With the arrival of the warmer months comes the annual reminders about the negative effects that heat stress can have on cows. But how often are you considering the potential negative impact on your calves?

It is important to understand the thermal neutral zone of a calf. This is the temperature range in which a calf is not expelling extra energy to stay warm, or keep cool. In calves 0-3 weeks old, their thermal neutral zone is 10-25 degrees C. Older calves range from 0-25 degrees C. It is interesting to know that calves that are experiencing heat stress actually have to expend energy to stay cool, and need 20-30% more energy to do this! That’s a big deal, especially when their appetites may significantly decrease in the heat. This can have a direct impact on the function of their immune system and can inhibit their ability to fight off any other health challenges that may arise. Many calves will also experience elevated levels of stress hormones during this time.


Signs and indicators of heat stress include: sluggish behavior, panting, and less willingness to eat. More severe consequences are sunken eyes, increased pulse rate, and dry gums and mucus membranes due to dehydration. While somewhat subjective, a skin tent test can be a good tool to determine the level of dehydration. A rectal temperature of over 39.4 also indicates heat stress.

Feeding Strategy

As with many things pertaining to calves – early intervention and preparation is key. It is best to continue with regular milk feedings as calves experiencing heat stress still need the calories for regular maintenance and growth. Since the energy demand is higher, it is a good idea to offer more frequent feedings to increase the total milk volume rather than trying to feed a larger volume twice a day. It is also not a good idea to mix more powder than suggested on the label as this can lead to unnecessarily high total solids, 13-15% is adequate. Also, be sure to keep the starter fresh.


Extra water may seem obvious, but should not be over looked. Calves experiencing heat stress will be losing water through panting and sweating in an effort to keep cool. Calves will normally drink 6-12L of water daily, while heat stressed calves will drink up to 20L to maintain normal hydration. Administering electrolytes partway through the day is a good strategy to help with rehydrating calves and replacing any lost minerals, and will also supply additional energy. Be sure not to feed with whole milk or milk replacer as this can lead to digestive upset.


Monitoring the calf environment is a large part of combating heat stress in calves. The current air exchange recommendation for calf barns in the winter is at least 4 per day, compare that to the summer recommendation which is minimum of 40 air exchanges per day. Removing humid, stagnant air containing ammonia and bacteria will greatly improve the air quality and reduce respiratory ailments. Calf hutch vents should all be open and back ends lifted to allow for maximum natural ventilation, providing shade can also go a long way in reducing the heat. Alternative bedding such as sand or shavings can also help to reduce the heat that straw bedding can generate in the summer. Keeping the bedding fresh and dry will also help to reduce fly populations.

Gentle Handling

The last point to make in regards to heat stress is avoiding any over exertion being put on calves. Gentle and quick handling for things like vaccinating, dehorning or moving calves should be done in the morning hours when the temperatures are cooler.