Hot summer temperatures, especially when coupled with high humidity, can affect cattle adversely, interfering with weight gain and reproduction, and sometimes it can become life threatening. There are a number of things producers can do, however, to prevent or alleviate heat stress. Tom Welsh, Texas A&M University Department of Animal Science, explains that risk for heat stress increases as temperature and humidity increase. "In some of the research on heat stress in cattle, environmentally controlled chambers are used, where we can monitor intake of water and feedstuffs at various temperature and humidity levels, to see how the cattle are affected. The heat and humidity index we use came out of research in Missouri, using these chambers," Welsh says. H.D. Johnson, now retired, got those chambers started at the University of Missouri, and Don Spiers, associate professor of animal science, has been continuing the research. Heat and humidity can be controlled for the studies. It can be like a sauna, or it can be a dry heat. They have animals on different diet treatments, and can look at the interaction of the environment with the type of feed, to see how well the animal does with various rations and energy levels at a certain humidity, compared with another animal in a different chamber with comfortable conditions. They can crank up the heat and/or humidity and crank it down, to look at how the animal goes through the transitions," Welsh says.
"Animals are usually able to regulate body temperature to stay within a safe zone. Things that affect body temperature include metabolism (the body's process producing heat), heat of digestion (fermentation in the rumen produces heat), etc. Researchers have been looking at whether cattle should be fed a high quality, or a low quality forage when experiencing heat stress. From what I understand, high quality forages generate less heat during digestion than the fermentation of low quality forages with more fiber. So using high quality forages can be a way to put less heat load on the animal," Welsh says.
"If we want to keep cattle eating at optimum levels, it's recommended they be fed at least two hours after the peak ambient temperature, so when they generate heat from digestion it's not during the highest temperatures of the day," he says. Cattle are not interested in eating or moving much when it's hot, and won't eat as well during mid day.
One of the ways cattle cool themselves is through sweating. Spiers says cattle are fairly good sweaters. "Many people think cattle don't have functional sweat glands, but they do sweat. A cow's body may not look wet, but the skin is damp and evaporating the water so fast that many times you won't see it.
"Cattle also increase their respiratory rate and breathe through their mouths to increase evaporative heat loss, especially if they've been active. They are getting rid of body heat through the respiratory tract. This air exchange works to help cool them even when the air temperature is higher than the cow's core body temperature, due to the evaporation created," Spiers explains.
If cattle get overheated, Welsh says that blood vessels dilate to try to get more blood to the extremities to exchange more heat with the environment in order to cool off. "They drink more water, and also try to conserve water if there is not enough available. There are some hormonal changes that take place to conserve water in the body, with less sweating, less urination, etc. If they do have adequate water, however, then they can exchange heat through urination, salivation, etc. If they can replenish the water loss, everything they can get rid of takes heat with it," says Welsh. This is why you'll see cattle drooling when hot, and they will also sling their heads to put saliva over their backs, which increases heat loss through evaporation from the skin.
Pastures and pens should be designed to have shade to help cattle keep their body temperature within the safe zone. Spiers says that animals who are constantly exposed to the sun get much hotter than those with access to shade. "If using a roof for shade, it should be insulated, especially if it's a metal roof. Otherwise you'll get radiant heating (like an oven). The usual recommendation is a roof at least 10 feet off the ground. The higher the better, because it allows more air movement underneath it," says Spiers.
Shade is important during hot days, so the cattle can get out of direct sunlight if they need to. Sometimes they need the shade, and sometimes, they don't, but it should be available to them. Welsh says, "We had a survey of our animal facilities here, some years ago, and it was pointed out that we didn't have enough shade for our beef cattle, according to the inspector. So shade structures were built. But in July and August the animals don't always go in under the shade. Sometimes, with no exertion, they are content to just rest in the sun."
Shade may or may not be the most comfortable place, depending on where it is. Shade along a brushy creek bottom may be swarming with biting flies, or have no wind movement due to the windbreak effect. Cattle often prefer to be out on a high spot or ridge with a breeze, away from the flies. Welsh points out that dairy producers use sprinklers, and chilled drinking water for the cattle. Feedlots also use some of these methods, while a Nebraska researcher is working with feedlot cattle to find ways to promote wind movement to cool the animals, without creating dust.
"Another thing to keep in mind during hot weather is to avoid working animals in the middle of the day. Do as much of the work as possible in early morning or late evening when it's cooler. Try to avoid bunching them and give them rest periods if it's hot. There's no air movement in solid panel corral chutes, and those get pretty hot as well as being physically and psychologically stressful, which raises the animals' temperature," Welsh says.
"If you work cattle, the activity and jostling, especially if they are not experienced and don't know what is expected of them to move through the facility, will elevate their body temperature 0.5 to 3.5° F just from the stress and exertion. So anything you can do to minimize stress will help. Move them in smaller groups, give them less standing time when they are tightly confined in the long alley or in the chute, etc.," Welsh advises.