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Raise healthier calves by monitoring and mitigating heat stress

Jodi Wallace and Bradley Bohemen for Progressive Dairy Published on 17 July 2020
calf in hutch

It’s hot enough these days to fry an egg on the hood of a tractor. Heat stress is a major issue, even in Canada. Yet not much is said about the negative effects of heat stress in calves. Most of our effects focus on cooling the adult milking cows. However, heat stress can also negatively impact the calves. If the calf’s dam was heat stressed during the dry period, the negative consequences could last a lifetime.

Our focus should be to recognize the signs of heat stress and take action for prevention. Calves start to feel heat stressed at 25ºC. Clinical signs of heat stress in calves are: 

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  • Increased body temperature resulting in reduced movement
  • Increased respiration rate (open mouth panting) 
  • Decreased consumption (milk, grain and hay)
  • Increased water intake

Calves will spend 20% to 30% more energy just to cool themselves. Couple this with an increased risk of dehydration, lower intake and impaired immune system, heat stress places the calf at risk for reduced average daily gains and increased risk of disease.

inside calf hutch

Data logger

Not convinced that heat stress in calves is an issue? We used the Kestrel Livestock heat stress monitor to evaluate the conditions in the calves’ environment. It’s a data logger that is placed in the calf’s pen or hutch and tracks and stores data, which is transferred wirelessly to a smartphone.

For dairy cows, we can use the temperature-humidity index (THI) to estimate the relative risk of heat stress to the herd. This is based on the combined effects of temperature and humidity. The THI does not take into account the effects of solar radiation or air movement. When the THI exceeds 72, cows start to experience heat stress. Over 78 THI, cows’ milk production is negatively affected, and over 82 THI, there are significant losses in milk production and risk of death.

However, there is little research for the THI cut points for calves. Everyone assumes they can tolerate more heat than cows since they are a monogastric and have yet to develop the heat-producing furnace called the rumen. Assuming they have a similar tolerance for heat, we placed two data loggers in two different calf hutches on different days – one inside and one outside. On “Farm A” we recorded data from July 6 to 7, and on “Farm B” we recorded on July 9 – some of the hottest days in July 2020 (Figure 1 and Figure 2).

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Figure 1 Calf heat stress

Figure 2 calf heat stress

The data collected showed that the THI inside and outside the calf hutches was hot enough to cause severe stress during the day. For example, the outside temperature on Farm B reached 37ºC on July 9. The THI shows that the calves spent approximately half to a full day under heat stress. There is nothing we can do to change the ambient temperature, but we can be proactive in our management to help reduce the effects of heat stress.

Corrective measures

There are many easy and economical ways to reduce heat stress in calves:

  • At all times, provide clean, fresh water.

  • For calves housed in hutches, raise the backs of the hutches off the ground. Prop up the backs by 15 to 20 centimetres to allow air flow through the hutch, and open up as many vents as possible. Provide shade – place the hutches in a shaded area or make a shade cover. Shade cloth can be placed 4 feet above the hutch. Be careful, a solid tin roof increases heat via conduction and won’t cool the calves. Also, the hutches can be moved to open to the north to avoid direct sunlight inside during summer months.

  • Consider using sand for bedding in the summer. Sand will keep calves cooler than straw. Its inorganic properties also help to reduce flies.

  • If calves are housed inside or in a naturally ventilated barn, consider using fans or positive-pressure summer tubes when the temperature is over 25ºC.

calf hutch

Don’t forget to cool the dry cows! Research has demonstrated that calves born from dams experiencing heat stress during the dry period had lower birthweights, compromised passive transfer and lower weaning weights, compared to calves from cooled dams. In addition, cooled calves had better survival rates and more milk in the first lactation.

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The take-home message: Take action to cool all your animals – your calves, dry cows and lactating cows. Keep everyone cool and content – it will pay dividends now and in the future.  end mark 

Dr. Jodi Wallace and Dr. Bradley Bohemen are veterinarians with the Ormstown Veterinary Hospital in Québec.

Jodi Wallace
  • Jodi Wallace

  • Veterinarian
  • Ormstown Veterinary Hospital
  • Email Jodi Wallace

PHOTOS: Photos provided by Jodi Wallace.

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