Editor’s note: This tip was written by Roy Williams, Dairy Calf and Heifer Association member.

Heat stress occurs when an animal's body temperature starts to rise above normal due to environmental factors. Air temperature, humidity, air flow around the animal, radiation from hot surfaces, direct and diffused sunlight, and ground temperature all influence the ability of the animal to control its internal body temperature. The animal's breed, age, health, lactation status, hair coat color, and diet also affect its ability to maintain its internal body temperature.  

 Worst-case scenarios
In July 1995, 3,750 cattle in one feedlot in Iowa died in 24 hours, due to heat stress. In the summer of 2006, more than 30,000 dairy cows died in California after several days of 100 degrees F. Estimated loss of milk production in U.S. due to heat stress in 2003 was calculated to be $900 million. Clearly, these are extreme cases, but nearly every farm will experience some negative effects of heat stress during the summer. 

Effect of heat on growing heifers
The economic costs of heat stress are very difficult to measure in a production environment, because there are so many different factors involved. When we look to the published research on heat stress, there are very few studies that have been conducted in a manner that has isolated a single factor. One of the best studies in this respect was reviewed in the Journal of Dairy Science in 2003 ("Effects of Heat-Stress on Production of Dairy Cattle", by J.W. West, vol. 86, pgs 2131-2144). This article reported on Holstein, Brown Swiss, and Jersey heifers raised for 13 months in environmentally controlled chambers with constant temperature of 50degrees F or 80degrees F. The weight difference was 18 pounds at 3 months and 67 pounds at 11 months. Heifers raised at the higher temperature took 1.5 months longer to reach 700 pounds. This is an important result, since it demonstrates that heifers need not be severely heat stressed to suffer decreased growth due to higher air temperatures. It also shows that heat stress can occur without changes in temperature: your heifers may be heat stressed even if there has been very little temperature change for several days or weeks.

Are your heifers heat stressed?
Any of the following is an indication of moderate to severe heat stress:

  • Slobbering
  • Rapid respiration/panting
  • Open mouth breathing
  • Lack of coordination
  • Trembling
  • 70 percent of cows have respiratory rate over 80 breaths per minute, or 50 percent over 100 breaths per minute
  • 70 percent of cows have body temperature over 103 degrees F
  • Dry matter intake reduced by at least 10 percent
  • Any healthy cows in herd with body temperatures over 104 degrees F
  • Coma
  • Death (will usually occur at body temperatures of 106 degrees F to 108 degrees F

Source: Dairy Calf and Heifer Association