Heat Stress

Heat stress

The effects of heat stress on dairy cow physiology and productivity have been well documented. The first signs of heat stress can be seen at 20°C, where sweating and accelerated respiration may be observed. Heat stress can cause milk yield to decrease by about 10 percent or more, and reduce reproductive performance, while heat stress during late gestation has been reported to reduce birth weight and subsequent milk production. Heat stress is a major health risk as it can dramatically affect hoof health as well as somatic cell count.


Temperature and humidity combined determine the level of heat stress (TH1 index). In a hot environment, a cow controls her metabolic heat production by reducing her feed intake, which leads to a decline in milk production. Heat stress occurs when a cow’s load exceeds her capacity to lose heat, cows exposed to heat have a lower rumen pH than cows exposed to more moderate temperatures.