Just as we humans don’t perform at our best in hot conditions, neither do livestock. Whether dairy cows, sheep, or any other animal, the effects of the often hot New Zealand summer can be devastating, particularly in the warmer north. And the most common effect of hot weather on animals is heat stress

Today we’ll take a closer look at the condition: why it’s dangerous, what causes it, how to identify it, and most importantly, how to prevent it.

What is heat stress?

Heat stress is a condition brought on by hot and humid weather, and is most prevalent in dairy cows. Having evolved and been predominantly bred in more temperate climates, they are unable to dissipate body heat efficiently. They have a limited ability to sweat, regulating their temperature instead through respiration. Generally speaking they accumulate a heat load during the day, and dissipate it at night.

The ambient temperature comfort zone for dairy cows is in the 4C-20C range. Go above that, particularly in humid conditions, and the cow begins to experience discomfort. If the heat and humidity gets high enough, it begins to put the animal’s health at risk. They are unable to dissipate enough heat, and intensifying symptoms lead to intensifying health effects.

Why is maintaining healthy cattle important?

First and foremost, every cattle or dairy farmer knows that you need to take care of your herd if you want them to take care of you. Heat stress can be a wildly uncomfortable and potentially dangerous experience for your animals, so minimising the discomfort and danger should be your primary goal.

But the reasons for maintaining a healthy herd go far beyond compassion. It perhaps goes without saying that a heat stressed cow is an unproductive cow. During particularly hot periods milk production can be reduced by up to 50%, while reproductive efficiency is also severely diminished, with insemination rates dropping as low as 10%-20%. The economic case for minimising heat stress is thus a compelling one.

Causes of heat stress

The likelihood of heat stress affecting your herd can’t be measured through mercury alone. To get a complete sense of the prevailing conditions, other factors such as relative humidity, ambient temperature and evaporation rate need to be taken into account.

The temperature humidity index (THI) combines all the relevant considerations into a simple number, with dairy cow heat stress occurring between 72 and 99.

  • A THI reading below 72 does not induce heat stress on dairy cattle.
  • A 25C, 50% relative humidity day is rated at 72, just enough to induce mild heat stress.
  • A THI of 78 is enough to significantly reduce milk yield
  • A 37C, 100% relative humidity day is rated at 99, inducing the most severe heat stress.
  • A THI reading of 100 or above will most likely result in death.

Check out this table for a complete view of THI and how these ratings affect cows.

In New Zealand, Northland unsurprisingly experiences the most days warm enough to reduce milk production, with Whangarei recording 129 such instances over the 2018/2019 season. Waimate in Canterbury, meanwhile, experienced just 39 such days over the same period.

Common heat stress signs and symptoms for cattle

As soon as conditions rise above a cow’s zone of comfort – a THI of 72 or more – a physiological response is induced, diverting energy from production to survival. At the cellular level, membranes go into lockdown mode, with some ceasing to operate altogether. Energy is instead almost exclusively used to prevent a rise in body temperature. 

There are a wealth of symptoms that can indicate an animal is suffering from heat stress. On a hot and humid day, look out for:

  • Lethargy.
  • Animals seeking shade and/or wind.
  • Agitation, restlessness and a refusal to lie down.
  • Accelerated and laboured breathing through an open mouth.
  • Sweating and excessive drooling.
  • Greatly increased drinking, decreased eating.
  • Increased urination (caused by electrolyte loss.)
  • A drop in milk yield and lower milk quality.
  • Emaciation.
  • Increasing instances of infection, disease and other health issues.
  • Lower fertility, increased embryo mortality and birth defects.
  • Reduced weight gain in growing animals.
  • Convulsions, collapse and instances of coma.

How to prevent heat stress

While you can’t change the weather, there are a number of mitigation strategies that can ensure the effects of heat stress on your herd are limited. A few of the most effective include:


With lactating cows requiring around 100L of water on an average day, you’ll need to ensure they have easy access to hydration on a hot one. Adjust the flow rates of your troughs to guarantee they’re never empty, and consider installing troughs in races so that cows can rehydrate immediately after milking.

Herd management

Decrease both the distance and the speed of the walk to the dairy, and cut back the time that animals spend in unshaded fields and yards. Keep a close eye on your herd, and act quickly if you see an animal with symptoms – isolate it, and provide it with shade, moving air and water. Avoid the heat of the day when milking.

Shade and cooling

Retaining trees in grazing paddocks is essential, although with each cow requiring around 5m² of shade, ensuring you have enough coverage can be a challenge. A more effective solution is to invest in a purpose-built shaded area, like a SmartShelters livestock shelter.

Heat stress is a particular concern for calves as heat stress will increase the rate of dehydration and lower the immune system. Both factors will lead to their energy being used to drive off heat vs. growing into strong and healthy animals. Our calf rearing shelters are a popular choice for this issue.

Fabric covered, perfectly ventilated and providing shade while making the most of natural light, a SmartShelters livestock shelter offers your animals a cool oasis in the heat of summer. With spans of up to 50m, these shelters can cover the biggest herds, while being incredibly quick and easy to construct.

Heat stress is a real concern for any New Zealand dairy farmer looking to maximise the productivity of their herd, and with temperatures set to rise into the future, the importance of heat stress management will only increase.

To discuss a truly smart livestock shelter solution, chat to our friendly team today!