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 dairy cow heat stress during the hottest and most humid times of the year66. Cows are known to become more resilient to dairy cow heat stress if they are exposed regularly and for longer periods. This could mean that if heat stress in cattle is less common in moderate climates and signs of stress in cows are only present for limited periods of time, heat stress cows may struggle to adapt more compared to cows living in tropical conditions72,73,74.
Signs of heat stress in cattle can already be seen at relatively low temeperatures 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, increasing the risk of heat stress in cows. A common way to quantify the risk of heat stress in cattle is the Temperature Humidity Index (THI, see figure 1).
This graph is based on dairy cow heat stress data from Arizona, where the threshold for heat stress in dairy cows is a THI of 72. In moderate climates, this threshold for heat stress in cattle can be as low as 6078. This clearly shows that even in countries with moderate climates, periods of heat stress in cattle can occur. Figure 2: Changes in milk yield in grazing first lactation cows related to THI. A drop of milk production as a result of dairy cow heat stress already occurs at a THI of around 60.