of the cows under evaporative cooling, which reflects the positive relationship between metabolic heat production and milk yield. During summer, feed intake is positively correlated with milk yield, and the decreased intake explains at least half of the reduction in milk yield by heat stress. These emphasize the importance of maintaining intake on productivity during summer. Although not entirely clear, mechanisms that mediate the reduced milk yield by heat stress in addition to intake may be multifactorial. These could include but are not limited to altered metabolism, potential activation of immune system and inflammation, changes in behavior, and altered mammary gland development and function.