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Heat stress in dairy calves

Serena Lamont for Progressive Dairy Published on 16 July 2021

During these hot summer months, you are likely aware of heat stress and the effect it has on your lactating herd. However, are you aware of the effect it has on your calves?

While heat stress and the effect it has on the lactating herd is fairly well-known, the effect it has on calves may be less common knowledge.



More research in the past few years has been looking at the effects of heat stress on dry cows and the calves they are carrying. Dry cows that experience heat stress have been shown to have decreased milk production of 3.6 kilograms per day in the following lactation and reduced immune function. This is the same for her calf. A calf born from a dam who experienced heat stress during her dry period has been shown to have up to a 4.6 kilogram lower birthweight, decreased milk production in her first lactation, decreased immunoglobulin (IgG) absorption, decreased disease resistance and reduced survival to her first lactation.

When a dry cow is subjected to heat stress, they have a shorter gestation and decreased oxygen flow to the placenta. This is one reason calves are born with lower birthweights. These lower birthweights extend all the way to 1 year old, and those calves are seen to have less lean body growth than calves born to dams who did not experience heat stress. By calving they catch up; however, these calves born to heat-stressed dams will have lower milk production for multiple lactations compared to calves born to dams who did not experience heat stress in their dry period. Research in 2020 determined that a calf whose dam experienced late-gestation heat stress had 2.2 kilograms, 2.3 kilograms and 6.5 kilograms per day less milk production in her first three lactations compared to a calf whose dam did not experience late-gestation heat stress.

It is well-known that to create a healthy calf, producers need to achieve passive transfer with their calves. Calves who are born to heat-stressed dams have a lower ability to absorb IgG and have lower serum protein. This lower ability is speculated to be from quicker gut closure. This effect is not from heat-stressed dams giving a poorer-quality colostrum but a physiological change within the calf due to prenatal stressors. These calves need colostrum with higher IgG content to increase their chances of having successful passive transfer.

We also need to be paying attention to heat stress in our calves during the milk-fed period. Heat-stressed calves have decreased feed intake and an increase in number of treatments. Calves do have a higher thermoneutral zone than milking cows at 25ºC to 31ºC temperature-heat index. This is about the same range as people. If you are feeling hot, then your calves are also feeling hot. Heat stress in calves induces gut inflammation due to blood being sent to the extremities. This pulls oxygen from the cells of the gut, causing the cells to shrink, leading to “leaky gut syndrome,” which increases the risk for infection.

It is important to be able to recognize the signs of heat stress in calves to be able to intervene as soon as possible. Reduced activity, lethargy, decreased feed intake, increased water consumption and panting are all signs. As an example in adult cows, shallow panting represents an approximate 7% increase in energy requirements – and if open mouth panting, a cow’s energy requirement increases by 25%.


Unlike cold stress, there are no exact calculations to determine the amount of additional energy required by the calf for heat abatement. But a recent study saw when calves were fed more milk replacer during heat stress, they had improved growth of 120 to 130 grams per day compared to calves that did not get additional milk. Increasing the amount of milk or milk replacer fed, coupled with free access to clean water and fresh starter feed, will help to counteract these challenges, maintaining desired growth and early rumen development.

Additional cooling measures may be needed to help alleviate heat stress in calves. For instance, providing a shade canopy over hutches and propping the back of the hutch up to improve air flow. For indoor housing, ensure that ventilation is correct for warm weather by increasing air turnover rate. Not to mention, installing extra fans and/or sprinklers in the dry cow pen to beat the heat right when it starts to affect the calf.

The best way to determine if your farm is struggling with heat stress in calves is the same way you would in your milking herd. Keeping records will help determine areas or times of the year where improvements can be made. Keeping track of birthweights can help in identifying trends throughout the year. It can also help in determining success of your milk-fed period. Are calves doubling their birthweight by 56 days? Is there a time of year calves are being medicated more than other times? There could be an environmental factor causing stress to your animals.

When thinking about how heat stress affects milk production, we need to look at more than just our milking herd. Dry cows, in utero calves, milk-fed calves and heifers are susceptible to heat stress. While the production drop might not be instant, it will affect the future of your herd’s performance. end mark

References omitted but are available upon request. Click here to email an editor.

Serena Lamont is a young animal specialist with Grober Nutrition Inc. Email Serena Lamont.