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.

Next to that, heat stress in dairy cows has a negative impact on cow fertility68 for a number of reasons:

• Systemic immune activation reduces fertility
• An increased body temperature as a result of heat stress has a negative effect on the quality of oocysts, embryonic growth, gonadotropin secretion, ovarian follicular growth steroidogenesis, development of the corpus luteum, and uterine endometrial responses67
• Dairy cows in heat stress do not express signs of heat due to lethargy because they want to reduce activity to the absolute minimum

Apart from a drop in milk production and a reduction of fertility, heat stress in dairy cows has a number of other negative effects. Heat stress increases the risk to develop mastitis80, if all cows in a herd group together in shaded areas of fields, the ground soon gets poached and contaminated with mastitis pathogens, which increases the risk of environmental mastitis. Similar effects can be seen in housed dairy cattle if they congregate and lie in inappropriate places.

There is also a higher percentage of lameness, as an increase of the THI results in an increase of standing time and a decrease of resting time75,

Heat stress results in an increase of mortality76. Lastly, in dry cows, heat stress results in a lower birth weight of calves and a reduced milk yield in the next lactation70,79.