Not cool: heat stress impacts cow health and milk production

Efficiency of thermoregulation mechanisms in cattle gets compromised as external temperature and humidity increase, and internal body temperature starts to rise. Heat stress has adverse effects on milk production, reproductive performance, and health. Extreme heat events might even lead to cattle mortality. The Temperature Humidity Index (THI) quantifies the intensity of weather stressors and helps stockbreeders monitor heat stress.

Just like humans, cattle are impacted by their thermal environment. As homeothermic animals, cows have a series of coping mechanisms to maintain their body temperature stable. However, as ambient temperature gets further from the optimum (the « thermoneutral zone »), more and more energy is spent by animals to adjust, resulting in thermal stress and impacting production and even survival.

Heat waves are already here : a monster in the US and Canada on June 2021!

The map shown below shows the THI this week from july 27 in the US. The extreme temperatures forecasted (up to 119°F in western states) and high humidity levels put the cattle under the threat of high heat stress and will probably have a negative impact on production and health if no measures are taken. It is the time to think about protecting your cattle!

How is cow behavior affected?

When intensity and duration of Heat Waves grows and temperature increases over the optimum, several physiological responses are activated:

Respiration rates increase up to the stage of panting. This thermoregulation mechanism involves an increase in evaporative cooling, but it might also lead to an increase in blood pH.
Sweating, another attempt to lose heat through evaporative cooling. It makes up for up to two thirds of evaporative water losses (panting makes up for the remaining third).
Vasodilation: higher blood flow toward the cow’s skin favors the dissipation of body heat.
Increased salivation, for wetting the animal’s body.

There are also behavioral adaptations that cows naturally adopt, such as standing against cool objects (heat dissipation through convection) or increasing water intake. Because digestion produces a large amount of metabolic heat, cows also decrease their feed intake and lower their rumination rates.

Milk production

Heat stress leads to a decrease in feed intake of dairy cows and thus reduces milk production. This effect is particularly evident in high yield breeds such as the Holstein, where heat stress can result in yield reduction of 10 – 40%. In addition to quantity, quality is also impacted. Many studies highlight decreased fat and protein contents in milk from heat stressed cows.

Reproduction

When cows are under heat stress, reproductive efficiency declines. Heat stress results in reduced duration and intensity of estrus, altered follicular development and less efficient sperm. In addition, even when fertilization takes place, heat stress can result in abnormal embryonic development.

Health

Recent research also shows that heat stress adversely affects immune function. When ambient temperature and humidity rise, the endocrine system responds to accommodate to the need to increase heat loss. In particular, cortisol and prolactin, both hormones known to affect the immune system, increase with heat stress. These changes negatively affect cow health and production.

All these metabolic adjustments might not be sufficient to contrast the negative effects of heat, so when heat events are really extreme and prolonged, they might lead to animal’s death.

Climate change is a major threat for dairy farms worldwide

Ongoing climate change means that heat events are becoming more severe and last more days. This tendency has been observed at a global level by the International Panel on Climate Change and is confirmed on a regional scale in different parts of the world. For example, the two figure below, based on climate change projections by MeteoFrance (scenarios RCP2.6 et RCP8.5, in orange and red respectively) shows a clear increase in duration (x axis) and intensity (y axis) of extreme heat events over the next decades. Figure 1 shows heat events for the first part of this century (2021-2050), Figure 2 for the end of the century (2071-2100).

The increase in air temperature is reflected by the increase in the number of heat stress days calculated using the DRIAS projections for Bretagne (figure below).

The impacts on cattle are already evident with many reports of cattle mass death and studies confirming a decreased production during heat waves in recent years. These events are relatively common in warm regions such as Australia or California but they might start to become more usual also in cooler regions due to global temperatures rising. Even when mass death does not occur, production and health are impacted during and after the heat spell. In Italy, a study shows that cow mortality increases significantly during and up to three days after heat spells. These adverse effects are more important for longer events, and when the events occur early in the summer.

How to monitor heat stress in your cattle?

With so much at stake, it is important for stockbreeders to be able to detect the first symptoms of heat stress and adopt measures to cool down their cattle. Visual detection of stress is important but other tools are available. Over the years, several weather-based indicators have been proposed to quantify the impact of excessive heat and humidity on cattle.

The most popular of these indicators is the Temperature Humidity Index, which considers the combined effects of environmental temperature and relative humidity. The index is expressed as a score. Dairy cows begin to show signs of heat stress at a THI of 68 or more.

Below is a table showing THI as a function of temperature and relative humidity

When the THI exceeds 72, cows are likely to begin experiencing heat stress
When the THI exceeds 78, cow’s milk production is seriously affected.
When the THI rises above 82, very significant losses in milk production are likely, cows show signs of severe stress and may ultimately die.

Whereas the THI gives an instantaneous picture of the severity of heat stressors, other factors need to be taken into consideration. Many studies have shown that the duration of heat events is also very important, with long heat spells gradually worsening stress and decreasing milk production.

ITK solution for heat stress monitoring: Farmlife – Heat Adapt

Farmlife, ITK’s tool for monitoring animal health, reproduction and well-being has recently integrated a spatialised heat stress forecast, Heat Adapt. This service allows stockbreeders to act at the first signs of heat stress to avoid a decrease in milk production and reproduction problems. Heat Adapt is a comprehensive solution that helps stockbreeders to anticipate, mitigate and adapt to heat stress events by an increased awareness of:

What’s coming: Follow risk forecast and adapt the daily practices before the first visible signs of heat stress
What is happening: Monitor how the herd is coping and adapt the mitigation measures.
What happened at the farm: Evaluate past risk exposure and the global efficiency of mitigation measures, to be better prepared for the future.

50,000 cows are already monitored for heat stress behavior in 2021.