The significance of the temperature and humidity index when managing heat stress in housed dairy cattle

Although temperature has historically been a key indicator of the risk of heat stress, humidity plays a critical role.

Heat stress is often thought of as a summer condition, but cows could become heat stressed at temperatures as low as 19°C especially if the humidity is high. High humidity means that any moisture on the cow’s skin cannot evaporate, which prevents them from cooling down.

Because of this, the temperature humidity index should be monitored to keep heat stress to a minimum.

The temperature humidity index (THI) combines the air temperature with the humidity in order to find the perceived temperature that the cow will feel.

What is heat stress?

Heat stress occurs when a cow’s core temperature gets above its normal range.

When the outside temperature and humidity are too high, they cannot cool down efficiently as cows don’t sweat as effectively as humans.

Heat-stressed cows will become lethargic and will stand with their heads bowed, panting. The cows will also tend to stand closely together in patches of shade, further increasing their temperature.

What’s the difference between temperature and humidity?

Temperature is a measure of how hot it is and humidity is a measure of how much water vapour is in the air.

Although outside temperatures are primarily decided by the sun’s energy, the temperature inside buildings can be determined by many factors.

Sun rays on the roof, the heat released by the animals and lack of ventilation can all cause the temperature inside a cattle shed to be higher than the outside temperature.

In hot weather, the air can hold more water so there is the potential for higher humidity than in the winter.

Although humidity doesn’t change the temperature, it can make it appear hotter than it is as sweat is not able to evaporate and cool down the animal.

As mammals, cows release moisture in their breath and sweat, but their waste products and decomposing bedding can also produce moisture which can increase humidity.

The importance of the temperature humidity index

The temperature-humidity index (THI) is used to measure the effects of both the temperature and humidity that cattle are exposed to and can be more accurate in predicting the risk of heat stress.

THI can also be used to predict how hot the inside of barns will be.

Figure 1 table from NADIS – https://nadis.org.uk/disease-a-z/cattle/managing-heat-stress-in-dairy-cows/

As the table shows, cows can develop moderate heat stress at just 22°C if the relative humidity is 100%.

Although the THI threshold for heat stress has long been considered to be 72, recent studies have shown that a THI of 68 is enough to induce moderate heat stress[1].

Humidity can easily reach 80% during the summer and can reach almost 100% in poorly designed buildings, meaning that cows can become moderately heat stressed at relatively low temperatures.

Even at lower THI levels, rumen function and fertility can be affected. As the THI increases, it can lead to major losses in milk production and calving rates, impacting profitability.

How to manage heat stress

There are three main ways to reduce heat stress.

 Cooling the animals with fans or tube ventilation

Fans or tube ventilation can help to cool animals down by wicking away heat from their skin. Positive Pressure Tube Ventilation systems like Galebreaker’s VentTube Cool have a lower running cost than fans and provide targeted cooling where cows need it most.

Increasing the nutritional concentration of the ration

Digestion produces heat, and often cows will reduce their dry matter intake (DMI) to stay cool.

Making sure the ration is high quality will increase the digestibility of the feed, which will produce less heat.

To increase the DMI, try feeding at cooler times (e.g. early morning and late evening), pushing up the forage more frequently and feeding smaller quantities more often.

Increase the availability of water

In hot weather, cows can drink up to 10% more water. Making sure they have access to clean, fresh water will prevent dehydration which can further reduce a cow’s ability to cope with heat.

[1] A Re-Evaluation of the Impact of Temperature Humidity Index (THI) and Black Globe Humidity Index (BGHI) on Milk Production in High Producing Dairy Cows. (researchgate.net)