Heat stress in dairy cows can also occur in moderate climates
Heat stress in dairy cows is often thought to occur only in arid or tropical regions of the world. This is a misconception. Dairy cows in areas with more temperate climates such as Europe and the coastal areas of Latin America and North America can also experience heat stress during the hottest and most humid times of the year66. Cows are known to become more resilient to heat stress if they are exposed regularly and for longer periods. This could mean that if cows in moderate climates are exposed to heat stress conditions for limited periods of time only, they may struggle to adapt more compared to cows living in tropical conditions72,73,74.
Dairy cows try to keep their body temperature at 38-39ºC (100-102ºF). Compared to monogastric species, dairy cows produce a lot of heat from metabolic processes. The thermoneutral zone of dairy cows depends on the relative humidity but is low compared to other species. A common way to quantify the risk of heat stress is the Temperature Humidity Index (THI, see figure 1).
This graph is based on data from Arizona, where the threshold for heat stress in dairy cows is a THI of 72. In moderate climates, this threshold can be as low as 6078. This clearly shows that even in countries with moderate climates, periods of heat stress in cattle can occur.