Young calves and heifers may be more heat tolerant than older animals in production, but heat stress can still have negative effects. Calf care expert Sam Leadley, PhD, Attica Veterinary Associates, Attica, N.Y., says heat stress can cause many negative effects on young calves.

If calves are born during a period of heat stress, Lance Baumgard, PhD, Iowa State University, says shade, plenty of colostrum and clean drinking water are important. “Heat-stressed animals have a compromised intestinal function,” he notes. Fortunately, he says, it is more difficult to heat stress smaller animals such as calves vs. lactating cows, primarily because the method of dissipating heat is largely dependent upon the surface area to mass ratio which is quite large in a neonatal calf.

But when calves are heat-stressed, primary negative effects include appetite and intake. Leadley says he always had his poorest rates of gain with calves born after the middle of June in western New York. “I never had any problems getting them to drink milk on a twice-a-day feeding system even when feeding four quarts each feeding of powder mixed at 15% solids (2.5 lbs. daily) between 3 and 5 weeks of age,” he says. “But, hot weather just killed calf starter grain intakes. Even when I cut back the milk powder by half during the fifth week (dropped the afternoon feeding) they just did not come up on grain in the summer heat like they did between December and March.”

Leadley says during times of heat stress he would try to coax calves into eating more at night by giving them fresh grain and fresh water at 6 p.m. “This worked somewhat but that long period of inactivity between 11 a.m. and 5 p.m. just hammered starter intakes.” And, if dry matter intakes were down, so was their rate of growth.

Heat stress and youngstockWhile Leadley says he has seen fewer pneumonia cases during hot weather, diarrhea cases could turn more deadly than at other times of the year. “With scours I really had to jump on dehydration issues because once a calf started with a severe diarrhea case she could be down and gone in 24 hours in blistering hot weather. So, I usually spend more time on ‘poop-patrol’ in very hot weather. If I missed a calf that needed fluids she could be down and gone fast.”

Heat stress impacts immunity
While there isn’t much research specifically looking at the impact of heat stress on immunity in the calf from birth to weaning, there is research indicating that heat stress can impact immune function in older cattle, so it’s logical to assume there is also an effect in the preweaned calf, notes says Amelia Woolums, DVM, MVSc, Dipl. ACVIM, Dipl. ACVM, University of Georgia.

“It’s important to note that much of the research on this subject has looked at how immune cells of heat stressed cattle respond to stimulation in the laboratory; less has been done to quantify disease resulting from the impact of heat stress on immune function,” Woolums says. “So it’s hard to say with certainty which diseases and which cattle are most affected by disease resulting from these effects. But the research showing that heat stress can affect several immune functions supports the concept that heat stress may indeed contribute, possibly with other factors, to suboptimal immunity or higher rates of disease in at least some situations.

Woolums says there is some research indicating that heat stress can affect the response of immune cells in young calves. “It’s interesting, though not really surprising given the complexity of the immune response, that some functions are affected more than others, and the same effects are not seen in all studies. However, heat stress can affect the response of blood lymphocytes to stimulation, with lymphocytes from heat-stressed calves sometimes showing lower responses than those from non-heat-stressed calves. This means that a heat-stressed calf may not be able to mount an immune response to infection or vaccination that is as effective as a nonheat-stressed calf.”

Heat stress has been shown to affect aspects of innate immunity, such as the production of oxygen radicals by neutrophils in response to stimulation. “This means that heat stress may impair the ability of cattle to fight off infection in the first hours or days after infection, when the innate immune  response is protecting the host while the acquired (memory) immune component of the response is gearing up,”Woolums explains. “And because the innate immune response interacts with the acquired immune response, the impact of heat stress on innate immunity may also translate to a suboptimal acquired immune response. Thus, heat stress at the wrong time could conceivably affect the long term immune function in cattle that were in the impact of heat stress on innate immunity may also translate to a suboptimal acquired immune response. Thus, heat stress at the wrong time could conceivably affect the long term immune function in cattle that were infected or vaccinated during significant heat stress.”

There is also some evidence that colostral antibody absorption is decreased in calves born to cows that are heat-stressed late in gestation. Woolums says in one study, calves born to dairy cows that were subjected to heat stress in the dry period had significantly lower apparent efficiency of absorption of antibodies from colostrum, and significantly lower serum total protein and serum IgG, than calves born to cows that were not heat stressed (Tao et al., JDS 95:7128, 2012). “While the calves in the heat-stressed group had serum antibody concentrations that were adequate, dam heat stress could be an effect that, with other additional factors, might lead to higher rates of calf disease in at least some situations,” she adds.

Housing and heat mitigation
Heat stress and youngstockTo prevent or mitigate heat stress for hutch-housed calves, the most common way is to open up the backs of hutches to capture air flow. The second most common is to raise the rear of hutches to allow more air flow. Leadley notes that concrete blocks work great for this. “Some folks use tires but they block a lot more of the opening than blocks do. I noticed last summer that calves that had been hutches much more than 4 weeks that were bedded with anything other than sand had pushed enough bedding to the rear of the hutches so that the opening at the bottom was pretty well blocked.”

Leadley has a few producers who use sand in the summer. “I think it works just fine as long a some extra is added starting around 3 or 4 weeks to keep things dry. I used to use fine crushed stone in my hutches because it was less expensive than sand in the locality where I lived.” If using organic bedding, Leadley prefers wood shavings over straw in the summer to help reduce fly populations.

Producers often have shaded spots you can suggest they move calf housing to when heat stress is at its worst. “I have a client in Georgia who starts moving his wire cages/huts from an open field where they have been since November back into the pecan groves,” Leadley says. “Under the pecan trees they benefit from not only the shade but also from transpiration cooling from the trees.”

And it’s not just the little calves that can be affected. Older calves can suffer from heat stress as well, but sometimes they are “forgotten about” as they grow. Heat stress can be a factor for them as well, even if they are on pasture. “Solar radiation (i.e. sunlight) is very intense,” Baumgard says. “Shade, airflow and plenty of drinking water is of primary importance for calves on pasture.”

Read more about mitigating dairy calf heat stress from Washington State University at


Sidebar: Communicate about heat stress
Sam Leadley, PhD, has had firsthand experience with a heat-stressed newborn calf that caused him to think differently about handling calves in those situations and the need for better communication on the dairy.

“To be effective in dealing with heat stress with newborns all of the care givers need to have heightened awareness of the danger,” Leadley explains. He notes a time when a calf was dropped off at his barn on a very hot day. The person dropping the calf off had picked up the calf in the trailer then dropped it off in the first open hutch he found.

Later, Leadley says, “I kicked myself around the block when I found her close to death. The person dropping her off wasn’t thinking about extreme heat stress when she wouldn’t stand up and never said anything to me about her being weak.” It was oppressively hot (close to 90F, 80%+ relative humidity) and Leadley says he should have gone out to check on her right after she arrived.

Looking back at that situation Leadley says though there is not a lot of information on heat stress and calves, there are some things he could have done had he known the calf’s condition. “We had two big maple trees in front of the barn and I could have put her on the lawn in the shade, maybe dumped a couple of buckets of well water over her, given her fluids either IV or SQ. My point is that the calf care team at some time needs to talk about heat stress and newborn care. That way our awareness is higher and we give some special attention to the babies.”

It’s imperative, especially with neonatal calves, says Leadley, to help them out during these times. “These little calves are not going to do anything themselves to make the situation better such as stand up or move to a different location.” Calves that are 4 to 8 weeks old will seek any available shade, will tend to stand more than the normal amount and will drink more water.