How To Recognize Heat Stress in Cattle
By Heather Smith Thomas
Heat stress — and death loss from heat stress — costs cattle producers about $369 million in losses every year, estimates Dr. Don Spiers, University of Missouri department of animal science.
Normal temperature in a beef cow is about 101.5 degrees. This temperature rises when the animal can’t dissipate body heat during hot weather. Individual animals have different levels of heat retention.
“We followed groups of animals through 4 periods of heat stress and found they always have their own ranking. Animals with low minimum temperatures always have low maximum temperatures,” he explains.
“Another clue how high body temperature will go during the day is how fast the air temperature rises in the morning. If it will be a hot day, the temperature increase between 6 a.m. and 8 a.m. shows how hot it’s going to get, and you have some warning of what’s coming,” he says. You have time to do something early in the day with at-risk cattle rather than responding after the fact when animals are starting to die.
Monitor respiration rate
“Even if you can’t measure body temperature, you can measure respiration rate — which is a more rapid response to heat than is body temperature,” says Spiers.
Anything below 40 breaths per minute is healthy. “Anything above 80 breaths per minute 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 with their tongues sticking out and drooling, they have a real problem,” he says. If you are moving cattle on a hot day and some start panting with mouths open and drooling, it’s time to halt and let them rest.
Dr. John Gaughan in Australia developed a panting score index, from 0 to 5. This involves counting respiration rate and observations about what the cattle are doing.
- At 0 there is no panting and breathing rate is below 40.
- Score 1 is 40 to 70 breaths per minute (and it’s 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 30 seconds and double it.
- Animals at score 2 are breathing 70 to 120 breaths per minute; they have a little drool, but the mouth is still closed.
- At score 3 there is drooling, open-mouth panting (120 to 160 breaths per minute), the neck is extended and head held upward.
- At panting score 4 the tongue is fully extended out of the mouth for long periods, with excessive drooling, 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 because by then the head is low and the animal is breathing from the flank and may stop drooling because it’s so dehydrated. At this point, risk of death is high. When they start going into heat stroke, they stop sweating because they are dehydrated and everything is shutting down,” Spiers explains.
“We published a paper on sweating rates and how it differs over different parts of the body — how much they sweat. The shoulder area has the highest amount of sweating. Rump and legs are less. If you use fans, aim them at the shoulder,” says Spiers.
“When we did studies on heat stress and put cattle in environmental chambers at the University of Missouri, we found sweating rate went up during the first few days, and then dropped. Their internal body temperatures were still high, but they sweated less. The body chose not to keep the sweating rate high. It was more important to conserve water (to prevent dehydration) than to maintain constant body temperature,” he says.
Enough water to allow cattle to eat
“You must look at different things when assessing the health of these animals. We check body temperature, but water intake is regulated separately by the body and seems to be more important. Animals always need adequate water. They will actually reduce their water loss after they’ve adapted to heat, and still keep their body temperature high.” This shows the importance of water, because once they become too dehydrated they will die.
“We did a study looking at dehydration, trying to come up with indicators of dehydration that a cattle producer could use. The major indicator of dehydration we found was decreased feed intake. They just stop eating.” When cattle can’t mix enough fluid with the feed, they can’t eat. They can’t produce saliva if the body is short on fluid.
“If you see an animal that’s not eating well, it might not be getting enough water. We did this study using environmental chambers and found that body temperature didn’t actually go up that much as heat in the chamber rose, but feed intake plummeted when they didn’t have access to water,” says Spiers. “We also saw that when they were dehydrated their sweat rates dropped. They didn’t have enough moisture to sweat.”
Various research groups have tried to come up with better indices of heat stress in feedlot cattle. “John Gaughan published a paper in 2007 showing that you can use ambient values to determine how hot the animals would be. He worked with several thousand cattle — Bos taurus as well as Bos indicus. He found he could use black globe temperature (which measures actual temperature of an object in the sunshine — which is generally hotter than air temperature), relative humidity and wind speed to come up with a heat load index (HLI) to use as a management tool,” says Spiers.
Dr. Tami Brown-Brandl, USDA, Clay Center, Neb., came up with a model to determine feedlot cattle susceptibility to heat stress by looking at cattle characteristics. There are 3 components. One is environmental conditions (black globe temperature, air temperature, humidity), second is management practices of the producer (whether cattle have access to water, shade, etc.) and third is susceptibility of the individual animal.
She listed 11 animal characteristics:
1. Color — A dark animal will be hotter than a light colored one.
2. Gender — Heifers are more susceptible to heat than steers, but this difference is very small.
3. Species — Bos indicus are more heat tolerant than Bos taurus.
4. Temperament — An excitable animal will overheat more quickly than a calm one.
5. Hair thickness — Thinly haired animals don’t get as hot as thickly haired animals. Hair creates insulation that slows heat dissipation from the body.
6. Previous exposure to heat — Whether the animal is acclimated or had only brief exposure to heat.
7. Age — The older the animal the more body mass and more difficulty dealing with heat.
8. Body condition — Whether thin, moderately conditioned, well conditioned or overly conditioned, finished cattle have the most insulation (body fat) and the most problem with heat stress.
9. Previous pneumonia — An animal that’s had pneumonia may have lung damage and respiratory insufficiency — less than optimum air exchange and less ability for cooling via respiration.
10. Other health issues
11. Current health
“She puts those 11 categories into a computer model to come up with a prediction of how these animals will deal with heat stress,” says Spiers. Stockmen could use a tool like this to identify the cattle most at risk and could put the at-risk individuals in a different environment where they could be watched, or a place where fans or water might be available to cool them if needed.
“She ranks those 11 categories as to which ones are more important. Whether the animal is male or female is not as important, for instance, as species, or whether they’ve had pneumonia,” says Spiers. Research has been beneficial to help stockmen identify signs of heat stress and determine which animals might need the most help to get through a severe heat wave.
“Heat Stress in Cattle” is excerpted from the August 2013 issue of The Cattleman magazine.