Heat Stress: 65 is the new 72

Hoard's Dairyman: 

Heat Stress: 65 is the new 72

Date: 
Tue, 06/14/2011

When summer arrives, we know how warm and uncomfortable we can become. We look forward to the shade of a big oak tree, a chance to have lunch in an air-conditioned home, or the feel of the breeze on our face as we drive a fast-moving tractor in the open air. Dairy cattle also feel the heat but rely on us to make them more comfortable.

June’s free monthly webinar was entitled, ”We’re learning more about heat stress” and was presented by Lance Baumgard of Iowa State University. Elanco sponsored this webinar.

For many years, cows were thought to experience heat stress at a THI (temperature humidity index) of 72. The THI is a combination of two factors, the temperature and the relative humidity. A chart which calculates the THI can be downloaded here.

However, new research, including that done by Dr. Baumgard, shows that the THI should be lowered to 65 - 68. This translates into cows needing to be cooled during hot and humid weather sooner (at lower temperatures and humidity) than we originally thought.

Cows with heat stress will stand more, eat less, sort though their feed, drool more, produce less saliva, and eat less frequently. All of these effects leads to less milk. However, the reduced feed intake only accounts for one-half of the loss in production. Other production losses are due to metabolic changes the cow is undergoing due to the temperature and humidity. In some regions of the country, such as the Midwest, cows will see more drastic swings in the temperatures. These cause more challenges for animal caretakers, as opposed to a more constant climate of the Southeast or West Coast.

A cow experiencing heat stress can lose 100 pounds of body weight in just seven to ten days. The key to reducing heat stress is to start addressing the issue BEFORE the 65-68 THI threshold is reached. Some producers use fans, shades, and misters to keep cows cooler. Others will alter rations and feeding schedules to provide fresh feed during the cooler parts of the day or feed smaller amounts more frequently. The holding area can be very uncomfortable on hot days and cause severe stress when a large group of cows are in close proximity for extended periods. The holding pen is a key place to put heat abatement items like fans or misters on your dairy.

But how do you determine humidity? Many producers have visible outdoor thermometers but not a reading for humidity. Using your best guess only may be causing additional heat stress on your dairy cows. For a small investment (of less than $25), a thermometer with humidity gauge can be purchased. (I checked with Nasco to get my price quote.) When hung near corrals or pens, the guesswork is removed, and you will know if your cows are approaching the critical 65 - 68 THI.

Past webinars are archived at www.hoards.com/webinars. The June webinar will be posted shortly.