Heat stress in dairy cows results in systemic immune activation

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 conditions^72,73,74.

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.

If heat stress in cattle is present, 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 response to dairy cow heat stress has several consequences:

A reduction in DMI in heat stress cows reduces the intake of nutrients required for milk production Restriction of the intestinal blood flow has a negative effect on gut wall integrity of heat stress dairy cows, resulting in “leaky gut”⁶⁹ Due to panting, heat stress in cattle reduces blood levels of CO₂, resulting in an increase of blood pH leading to respiratory alkalosis in heat stress cows. To reduce the pH, the heat stress cows will excrete HCO₃^- in their urine. Heat stress in dairy cows thus results in a reduction of buffering capacity of the rumen, increasing the risk of rumen acidosis. This effect of heat stress in cattle on rumen acidosis is exacerbated by cows drooling and losing saliva

“Leaky gut” and rumen acidosis as a result of heat stress in cattle lead into systemic immune activation and inflammation, which has a negative impact on milk production of heat stress cows. The reduced supply of nutrients to the udder results in a further decrease of milk production as a result of heat stress in dairy cows⁷⁴, see Figure 2.

A drop in milk production is the most obvious dairy cow heat stress symptom. Next to that, heat stress in dairy cows has a negative impact on cow fertility68 for a number of reasons:

• Systemic immune activation as a result of heat stress in cattle reduces fertility
• An increased body temperature as a result of heat stress in dairy cows 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
• Heat stress dairy cows do not express signs of heat due to lethargy because they want to reduce activity to the absolute minimum

Elevated temperatures lead to signs of stress in dairy cows. Heat stress cows are breathing rapidly (“panting”). The animal in this video is drooling and leaking milk as a result of heat stress in cattle.

How to recognise heat stress in cows? Clinical signs of heat stress in cattle 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 in cattle increases the risk to develop mastitis80. If heat stress in dairy cows is present, animals 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 in heat stress cows.

Similar effects can be seen in housed heat stress dairy cows if they congregate and lie in inappropriate places. Heat stress in cattle also results in a higher percentage of lameness, as an increase of the THI leading to heat stress in dairy cows results in an increase of standing time and a decrease of resting time⁷⁵,

Heat stress in cows results in an increase of mortality⁷⁶. Lastly, during the dry cow period, dairy cow heat stress results in a lower birth weight of calves and a reduced milk yield in the next lactation^70,79.

In case of risk for heat stress in cattle, 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 increasing the risk of heat stress in cows.

Production parameters may already change before any clinical signs of heat stress in dairy cows become visible. It usually takes 24-48 hours before heat stress in dairy cows results in a drop of milk production. Milk composition in heat stress cows changes at an earlier stage and may thus be a better indicator for early diagnosis of heat stress in cattle⁶⁶.

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

Heat stress in cattle can be reduced by using fans to cool the animals, with or without cooling the air by adding water. An alternative method to reduce heat stress in cows is wetting the cows with sprinklers. Heat stress in dairy cows increases water consumption by 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 heat stress cows can drink when they want to.

Cooling the drinking water to 10 °C can reduce signs of stress in cows. Provide shade to all the heat stress cows and optimise quality of the flooring, to reduce the risk of lameness when heat stress dairy cows are standing for longer periods of time. In case of heat stress in cattle, consider timing of feeding and provide fresh feed during the cooler periods of the day. Reformulat of the diet of heat stress cows in favor of glucogenic energy and ensure cooling at the feed bunk. Supplementation with feed additives that normalise rumen pH and reduce systemic immune activation can reduce the risk of problems as a result of heat stress in dairy cows⁷¹. Reduce the risk of hindgut acidosis as a result of heat stress in dairy cows with prebiotics such as Selko LactiBute.

Ideally, a programme for heat stress in cattle should be implemented about three weeks before the cows become affected by dairy cow heat stress to give the animals enough time to adjust to the changes.