by: Heather Smith Thomas

Hot weather can be very hard on cattle, especially when humidity is high (which makes it more difficult for the animal to dissipate body heat), or when cattle exert and overheat. Moving or working cattle in hot weather can become life-threatening if the stockman doesn't pay attention to signs of heat stress. The fattest animals and those with the most muscle mass or thick hair coat are usually most at risk because they overheat more readily than a lean or thinly haired animal that has less body insulation. Dark colored animals also suffer more readily from heat stress than lighter colored animals.

Dr. Don Spiers, University of Missouri, says the comfort zone of cattle has an upper limit of about 75 degrees, but this can vary depending on how large the animal is, and its color. “If an animal is dark skinned, chances of heat stress are greater. Studies have shown that about 70 percent of feedlot deaths due to intense heat is in animals with dark hide. Researchers found that animals with black hide get higher body temperature, up to 104 degrees during hot weather—as opposed to light-colored cattle that are usually 101 to 102 degrees, which is normal,” explains Spiers.

Cattle with lots of muscle mass also generate more body heat than cattle with less muscle, and the bulky muscles also have more heat to get rid of. “If body temperature climbs to 107 degrees, cattle may suffer heart failure,” he says.

Dr. Tom Welsh, Texas A&M, advises against working animals in the middle of the day during hot weather. “It's safer to do as much of the work as possible in the early morning or late evening when it's cooler. Try to avoid bunching up the cattle (which hinders air movement). Give them rest periods periodically, if it's hot,” he says. If you are working in a corral, there's limited air movement in solid panel corral chutes. These chutes and alleys can get pretty hot, as well as being physically and psychologically stressful to the animals, which raised their temperatures.

“If you are working cattle, the activity and jostling—especially if they are not experienced and don't know what is expected of them when moving through the facility—will elevate their body temperature anywhere from .5 to 3.5 degrees, just from the stress and exertion,” says Welsh. Anything you can do to minimize stress will help keep them from overheating so much.

Move them in smaller groups, give them less standing time when they are confined in the long alleyway or chute, for instance. “We try to work cattle in very small groups if we have enough extra holding pens and enough people,” explains Welsh.

If you are moving cattle on a hot day, take them slowly. The best thing is to allow them to choose their own pace and graze a little as they go. One problem with taking cattle from one pasture to a new one is that often the cows will eagerly travel at a speed too fast for their calves, and if they have to go very far the calves may be overheated before they get there. The fat young calves may be breathing with their mouths open, or falling behind, or trying to lie down.

If the herd is not slowed down, or made to stop and rest at the first signs of heat stress, the calves (or fat yearlings) may become overheated to the point of dying. If range cattle are being moved in rugged terrain and are not allowed to rest when they start to get tired or overheated, the risks are increased. This is especially true if it's a long cattle drive with some uphill climbs—if the cowboys don't give the animals periodic rest stops.

Signs of heat stress include increased salivation and drooling. Cattle drool to get rid of more heat via saliva, and also sling it over their backs to provide dampness for more evaporation. Cattle in a pen or pasture will become anxious and nervous. If they have access to water they will stand in water, or stand next to a water trough. They won't eat, just spending all their time trying to stay cool.

Spiers says that high respiration rate is a sign of heat stress, as is breathing with the mouth open. “When you see the head elevated, this means they are having trouble breathing, trying to breathe easier—which is a sign they have a problem,” he explains.

Anything below 40 breaths per minute is indicative of healthy, safe temperature and the animal is fine. “When they get up to 80 breaths per minute this is a sign of heat stress. When they get up to 120 it's more serious, and by the time they get up to 160 breaths per minute and their tongues are sticking out and they're drooling, they have a real problem,” he says. If a person is moving cattle on a hot day and some of them start panting rapidly, with mouths open and drooling, it's time to halt and let them rest.

A number of people around the world have been studying heat stress in cattle. “Dr. John Gaughan in Australia (University of Queensland), in collaboration with Dr. Terry Mader (University of Nebraska), developed a panting score index. Australia has had a lot more hot weather than we have, and they have devoted more resources toward studying all aspects of heat stress in their animals,” says Spiers.

A booklet called Heat Load in Feedlot Cattle was published by the Meat and Livestock Association of Australia and discusses the panting score index. Panting scores go from 0 to 5 and involve a combination of counting respiration rate, and observations about what the cattle are doing. At 0 there is no panting and the breathing rate is below 40. Score 1 denotes faster respiration (40 to 70 breaths per minute, and it is easy to see the chest movements) but the mouth is closed and there is no drooling. You don't have to count for a full minute to check respiration rate; you can count for 15 seconds and multiply by 4, or for 30 seconds and double it—to come up with the respiration rate.

When animals get up to score 2 they are breathing rapidly (70 to 120 breaths per minute), they are drooling a little, but the mouth is still closed. At score 3 there is more drooling, open mouth panting (120 to 160 breaths per minute), the neck is extended and the head is usually held upward. At a panting score of 4 the tongue is fully extended out of the mouth for long periods of time, with excessive drooling, and the head is still extended upward, with breaths more than 160 per minute. You can't even count the respiration rate when it's this rapid.

“The critical level is score 4.5. By then the head is low, the animal is breathing from the flank and may actually stop drooling because it's so dehydrated,” says Spiers. At this point, risk of death is high. “You never want to let cattle get to this level, because score 5 is when they die. When they start going into heat stroke (and this applies to humans as well), they stop sweating because they are dehydrated and everything is shutting down,” explains Spiers. It is important to keep track of respiration rate and other signs of heat stress and try to do something before the animals get into danger.

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