“Shirt-sleeve temperature (ambient) for cattle with any kind of hair coat is about 55° F.,” says Dee Griffin, feedlot production management veterinarian at the Great Plains Veterinary Educational Center. He was discussing heat stress at the recent International Beef Cattle Welfare Symposium hosted by Kansas State University. “At 82° they enter the upward critical temperature.”
That’s a timely reminder as the southern High Plains swelters amid the season’s first heat wave.

“When humans are uncomfortable at 80° F. and feel hot at 90° F., cattle may well be in the danger zone for extreme heat stress,” says Glenn Selk, Oklahoma State University Extension cattle reproduction specialist, in his weekly Cow-calf Corner newsletter. “Humidity is an additional stressor that intensifies the heat by making body heat dissipation more difficult.”

Do more than skim the surface, and an extraordinary number of factors make some cattle more predisposed to heat stress than others. Among environmental factors, Griffin cites ambient temperature, solar radiation, humidity, wind speed, saturated soils and overnight temperatures that remain above 70° F.

According to Griffin, animal factors contributing to heat-stress susceptibility include genetics – hair color and temperament – and also temperament resulting from how cattle are handled. What Griffin calls transient factors include hair thickness, age and acclimation, nutritional management and cattle health. Origin and previous management is obviously part of it, too.

All those considerations can help gauge the susceptibility of cattle. As for the weather itself, there are a number of tools, indices and equations that help forecast heat danger levels.

For instance, there is the broadly used Heat Index (HI) and the Temperature and Humidity Index (THI). There’s also the Heat Stress Forecast Maps provided through a partnership between USDA’s Agricultural Research Service and the National Oceanic and Atmospheric Administration available at www.ars.usda.gov/. The maps account for predicted ambient temperature, humidity, wind speed and cloud cover. The result is a general guideline for expected cattle heat stress.

Griffin points out cattle core temperature follows ambient temperature by some number of hours. In other words, once the heat of the day has passed and you get a hankering to handle them, understand that’s about the time cattle core temperature is the hottest.

The U.S. Meat Animal Research Center developed a breathing rate equation to determine cattle heat stress. For temperatures higher than 80° F., the equation is: breathing rate = (2.83 x temperature) + (0.58 x humidity) - (0.76 x wind speed) + (0.039 x solar radiation) - 196.4. Without access to onsite weather data, they say to use 1,000 watts/meter2 for the solar radiation portion of the equation. The resulting predicted breathing rate places cattle into one of four heat stress risk categories (see Table 1).

When it comes to heat stress and keeping cattle alive, Griffin says, if you do anything, make sure they have air flow and water. No one can make the wind blow, but everyone can identify and work to remove obstructions to air flow and give cattle access to areas outside of dead air zones. That’s one reason Griffin is such a fan of mounds in feedlots – the higher cattle get from the surface of the ground, the more air flow there is.

As for water, Griffin says it’s about more than access. He says available reserve capacity needs to be large enough to provide half of the daily water needs of cattle in an hour. Figure average needs of 20 gals./day; that means there needs to be enough reserve capacity for a given set of cattle that they could each consume 10 gals. in an hour’s time.

“Overheating in cattle can be prevented under most management conditions,” Selk says. “Allowing animals access to cool water and mineral supplements is a must in very hot summer weather. Shade and free air circulation should be provided if at all possible. Avoid working cattle during very hot parts of the day. Very excitable cattle will be even more prone to heat stress if handled at high environmental temperatures. If animals are going to have limited access to water under stressful conditions such as shipping by truck or trailer, they should be allowed water prior to further stressful situations.”

Selk offering these commonsense recommendations to prevent heat stress in cattle:

  • Cattle that must be handled during hot weather should spend less than 30 minutes in the working facility. Dry lot pens and corrals loaded with cattle will have very little if any air movement. Cattle will gain heat constantly while they are in these areas. Therefore, a time limit of one-half hour in the confined cattle working area should limit the heat gain and, therefore, the heat stress.

  • Make every effort to see that cool, fresh water is available to cattle in close confined areas for any length of time. During hot weather conditions, cattle will drink more than 1% of their body weight/hour. Producers need to be certain that the water supply lines are capable of keeping up with demand, if working cattle during hot weather.