Article
Preventing heat stress in calves will protect growth rates
Calves are increasingly vulnerable to heat stress as hot summers become more common in the UK. However, there are steps rearers can take to mitigate the impact on their youngstock, such as including additives in calf milk replacers (CMR).
How to measure heat stress
We spoke to Dr Dunn from Bonanza Calf Nutrition about heat stress in calves and how to measure it.
When the ambient temperature rises to around 25°C or higher – a calf’s upper critical temperature – the animal becomes heat stressed. To dispel heat to keep cool and to thermo-neutralise, the calf uses additional energy.
Dr Dunn says the temperature humidity index (THI), a combination of heat and humidity, is used to measure heat stress. “Our climate in the UK is usually warm and wet with sunny spells with humidity ranging from 70 – 90+ % during summer months,’’ she says.
Here she sets out the THI Formula and associated data:
(1.8 x T° + 32) – (0.55 – 0.0055 x % of humidity) x (1.8 x T° – 26) (Source NRC)
Heat Map : Temperature Humidity Index
Heat map showing THI, where humidity is in % and Temperature is in °C

Consequences of heat stress for calves

At a THI higher than 80, young calves can no longer efficiently regulate their own temperature, Dr Dunn warns. Calves quickly dispel unwanted heat from their skin by redistributing their blood flow to decrease the amount of oxygen and nutrients reaching their gastrointestinal tract. This limits the supply of oxygen and nutrients, blunts intestinal mucosa development, increases epithelial stress, and alters intestinal barrier function.
The weakening of the intestinal barrier allows bacteria to enter, causing the activation of the innate immune system. Consequently, the metabolism of heat-stressed animals no longer prioritises growth, production, reproduction, and health.
During warm conditions, calves eat less dry feed such as starter pellets and forage. This, coupled with higher energy demands, often results in lower daily liveweight gains, delayed growth targets and impaired rumen development making weaning trickier.
Tips for combating heat stress in calves
Feed additives can be incorporated into calf milk replacer to help support calves during periods of heat stress. Dr Dunn says supplementation with certain additives can significantly reduce respiratory rate, increase growth, and feed efficiency; their effectiveness is evidenced by lower rectal temperatures.
“Calves have also shown a significant improvement in their antioxidant capacity,’’ she explains. This results in calves having greater resistance to oxidative stress caused by long periods of heat stress which otherwise could lead to suppression of necessary immune responses and potential intestinal diseases.
Calf behaviour is often indicative of their comfort. Frequently changing posture, panting, and looking for shade can all be signs that it is too hot. Dr Dunn recommends taking a few minutes each day to observe calves. “Watching a calf can tell you a lot,’’ she suggests.
A healthy calf’s rectal temperature should be in the region of 38.5°C – 39.1°C. Research involving calves exposed to high ambient temperatures showed readings as high as 39.7°C, 39.8°C, 40.1°C and 40.4°C.
Calf housing also plays a big role when it comes to heat stress. Hutches are a popular choice for calves reared outdoors, but in summer the microclimate within these is generally hotter than in a purpose-built shed.
Dr Dunn advises that vents should be at the back of a hutch, which can then be opened to allow air to circulate. Research has also shown that elevating the back of the hutch reduces the inside temperature and lowers the level of airborne bacteria. Providing a shaded area over the hutches is very beneficial. In a study, calves in shaded hutches had a reduced heart rate, respiratory and cortisol levels, and lower rectal and ear temperatures compared to those in unshaded hutches.
Calves in south-facing hutches also had greater respiration rates than those placed at a different orientation. In a shed situation, a fan can work effectively to cool calves down, says Dr Dunn. Consideration also needs to be given to bedding type, she adds. “Straw is usually the go-to bedding for keeping calves comfortable, however, it is one of the best insulators, during summer months, switching to sawdust, woodchips or sand is advisable. These materials also tend to attract less flies.’’
Although rearers cannot control the outside temperature, they can make calves more comfortable through good management, nutrition, and housing.
Leave a Reply