Impact of Heat Stress on Calves

Extreme Weather

This summer has seen some of the hottest ever temperatures that the UK has ever seen- many places had over 30°C for several days running. These heatwaves are becoming more common and it could well soon be the case that we expect them every year before long.

The effect of heat stress on dairy cows is well known, with lower yields and an impact on fertility. Livestock buildings are generally designed to aid ventilation, with hot air rising because of the stack effect and exiting through the ridge, with cooler air being drawn in below. This constant airflow has benefits in both winter and summer, as the removal of stale air helps to reduce disease risk as well as keeping temperature constant.

In the UK, it is less common for calves to be reared in specialist buildings- often they will be in older buildings which have been repurposed or in specialist hutches. For this article, we want to have a look at the conditions inside calf housing at a farm where we have placed some sensors.

Local Situation

This farm is situated in Scotland, so it did not receive the same extreme of temperatures as in the south of England- the maximum over the affected period was 26°C. The calves were in pens with a hutch and an open straw area under a shed which had open sides, allowing good airflow. Hutches were on one side of the calf building facing south-west. The shed sensor was placed on a stanchion in the corner, away from direct sunlight.

Sensors were placed inside the hutches under the roof hatch. The local environmental temperature was gathered from a nearby weather station.

Outside temperature and shed temperature

At sunrise, the light hits the part of the shed where the sensor is placed. Even though this area is in shade well before midday, the higher temperature remains until well into the evening. It reaches peak temperature at around 11am, before falling close to the outside temperature in the late afternoon.

Fig1: Shed temperature consistently rises above the air temperature, with the difference being caused by direct sunlight during the late morning and early afternoon.

Here we can see that although the temperature outside peaked at 26, inside the shed it reached 32C, a massive 6 degrees Celsius difference! This makes a significant difference to the heat stress placed upon the calves. The temperature also stayed above 20C in the shed for nearly all the period shown, which exacerbates the effect of the heat as the calves have little time to recover. 

Fig2: There is a consistent difference between inside and outside maximum temperature through the heatwave, generally of at least 5 degrees.

The graph above shows that this raised shed temperature was a feature throughout the hot period. This suggests that the airflow internally was not very efficient at losing heat during the day.

UK Humidity

The UK generally has a humid, maritime climate. Whilst this generally helps to limit the extremes of temperature, during a heatwave this humidity becomes a problem. On the hottest day of the recent heatwave, August 12, humidity levels in the shed were as below:

Fig3: Humidity levels only reduced to 70% in the shed, limiting the amount of heat which can be lost through sweating.

One of the main methods which calves use to disperse excess heat is through panting and sweating. Both of these rely on using the evaporation of water moisture to get rid of heat and cool down. Unfortunately, when humidity is high (as above), this method becomes much less efficient.


Even in buildings with open sides, during periods of hot weather it can’t be taken for granted that the temperature and humidity are the same internally as externally. Make sure to provide plenty of water and shade for the animals! We will look at what other factors might have been able to help in more detail later.

Part 3:

Next time, we will examine the microclimate in the hutch as well as look at the factors which can help to keep the hutch and shed as cool as possible and help to minimise heat stress.