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.

Dairy cows cool themselves by reducing dry matter intake (thus reducing heat production through rumen fermentation), by redirecting blood away from their organs into their skin and by increasing evaporation through sweating and through rapid breathing (“panting”). This has several consequences:

• A reduction in DMI reduces the intake of nutrients required for milk production
• Restriction of the intestinal blood flow has a negative effect on gut wall integrity, resulting in “leaky gut”69
• Due to panting, blood levels of CO2 decrease and blood pH increases, resulting in respiratory alkalosis. To reduce the pH, the cow will excrete HCO3- in her urine. This results in a reduction of buffering capacity of the rumen, increasing the risk of rumen acidosis. This effect on rumen acidosis is exacerbated by cows drooling and losing saliva

“Leaky gut” and rumen acidosis result in systemic immune activation and inflammation, which has a negative impact on milk production. The reduced supply of nutrients to the udder results in a further decrease of milk production74, see Figure 2.

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

Clinical signs of heat stress are:

• Panting
• Drooling
• Milk leakage
• A drop in milk production
• Standing up for long periods of time

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.

Production parameters may already change before any clinical signs of heat stress become visible. It usually takes 24-48 hours before heat stress results in a drop of milk production. Milk composition changes at an earlier stage and may thus be a better indicator for early diagnosis of heat stress66.

Managing heat stress in dairy cows requires a holistic approach 66,70

• Using fans to cool the animals, with or without cooling the air by adding water
• Wetting the cows with sprinklers
• Water consumption increases 1.2 kg/day per 1°C rise in environmental temperature77, therefore, ample water points should be available across the enclosures or pasture to ensure all cows can drink when they want to
• Cooling the drinking water to 10 °C
• Provision of shade accessible to all the cows
• Optimise quality of the flooring, to reduce the risk of lameness when cows are standing for longer periods of time
• Timing of feeding, provide fresh feed during the cooler periods of the day
• Reformulation of the diets in favor of glucogenic energy
• Cooling at the feed bunk
• Supplementation with feed additives that normalise rumen pH and reduce systemic immune activation71
• Reducing the risk of hindgut acidosis with prebiotics

Ideally, the heat stress programme should be implemented about three weeks before the cows become affected by heat stress to give them enough time to adjust to the changes.