Watch for Signs of Heat Stress This Week

With the first major wave of heat stress rolling into the Midwest this week, now is the time to check cooling systems for your herd, including dry cows, replacements and calves.

Declines in feed intake, milk yield and milk fat are obvious signs of heat stress in lactating cows. However, dry cows, calves and heifers can also experience heat stress.

Normal body temps for cows and calves is 101.5°F. “Body temperatures above 103°F indicate heat stress, and above 104°F indicate severe heat stress,” says John Bernard, a dairy nutrition and management specialist with the University of Georgia.

Respiration rates will also increase with heat stress, he says. Normal respiration rates for cows is 40 breaths per minute. To evaluate heat stress, count the respiration rate of 10 cows. “A respiration rate greater than 75 breaths per minute for seven cows indicates that the cows are experiencing heat stress,” Bernard says. “If more than 5 cows display open mouth breathing or respiration rates greater than 100 per minute suggest severe heat stress.”

For calves and older heifers, the normal respiration rate is 30 breaths per minute. If the respiration doubles (more than 60 breaths per minute), the animals are likely experiencing heat stress. “While heifers are generally considered to be able to handle heat stress better than lactating cows, body weight gain will be lower,” Bernard says.

Animals experiencing heat stress will tend to stand more to facilitate breathing. They will also congregate around soaker lines or water troughs. On pasture, cattle will seek shade rather than graze.

“If you observe any of these symptoms, evaluate your heat abatement system,” says Bernard. Check the following:

• Is there adequate shade and natural air flow for animals outside?

• In buildings, are fans clean and moving adequate volumes of air?

• Is there any blockage of fresh air flow into buildings?

• Are your soaker or mister systems operating as designed?

• Are your building ridge vents open and unimpeded?

• Is radiant heat load from building roofs adding to the heat load of cattle in barns?

• Is there adequate drinking water and space available for cattle?

“If heat stress abatement is not ideal,” says Bernard, “now is the time to make adjustments to minimize the impact during the remainder of the summer.”