Dairy production: Keep the heat down on your dairy farm

Question: I’ve heard that dairy cows can have serious problems with heat stress. Is it serious and what can I do to prevent it?

Heat stress in dairy cattle can be prevented, but this condition still causes significant financial losses every year.

Heat stress in dairy herds costs the milk industry millions annually. Dr. Jan du Preez, Managing Director of the Institute for Dairy Technology in South Africa, estimates that more than 90% of dairy cattle are affected by heat stress.

“Heat stress is a huge problem, but it can be prevented. All farmers have to do is be proactive and make slight adjustments to farm management practices when it’s very hot.”

Also read: How to run a dairy farm

Du Preez likens heat stress to a thermostat. When this device breaks, it is no longer possible to regulate temperature. Similarly, dairy cattle rely on an internal heat regulatory centre.

“During the hottest times of the year, dairy cattle are susceptible to heat stress if exposed to more heat than they can handle. This compromises their ability to regulate their body temperature,” he explains.

Dairy cows try to maintain a body temperature of 38ºC to 38.5˚C, because they are warm-blooded (homoeothermic) animals. The body temperature of dairy cows who suffer from heat stress increases from 38.5ºC to 39.5ºC, and can even rise as high as 40ºC.

This may seem like a very slight increase, but the 1ºC to 1.5˚C fluctuation is risky, to say the least. “Anything above 40°C is dangerous and can be fatal,” Du Preez says.


“If a person feels uncomfortable in the heat, chances are that the dairy cow will feel it, too. The only difference is that cows are even more sensitive to the heat,” he says.

Other signs of heat stress include panting and drooling and changes in the animal’s behaviour. For example, a cow might not move around as much, lick her skin to dampen it and eat less (roughage, in particular).

A temperature/humidity formula can be used to determine the level of heat stress in animals, according to Du Preez. Temperature, height above sea level, wind speed, humidity and dew point temperature are factors that influence heat stress.

Without using a formula, he says, farmers can safely assume that if the temperature increases to above 21°C, and is coupled with humid conditions, dairy cows will start experiencing heat stress.

Although temperature and relative humidity aren’t the only factors that determine heat stress, this is a good rule of thumb. (Dairy cows perform optimally between 0ºC and 15ºC.)


  • One of the easiest preventative measures is to plan ahead and anticipate hot days.
  • Farmers should also look out for unexpectedly hot days, even during cooler times of the year.
  • To enhance the cow’s natural mechanism for heat loss, farmers could use natural or artificial (permanent or portable shade) shelters or well-ventilated barns.
  • Ensure that cool drinking water is available in shaded troughs.

“Trees are an excellent source of shade and, given the choice, cows generally seek the protection of trees rather than man-made structures.”

  • Farmers can also reduce the ambient temperature by using large industrial fans, sprinklers, foggers, misters or showers.
  • Foggers disperse very fine droplets of water, which quickly evaporate, cooling the surrounding air in the process.
  • Fogging systems are very efficient at cooling air, but they’re expensive and often require a lot of maintenance.
  • Farmers could also use a hosepipe to spray water over the animals.

“These methods, or a combination thereof, will instantly help to bring down the animal’s temperature. Even if it’s just for an hour or 2, it will make a huge difference.”

  • Farmers should, however, ensure that the method used is effective for the level of heat stress the animals are experiencing.
  • Ensure that these methods are used once a day, at the hottest time (between 12:00 and 14:00).
  • Farmers should limit any stressful handling of animals during the hottest time of the day.
  • The milking routine (especially if there are three milkings per day), vaccination, dipping and dosing should also occur during the cooler period of the day or night.


  • In summer, farmers should increase the number of daily feeds, and offer fresh feed during the cooler night-time hours.
  • Heat stress reduces feed intake, so farmers should increase the energy and nutrient density of diets when it’s hot.
  • Farmers can achieve this by reducing the forage-to-concentrate ratio while ensuring that adequate digestible dietary fibre is provided to promote the normal ruminal digestive function.
  • Overfeeding highly degradable protein, such as lucerne and soya, when it is very hot, sometimes reduces feed intake and milk production.
  • Always provide enough clean drinking water when it’s very hot.

Also read: Livestock production: The secret to making good silage


Certain breeds are more susceptible to heat stress than others. Friesian dairy cows, for example, don’t tolerate heat stress as well as Jerseys do. But, says Du Preez, the improved tolerance level is only marginal.

“Breeding can help to make animals more resilient, but this isn’t the real solution to the problem,” he says. Planning facilities carefully and adopting highly adaptable herd management practices are required to protect dairy cattle from heat stress.

Also read: Milking indigenous cattle breeds


  • Dairy cows suffering from heat stress can have a 5 to 6 litre drop in daily milk production. This could cause a farmer to suffer huge production losses.
  • Heat stress affects the appetite of dairy cows, and in the process can cause a drop in the butter fat content of milk.
  • Heat stress suppresses the animal’s immune system, making it more susceptible to diseases and infections.
  • There’s also a greater chance that dairy cows will develop udder infections and mastitis.
  • Conception rates can decline by up to 40% in heat stress conditions.
  • This will force the farmer to use Artificial Insemination (AI) more often than usual.
  • There’s also an increased risk of pregnant cows calving before their due date.
  • Unlike full-term calves, early calves usually have a lower body weight, weaker immune system and are more vulnerable to disease.
  • The cow’s oestrus (the time when female animals are ready to mate) may also be affected. The period can be shorter, lasting only 4 to 8 hours, instead of 16 hours.
  • Heat stress could also increase the period between oestrus cycles: instead of 21 days, it could increase to 25 days or even 28 days.

Also read: Dairy production: Another route to dairy farming

  • This article was written by Wilma den Hartigh and first appeared in Farming SA.

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