What is heat stress?

One of the most common mistakes when managing heat stress is not knowing when heat stress begins. Animals will experience heat stress sooner than people, so it’s important to understand what’s too hot for dairy cattle. In Part 1 of the Heat stress & facilities podcast, UW-Extension and industry talk about what heat stress is and when it occurs.


Liz Binversie, UW-Extension Brown County Agriculture Educator


Mark Mayer, UW-Extension Green County Agriculture Agent
David Kammel, UW-Madison/UW-Extension  Biological Systems Engineering professor
Dr. Vicky Lauer, Professional Services Veterinarian, ANIMART


Time: 6:10 minutes


Liz Binversie: Greetings, I’m Liz Binversie, agriculture educator for UW-Extension in Brown County, Wisconsin. Today we will be discussing heat stress in dairy cattle: what it is and what can we do now to prepare for it. With the low milk prices, it will be even more important to manage heat stress this summer so you can maintain a healthy and productive herd. I am pleased to have with me on the panel: David Kammel, Biological Systems Engineering Professor for UW-Madison/UW-Extension, Mark Mayer, agriculture agent for UW-Extension Green County, and Dr. Vicky Lauer, Professional Services Veterinarian for ANIMART.

Heat stress can be a major issue on farms because it can decrease milk production by as much as 50% when temperatures reach 104 degrees or more, according to University of Arkansas. Now, we here in Wisconsin don’t usually have such high temperatures but heat stress can still be a significant problem if not managed properly. The first step in managing heat stress is knowing what you’re looking for, so Mark, for our listeners at home, what exactly is heat stress?

Mark Mayer: Well, in Wisconsin, even though we don’t have a lot of temperatures in the upper 90’s and 100’s, we certainly have a lot of temperatures in the 80’s and 90’s with high relative humidity. Heat stress is certainly one of the largest profit robbers on dairy farms with annual loses of over $200 per cow seen on many dairies. These losses are not just from lower milk production but also could include things like loss of body condition, embryonic mortality, lower fat test, missed heats, feet problems that are related to acidosis and increased standing. We need to remember that the ideal temperature for a dairy cow is between 40 and 60 degrees Fahrenheit. That means that heat stress occurs in cows much earlier than it does in humans, and therefore, it’s important for producers not to wait until they feel uncomfortable to turn on and implement heat abatement systems for their milk cows. We use a Temperature Heat Index called a THI index, and the first one of those was developed over 60 years ago, and that’s been recently updated because that data was using cows producing only 30-35 pounds of milk. The new index has been updated and now shows that cows will be stressed at a THI level of 68, and to put that in perspective, that’s a temperature of 72 degrees Fahrenheit with a humidity level of 50. That equals a THI of 68. Dr. Vicky, do you have anything to add?

Dr. Vicky Lauer: Just one thing, talking about cows, also dry cows, so most of the time we put a lot of consideration into our milking cows, but that heat stress will affect dry cows as well. They’ve done some research and showed that cows that are cooled during their dry period will actually produce more milk, they’ll have better immune function, a lower somatic cell [count], and thus a healthier cow, and then also calves that are born to those dry cows that have been cooled, they’ll be born heavier. They will maintain that weight gain through weaning. The calves will absorb colostrum better. They’ll have better immune function, and then those same heifers will conceive one service faster, and then once they went on to go in milk, they actually produced 6 pounds more milk per day for 35 weeks, so very important: don’t forget about the dry cows, and then also don’t forget about calves. Calves are less susceptible to heat stress due to their larger body surface area, but they do gain less weight during the summer, and their weight gain is about the same as what it would be in the winter, so definitely make sure you cool those calves too. David, do you have anything to add?

David Kammel: Yeah, Dr. Vicky, I’m glad you mentioned the dry cows. I was going to talk about calves and also heifers because they can also experience heat stress. The implementation of heat stress mitigation, things like sprinklers and fans, usually always kind of focuses on dairy cows, but we need to think about some of the other facilities on the farm, especially if there’s heifers on pasture without shade, for example, or calf hutches out in the open sun with minimal shade, or open pens where they don’t have a chance to get out of the sun. That solar radiation can really impact the animal even though it might seem reasonable or feel not too bad under the barn because they’re under shade, but you have some other animal groups that may not be thought of on the farm because they’re out on other farms, other facilities, and they’re not, kind of, in our eye sight. Keep in mind all animals have the potential for a heat stress period. Probably the other thing that we experience in Wisconsin is we have prolonged heat periods. Animals can tolerate some high temperatures during the day if it cools off at night. If we don’t cool off at night and that happens over, say, a 2 to 4 day period, that second or third day is when things are really going to shake loose as far as what’s happening to the animal. We have the ability in Wisconsin, most of the times, to get hot in the day but actually cool off at night, have a breeze. The animals can acclimate to that. On the other hand, if it is hot during the day and it doesn’t cool off at night and we don’t have any wind, and a couple of days of that, we’re going to have some significant issues occurring.

Liz Binversie: Thank you again to our wonderful panel today. If you have questions or would like to reach any of our panelists, you can reach them by email. Tune in next time for Part 2: Heat stress and the warning signs.

David Kammel

Mark Mayer

Dr. Vicky Lauer

Liz Binversie