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Heat stress is coming – Is your calf program ready?

Stephen Hayes Published on 06 May 2014

Heat stress and its effect on calves is not fully understood, but we do know that calves are stressed in high temperatures and humidity. These effects can be severe, and it is up to the people working with the calves to recognize the signs of heat stress and take appropriate action to prevent problems from occurring.

Calves under heat stress will have:



• An increased body temperature (higher than 103°F)

• Increased rate of respiration (more than 50 per minute)

• Manure may become firmer and urine output may decline

• Decreased activity

• Less appetite


• Increased water consumption

The likelihood of calves being susceptible to heat stress depends on the temperature-humidity index (THI). Cows tend to exhibit signs of heat stress when the THI gets above 72. Calves can tolerate a slightly higher THI because they have a greater surface area for heat dispersal and the heat increment generated in the rumen of a calf is less than in a cow. Table 1 shows the differences between a THI of 72 and 76.

differences between a temperature-humidity index of 72 and 76

A key side effect of high ambient temperatures on calves is: They will eat less feed when they are hot. We do not have to be animal scientists to know that if calves are not eating their feed and are under stress, their weight gains and performance will be less than desired.

The University of Minnesota did a compilation of data showing calf growth and starter intakes for summer and winter. They found approximately a 15 percent reduction in starter intakes and growth rates for calves in the summer versus the winter. If calves eat less, they will grow less. Write that down.

Here are 10 steps to help minimize heat stress on calves this coming summer:


1. Look at the dry cows and maternity pens. Keep cows comfortable and start monitoring colostrum quality.

• Colostrum quality often goes down in the summer heat, so by monitoring colostrum, we can make sure to feed only high-quality colostrum or purchase a colostrum replacer and use it appropriately during these stressful times.

• Bedding packs in the maternity pens can often become dirty, and as the summer continues, pathogen load in these bedding packs can become very high. Change the packs regularly and remove the calf as soon as possible to avoid exposure.

• If a calf is born and it is really hot, you may need to help the calf cool down. A newborn calf is poor at thermoregulation in high-heat situations. Fans and shade can be used to cool the calf after birth.

2. Feed good-quality colostrum quickly. Follow the guideline of feeding 10 percent of bodyweight within two hours of birth. Passive transfer will be challenging on a stressed calf, so getting colostrum into a calf early can help.

• Monitor the temperature of colostrum being fed. Too-hot colostrum being tube-fed into a calf in heat stress is not helping. A colostrum temperature of 102°F should work fine.

3. Properly transport calves. This relates to calf ranches picking up at dairies or to dairies that raise their own calves but need to move them to a new location. Make sure the trailer is not overheating during transport. Have vents with good airflow so the calves’ body temperatures do not go up during transport.

4. Pay attention to the first feeding of newly arrived calves at the calf-raising facility. There are many ways to do this, but there are two primary methods that seem to work well.

• Feed a 2-quart bottle of properly mixed electrolytes to help rehydrate the calf. The temperature of this feeding should be body temperature. It is not uncommon to see successful calf raisers use their electrolytes at a “50 percent use rate” on this incoming bottle. This means if there is supposed to be two scoops per 2-quart bottle, they will use one scoop for this feeding.

• Feed a 2-quart bottle of a “transition milk type of diet.” This is a way for calves to switch their intestinal tract from colostrum to milk or milk replacer feeding. This is often a 50 percent blend of a second milking colostrum and milk or milk replacer.

A common alternative is to use a commercial colostrum supplement as this first feeding. This diet can help transition calves from colostrum to their next feeding of milk or milk replacer. There may be some “gut cleansing” effect that helps to reduce any pathogens picked up in the maternity pen.

• Make sure free-choice water is always available.

5. Increase the level of sanitation and cleanliness. Do not settle for “almost clean.” As the summer heat progresses, pathogen loads are growing and calves are getting sicker. Start early in the season by regularly cleaning and disinfecting any and all cleaning equipment and housing.

6. Look at the housing and environment. Maximize shade and increase air flow. In outside hutches, put cement blocks under the backs and turn the fronts to the north. Open all the vents. Do whatever you can to make calves more comfortable. Avoid heat-retention bedding such as straw. Sprinkling water on calves that are overheating can help to cool them down.

7. Hydration is vital. Have fresh, free-choice water always available. Have the buckets clean, and if the water in the bucket gets hot, it is time to dump it and refresh with cooler clean water. Remember the “half rate” of electrolytes we discussed under Step 4?

Many raisers will try a dilute solution of electrolytes in the middle of the day. This idea should not replace free-choice water. This is something offered once a day for a period of time and then replaced with free-choice water.

8. Review the feeding program. If calves are not eating much dry feed, then we need to maximize their nutrition through milk feeding. Heat-stress feeding is not much different than winter-stress feeding except starter intakes go down in the summer heat. If we want to get energy into a heat-stressed calf, and they are not eating dry feed, milk is the best way to do this.

• Weaning may need to be delayed a week or more. It all depends on how calves are eating their starter feed.

• Grain needs to be replaced regularly so as not to get overheated, stale, moldy or undesirable to consume.

9. Consider processing of calves with environmental conditions. Do not vaccinate and process calves in the heat of the day. In many situations, calf processing is only done in the very early hours of the day. In extreme heat, processing may simply need to be delayed to avoid stress to calves and people. Use common sense on this.

10. Review all nine points above and prioritize what is important for your operation, make a plan and get started.

The summer heat is coming, and actions you take today can help your calf program produce a better calf this summer. PD

Stephen Hayes is a veterinary consultant with Day 1 Technology, based in Winona, Minnesota.

steve hayes

Stephen Hayes
DAY 1 Technology, LLC