Not Cool: Heat Stress Happens Sooner Than We Thought

New research has shown that cows can suffer the negative impacts of heat stress sooner than we previously thought.

Experts previously believed that mild heat stress began at a temperature humidity index (or THI, the “feels like” temperature) of 72, with heat stress fatalities occurring around a THI of 99. It is now known that the average cow shows signs of heat stress at a THI of 68, with high-producing cows feeling the heat even sooner.


Why are high-producing cows affected more adversely? It all has to do with feed intake, explains Bob Collier, professor at the University of Arizona’s Agricultural Research Center. Cows that produce more milk require more feed; cows who convert more feed into milk produce more body heat than lower producing cows.

While shades, fans, and plenty of drinking water should be made available to all livestock, Collier recommends keeping a closer eye on high-producing and just-fresh groups, as these populations are more likely to fall victim to heat stress. Experts also recommend evaluating your livestock’s diet, as simple, lower-fiber modifications can help reduce the amount of heat produced during digestion.

Heat stress can have many negative impacts on the milking herd, including decreased milk production, lower reproduction rates, weakened immune systems, and higher somatic cell counts.



  • Halladay, Dennis. “Cows get hot quicker than we thought.” Hoard’s Dairyman 14 April 2014.
  • “Manage heat stress to protect dairy cattle immune function.” Dairy Herd Management 13 May 2014.

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