The temperature-humidity index (THI) is a frequently used measure of heat stress in dairy cattle. It is calculated using the ambient temperature and relative humidity. Heat stress in dairy cattle starts at a THI of 72, which corresponds to 72°F at 100% humidity, 77°F at 50% humidity, or 82°F at 20% humidity, explains Albert De Vries with the Department of Animal Sciences at the University of Florida.
“Lately, even lower thresholds for THI have been identified in high-producing dairy cows, such as 68°F,” De Vries says in a recently updated eXtension article. “Heat stress reduces milk production, feed intake and reproduction and increases the risks of lameness and culling.”
In the article, De Vries reviews some estimates of the economic losses from heat stress on dairy cattle production and discusses some implications of heat stress on reproductive management.
Source: "Economics of Heat Stress: Implications for Management" at eXtension.org