(Reprinted with permission from the July 2001 Charolais Journal)
Heat stress can affect your next calf crop, however there are some things you can do to help prevent it. Dennis Maxwell, Iowa State University McNay Research Farm, recalls a project a few years ago at the University of Missouri in which the calving season was moved to early summer, "to calve in more natural conditions when there's lots of green grass. But that put the breeding season in late summer when temperatures were high, and the cows had very poor conception rates."
"Here in Iowa we had some problems with heat in our own cow calf herd a couple years ago, with some early embryonic losses, when we had an extremely hot period in mid July. There can be several kinds of reproductive problems when you have a late breeding season. Many producers, and myself here at the Research Farm, have such a problem with mud and calf health in early spring, that a lot of us are moving our calving season later. But then you get into a situation where you are dealing with hot weather during the breeding season, so it's a Catch 22 situation," says Maxwell.
"We also have a fall calving herd, and these cows start calving the middle of August. If it's extremely hot and humid and there's no shade, you can have problems with baby calves. They can also become easily dehydrated in hot weather if they happen to get sick. Shade and water are very important, and the color of the calves can also make a difference. If you have a black calf out in the middle of a pasture with no breeze and it's 100 degrees, that could be a problem. The main thing is to plan the breeding season for a time of year that works best for your region and situation," according to Maxwell.
Curtis Youngs, Iowa State University reproductive specialist says, "Our primary concern about heat stress involves the time of breeding. If a cow has been bred and she's under heat stress, even if fertilization does occur, the embryo is very likely to die and she comes back into heat again." Heat stress can cause failure of the embryo to attach to the uterus.
"The most common time for affects of heat stress to cause embryonic loss is probably in the first week after mating, although in some cases a pregnancy can be lost even up to the first month. Mother nature doesn't want to add more stress to a cow who is already stressed, so if the heat stressed cow has just been bred, the pregnancy is often sacrificed," Youngs explains.
"The other time we see adverse affects on pregnancy from heat stress is in late gestation. Anything that stresses the cow or fetus at that time can then trigger a premature birth, which usually leads to death of the calf," he says. Youngs advises timing of calving and breeding to avoid the worst heat of summer. "The bull can also be affected by heat. The sperm are created in the testes, which hang down away from the bull's body since they need to be about 7° cooler than body temperature. Nature is pretty ingenious in keeping the testicles cool; the bull has three avenues to help regulate their temperature. The scrotum itself has the ability to have a thick or thin wall. It can relax or contract to control the thickness of the wall, to serve as insulation when thick or to more effectively allow heat to escape when thin. The scrotal wall is thick in winter when the muscle is contracted, and thin in summer when it's relaxed," he says.
"Then the cremaster muscles raise or lower the testes within the scrotum itself. When it's cold, the testes are pulled up next to the body for warmth, and when weather is warm they are suspended farther away from the body," explains Youngs.
"The third mechanism the bull has is a very interesting series of blood vessels called the panpiniform plexus. As blood is on its way to the scrotum and testes, it is at body temperature. But in the panpiniform plexus the blood is also leaving the testes and that blood is cooler. As the warm blood comes into the testes, it goes right next to the blood vessels carrying away the cooler blood, so there's a heat transfer. This countercurrent heat exchange insures that the blood leaving the testes gets warmed back up to normal body temperature before it gets back into the body, and that the warm blood coming into the testes gets cooled down before it gets there," he says.
"There are some things in terms of bull management in which people just need to use common sense, such as to provide shade and plenty of water. The bulls may not want to be in direct sunlight during the hottest part of the day. Ideally they need ventilation if they are in a barn or under a roof; there must be good air movement through the structure," advises Youngs.
"If a bull does get heat stress, you usually end up with fertility problems. These can be mild or you can end up having a complete wipeout of the whole population of spermatozoa that are in the testes and being stored in the epididymis. This causes a true sterility for 45 to 60 days, the length of time depending on how severe and how prolonged the heat stress was," he says.
"Severe heat stress will affect mature sperm that are being stored, so there will be some immediate effects. But the long term problems that we run into are from damage to the sperm that are actually in the process of being formed inside the testes. These damaged immature sperm cells will take the same length of time to get through the system, but once they get there they are no good. The bull could be infertile for up to two months after severe and prolonged heat stress," according to Youngs.
"Sometimes what happens is a bull suffers heat stress and the first few matings afterward are fine. Then there's a big string of matings when no cows become pregnant. Once the heat damaged sperm work their way through the system and are gone, you start getting normal sperm again and fertility picks up. It can be a frustrating problem for producers. The first cows bred may settle, then there is a big gap when none get pregnant, then it's like someone turns a switch on again and they all get pregnant."
One strategy (if a producer must have the breeding season in hot weather) is labor intensive, according to Youngs, and that's to do night only breeding. "Turn the bull out with the cows in late evening, leave him with the cows all night, then entice him back in with a bucket of grain in the morning to lock him away from the cows during the heat of the day. Here in the Midwest where we have smaller herd sizes and opportunities for more facilities and intense levels of management (as opposed to large range pastures) this can be an option, if the facilities and the disposition of the bull permit," he says.
"Here in Iowa, we do have hot days and humidity, but not as severe or prolonged as what they experience farther south. Some of the best research on the effects of reproduction has been done on dairy cattle in Florida. Part of those findings we try to extrapolate to beef, but it doesn't always make a direct crossover. But it does help a person try to realize what problems might occur, so steps can be taken to prevent them, rather than waiting until calving season and finding out you don't have as many pregnant cows as you thought. We try to encourage all producers to have their cows pregnancy checked," says Youngs.
"It also helps to have bulls in good body condition before the breeding season begins. If a bull is thin going into the breeding season, and gets hit with a heat wave, he won't have much appetite and will lose even more weight. This is not a good situation because he could lose so much that it interferes with his energy and breeding ability," he says.
"Breeding soundness exams are also wise, to check the general health of the bull, check feet and leg soundness, and evaluate a semen sample. This is no guarantee he'll be fertile throughout the season, but can show if he has healthy sperm at this point in time in the short term he should be a good breeder. If you collect a sample and his sperm are no good, you want to know about it early enough to get another bull lined up so the cows can be bred."
Youngs says salt and mineral are also important. "There's a tremendous interaction between nutrition and reproduction. Salt is very important in hot weather." If the animals can't replace the electrolytes lost through sweat and other body fluids, they are more stressed in hot weather.
Well managed animals can do a better job of tolerating short term stresses. "If we only pay attention to them during breeding season, they won't be in the right kind of shape to tolerate those stresses. Don't ignore the bull for 10 months of the year and then expect him to be a superstar during breeding season. Good management and common sense can go a long way toward preventing problems," Youngs concludes.