Factors that Influence Heat Stress
Watch for heat waves during which nighttime temperatures don’t dip below 70°.
Don Spiers, professor of environmental physiology within the Department of Animal Science at the University of Missouri, has been doing studies on heat stress for many years.
“When assessing the factors that lead to heat stress, we know that the nighttime temperature is very important. If it doesn’t drop down below 70° [F] during the night, cattle start getting too hot,” he says. The heat is cumulative, and if they can’t dissipate heat into the cooler night air, their body temperature will slowly rise during a multi-day heat wave.
“If ambient air temperature drops below 70°, the animals have a window for heat loss, and they can often recover. If it stays in the upper 70s or higher, they start to accumulate heat,” explains Spiers. When a heat wave lasts longer than three days, it can be fatal to livestock, he warns.
“If you can create a break in the heat before you get to that three-day point, you may prevent losses,” he advises. If nature doesn’t provide that break, producers may need to intervene with sprinklers, shade, fans or other means.
“If cattle are outdoors you can hope for clear nights with no clouds to get some radiant heat loss,” Spiers says. “The sky is a heat sink if you have clear nights, but if it’s cloudy the heat sink is blocked and the cattle can’t get rid of the heat.”
The normal body temperature of a cow is about 101.5° and her normal respiration rate is about 40 breaths per minute. If the animal’s body temperature increases to 102.5° or higher and her respiration doubles to 80 or higher, she is definitely heat-stressed, Spiers says. “If the animals are breathing open-mouthed, especially if they are drooling saliva, they have a big problem and you need to do something (fans, misters) to cool them.”
Spiers notes that the comfort zone of cattle has an upper limit of about 75°. This varies, depending on how large the animal is and whether it’s dark or light in color.
“Cattle with lots of muscle also generate more heat,” he says. “The bulky muscles have more heat to get rid of.”
Body temperature above 107° is dangerous, and heart failure is a possibility.
“Once it gets that high, you’re dealing with major heat stress,” Spiers says. “Animals may have circulatory collapse and go into heat stroke if they can’t get enough water to maintain blood pressure. The vascular system collapses and the heart stops.”
Heat stress has many health implications, including reduced immune function, increased energy expenditure, acidosis, he explains. “The more you stress an animal (or a human), the incidence of disease goes up.”
For ways to mitigate heat stress, see “Minimize Heat Stress” on the Management page of this month’s Angus Beef Bulletin EXTRA.
Editor’s Note: Heather Smith Thomas is a freelance writer and cattlewoman from Salmon, Idaho.