AMES, Iowa — The USDA’s Meat Animal Research Center (MARC) warned U.S. cattle producers that heat stress would be at “dangerous” to “emergency” levels for three straight days last week, with the National Weather Service in the Quad Cities having forecast temperatures in the mid- to upper 90s.

“It’s clear that we will have temperatures near 100 degrees (June 27-29),” said Matt Deppe, CEO of the Iowa Cattlemen’s Assoc. (ICA). “This will clearly put all cattle at risk. Compared to other animals, cattle rely on respiration more than sweating to cool down.”

Deppe said estimates of heat stress are based on four specific weather factors: temperature, wind speed, humidity and solar radiation. “Wind and cool nights can help, but when temperatures are this high, producers must also consider other ways to keep their livestock comfortable.”

Grant Dewell, Iowa State University extension beef veterinarian, said during extreme weather conditions with insufficient cooling at night, cattle will accumulate heat they cannot disperse.

“A temperature-humidity index (THI) alone may not predict cattle heat stress because it does not account for accumulated heat load,” he said. “Another shortfall of THI is that it does not account for solar radiation and wind speed, which can affect heat load of cattle.”

Dewell said cattle shouldn’t be worked during times of extreme heat and only early in the morning when it’s not too hot – especially since working cattle will elevate their body temperature.

“Cattle should not wait in processing areas longer than 30 minutes when it is hot,” he said. “Do not work cattle in the evening, even if it has cooled off a little. Cattle’s core temperature peaks two hours after peak environmental temperature. It also takes at least six hours for cattle to dissipate their heat load.”

In addition, if peak temperature has occurred at 4 p.m., cattle will not have recovered from that heat load until after midnight, and it will be later than that before cattle have fully recovered from the entire day’s heat load.

“Feedlots should evaluate cattle daily, especially during July and August, for evidence of heat stress,” Dewell said. “Special attention should be paid to cattle with increased risk of heat stress, including heavy cattle, black cattle and respiratory-compromised animals.”

Dewell said heavy cattle cannot handle heat stress compared to lighter-weight cattle because increased fat deposition prevents cattle from regulating their heat effectively.

“Solar radiation is a critical component that can lead to death loss from heat stress,” he said. “Proportionally, more black-hided cattle die during heat waves then other hide colors

“Since cattle rely on respiration as a method to manage heat respiratory function, cattle that had severe respiratory disease early in the feeding period will have a decreased ability to regulate their heat load.”

The water requirements of cattle also increases during heat stress, Dewell said, since cattle lose water from increased respiration and perspiration – which is why consumption of water is the quickest method for cattle to reduce their core body temperature.

“Rule of thumb is that cattle need three inches of linear water space per head during the summer,” he said. “Extra water tanks should be introduced prior to extreme heat events so that cattle become accustomed to them. Waterers need to be kept clean to encourage cattle to consume adequate water.

“The water supply should be able to deliver 1.1 percent of body weight of the cattle per hour.” 

For example, “a 1,000-pound animal needs about 1.5 gallons of water per hour.”

Pastured cattle aren’t as susceptible to heat stress as feedlot cattle since they have the ability to seek shade, water and air movement to cool themselves, Dewell added. In addition, radiant heat from dirt or concrete surface is increased for feedlot cattle. At temperatures above 80 degrees Fahrenheit, cattle endure physiologic stress trying to deal with their heat load.

“Although cattle at this temperature are not at risk of dying, they will have an increased maintenance requirement to cope with the heat,” Dewell said. “Feedlot operators should have a plan to manage heat to not only prevent death loss, but also performance loss from decreased efficiency and feed intake.”

He recommended cattle producers provide the following this summer:

•Clean, fresh water – consumption can double during heat events. Cattle need at least two gallons per 100 pounds per day during heat events.

•Shift to feeding a higher percentage of feed in the afternoon and lower the energy content by 5 percent.

•Provide shade if possible. UV radiation is many times the critical factor for livestock losses due to heat stress.

•If necessary, sprinkle cattle with water if signs of heat stress are evident.

“Eventually, cattle will begin to group together,” Dewell said. “In severe heat stress, cattle will be open-mouth breathing with a labored effort. Feedlots need to monitor for heat stress and implement strategies to minimize impact on cattle to prevent severe death from heat stress.”