Editor's note: Geni Wren is former editor of Bovine Veterinarian, a sister publication of Dairy Herd Management. She now works for the American Association of Bovine Practitioners.

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, of Attica (N.Y.) Veterinary Associates, says heat stress can have many negative effects on young calves.

When calves are heat-stressed, negative effects include suppressed 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 4 quarts each feeding of powder mixed at 15 percent solids (2.5 lbs. daily) between three and five 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.”

And, if dry matter intakes are down, so is their rate of growth.

While 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.”

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 pre-weaned calf, says Amelia Woolums, veterinary researcher at the University of Georgia.

Woolums says there is some research indicating that heat stress can affect the response of immune cells in young calves.

For instance, lymphocytes from heat-stressed calves sometimes show lower responses than those from nonheat- stressed calves. This means that a heat-stressed calf may not be able to mount an immune response to infection or vaccination as effectively as a non-heat-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.

Heat stress at the wrong time could conceivably affect the longterm immune function in cattle that were infected or vaccinated during significant heat stress.

Neonatal calves need extra help

There is also some evidence that colostral antibody absorption is decreased in calves born to cows that are heat-stressed late in gestation.

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.”

And it’s not just the little calves that can be affected, points out Lance Baumgard, associate professor of animal science at Iowa State University.

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 (sunlight) is very intense,” Baumgard says. “Shade, airflow and plenty of drinking water is of primary importance for calves on pasture.”

So, it’s imperative to put these animals in situations where they can avoid the deleterious effects of heat stress.


Sam Leadley has had first-hand 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,” explains Leadley, a calf-care expert affiliated with Attica Veterinary Associates in Attica, N.Y.

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, and 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 90 degrees F, 80-percent-plus 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.”

Sidebar: Housing and heat mitigation

To 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. Calf-care expert Sam Leadley, of Attica (N.Y.) Veterinary Associates, notes that concrete blocks work great for this.

“Some folks use tires, but they block a lot more of the opening than blocks do,” he adds.

Leadley has a few producers who use sand in the summer. “I think it works just fine as long as some extra is added starting around three or four 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.”

During times of heat stress, Leadley 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.”

Read more about mitigating dairy calf heat stress from Washington State University at http://extension.wsu.edu/vetextension/Documents/CalfHeat-StressTrial%202....