Heat stress & reproduction: Part 1 What happens to reproduction during heat stress?

Heat stress can have a big effect on dairy cattle. One thing that we see a lot of are decreased conception rates during the hot, summer months but why is that? In this month’s podcast series, UW-Extension specialists and industry talk about why you’re seeing reproductive losses, how to minimize them during the summer, and some helpful do’s and don’ts when it comes to reproductive management.


Liz Binversie, UW-Extension Brown County Agriculture Educator


Paul Fricke, PhD, Dairy Cattle Reproductive Specialist, UW-Madison/UW-Extension

Heather Schlesser, PhD, UW-Extension Marathon County Dairy Agent

Dr. Vicky Lauer, Professional Services Veterinarian, ANIMART


Time: 6:52 minutes



Liz Binversie: Greetings, I’m Liz Binversie, Agriculture Educator for UW-Extension in Brown County, Wisconsin. This week we are beginning a new podcast series about heat stress and how it affects reproduction in dairy cattle. On the panel today is Paul Fricke, Dairy Cattle Reproductive Specialist for UW-Madison and UW-Extension; Heather Schlesser, Dairy Agent for UW-Extension in Marathon County; and Dr. Vicky Lauer, Professional Services Veterinarian for ANIMART.

First off, Paul, how does heat stress affect reproduction in dairy cattle both in the short and long run?

Paul Fricke: Yeah, that’s a really good question. I’ll just start with the concept that high producing dairy cows produce a lot of internal heat simply as a byproduct of the increased metabolism associated with this very high level of milk production. What we want to have happen is that cow has to be able to dissipate that heat. She has to be able to lose that heat to her environment. At lower environmental temperatures, she’s able to do that. She’s able to basically control her body temperature by losing this excess heat from metabolism and just normal functions of the body to the environment. What happens then is when the environmental temperatures increase, that cow starts to lose the ability to dissipate that heat. There’s just a few ways she can do that. She can basically increase her respiration so you see cows that are heat stressed that are panting. Evaporative cooling is another thing that she can do, but these cows will experience this heat stress phenomenon. What actually happens is the normal body temperature of the cow is about 101.5 degrees Fahrenheit. You’ll see that body temperature actually increase. That’s really what the underlying physiology and the problems that happen due to heat stress is due to this increased body temperature of the cow. In times of heat stress and severe heat stress, sometimes body temperatures in these cows can get to 105 or 106 degrees Fahrenheit. As body temperature increases, we start to see all kinds of problems with the physiology of reproduction. Two of the major problems that we see with reproduction, one of them just has to do with estrus expression. Every 21 days or so a non-pregnant cow will show behavioral estrus and she’ll come in and she’ll stand to be mounted by herd mates. That’s typically what we use as an indicator of when to breed that cow. One of the problems with reproduction is under periods of heat stress, the estrus expression, the duration, and intensity of estrus expression is dramatically decreased. It becomes harder and harder to see cows in estrus, so that we can actually inseminate those cows. That has a negative effect on reproduction. The other problem with heat stress is simply on the actual fertility of the dairy cow. What happens is early during embryonic development—basically the last few days right before a cow ovulates and the first couple days after ovulation—the early embryo is highly susceptible to these increased body temperatures that the heat stressed female is going to impose on it. That’s simply because the genome of that embryo is not been turned on. There’s a class of proteins called heat-shock proteins that can actually prevent the problem but because in this early stage of development, those proteins aren’t able to be synthesized. This early embryo is highly susceptible to the elevated maternal body temperature. What we actually see is a decrease in conception rate, which is associated essentially with the increased pregnancy loss. The other main factor that happens with heat stress is—and we have some pretty good data from different experiments that have shown this—fertilization rates drops quite a bit as well. If you just look at the proportion of oocytes that are fertilized after an insemination, the fertilization rate will dramatically drop due to heat stress. There’s some pretty severe things that can happen on the female side that really impact reproduction and decrease reproduction performance in these lactating dairy cows. Dr. Vicky, do you have anything you want to add to the impact of heat stress on reproductive performance in dairy cows?

Dr. Vicky Lauer: Those were all really great comments. One question I get from producers pretty commonly is, “Well how long is this heat stress going to last?” They’ve done pretty good studies that show that cows have decreased fertility for 40 to 50 days later, even after that heat stress is gone, just due to the effect of some of those follicles on the ovary that hadn’t developed yet. I do have a farms where they are still using bulls. You can’t forget that heat stress is going to have the same effect on bulls as it does on cows. They’re going to have considerably decreased fertility as well, so something to think about. Heather, do you have anything to add?

Heather Schlesser: Thanks Dr. Vicky and Paul. I just wanted to mirror sort of what Dr. Vicky said about the bulls. We have to keep into consideration that it takes the bulls about 60 days or more days to go through one round of spermatogenesis or the production of sperm. Because of that, you’re going to see a prolonged effects of heat stress similar to what you see in the dairy cows. After a heat stress event, you can expect to have decreased fertility in that sperm for about 6 to 12 weeks. It’s a long period of time in which that bull’s going to need to recover and recoup. Also, during a heat stress period, bulls are going to be more sedentary. They’re going to seek out shade. They’re not going to work as hard at mating the female as they would normally, which will also cause a decrease in conception rate just because they’re not mating.

Liz Binversie: Thank you to our wonderful panel. Tune in next time to hear about what you can do to minimize reproductive losses this summer.