Heat stress results in decreased milk production, reproductive performance, and immune function in both milking and dry dairy cows.

Both environmental temperature and humidity impact the amount of heat stress that dairy cows undergo. Recent research has shown that milking dairy cows start to decrease milk production when the temperature-humidity index (THI) exceeds 68 (i.e., temperature of 72°F with 45% relative humidity, or 80°F with no humidity) and not 72 as shown in previous research with lower-producing dairy cows.

The detrimental effects on the estrus expression, conception rates, and early embryo survivability occur before declines in milk production are observed and may occur at a temperature-humidity index as low as 55 to 60. Generally, the maximum declines in milk production as a result of heat stress are not seen until 36 to 48 hours after the initial heat stress event. Older dairy cows seem to be more severely affected compared to younger cows, and not all cows respond to heat stress in a similar manner.

Dry cows also are negatively affected by heat stress. Heat-stressed dry cows produce 1,000 to 2,000 pounds less milk during the next lactation. In addition, fetal growth is reduced because of reduced blood flow to the uterus, resulting in a decreased supply of nutrients for the rapidly developing fetus. These effects result in smaller calves being born to dams subjected to heat stress during late pregnancy.

Thus, proper management practices, facilities, and, to a lesser extent, nutrition are needed to mitigate the effects of heat stress not only in milking dairy cows but just as importantly in dry cows.

Read more in “Dairy Feeding and Management Considerations during Heat Stress” by Donna Amaral-Phillips, extension dairy specialist with the University of Kentucky.