HealthyLife
Programme for Sustainable Dairy Farming
Heat stress in cows results in systemic immune activation
HealthyLife
Programme for Sustainable Dairy Farming
Heat stress in cows results in systemic immune activation
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.
Figure 1, relation between temperature, humidity and THI. When THI exceeds 72, cows are likely to begin experiencing heat stress and fertility will be affected. When THI exceeds 78, milk production is seriously affected. When THI rises above 82, very significant losses in milk production are likely, cows show signs of severe stress and may ultimately die.
Why does heat stress in dairy cows result in loss of production?
Figure 2: Changes in milk yield in grazing first lactation cows related to THI
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:
“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.
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Heat stress in dairy cows also has a negative impact on cow fertility
Next to that, heat stress in dairy cows has a negative impact on cow fertility68 for a number of reasons:
What are the signs of heat stress in dairy cows?
Dairy cow in heat stress. The animal is breathing rapidly (“panting”), is drooling and is leaking milk as a result of heats stress
Clinical signs of heat stress are:
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What are the consequences of heat stress in dairy cows?
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.
It is important to recognize signs of heat stress in dairy cows as early as possible
Ensure sufficient shade is available to all animals. If there is not enough space for all animals, the high ranking cows in the group will occupy all space in the shade and the animals low in rank will be exposed to direct sunlight continuously.
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
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.
Learn more about the holistic approach of managing heat stress
Heat stress in dairy cows can result in systemic immune activation. It is a misconception to think heat stress with dairy cows only occurs in arid or tropical regions of the world. Dairy cows in areas with more temperate climates can also experience heat stress during the hottest and most humid times of the year. Heat stress can have several consequences that create a substantial loss of production.
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Heat stress in dairy cows can result in systemic immune activation. It is a misconception to think heat stress with dairy cows only occurs in arid or tropical regions of the world. Dairy cows in areas with more temperate climates can also experience heat stress during the hottest and most humid times of the year. Heat stress can have several consequences that create a substantial loss of production.
