It’s relatively easy to determine if the body temperature of your cows is in the heat stress zone.
 

Now that we’re entering the heat stress season, a simple heat stress diagnostic tool, which I have discussed before, bears repeating.

Body temperatures as low as 102.2 F are indicative of a heat-stressed cow. At this body temperature, detrimental effects on the developing embryo that may result in early pregnancy losses have been shown. 

It’s relatively easy to determine if the body temperature of your cows is in the heat stress zone. You don’t have to temp cows or use a recording device. Respiration rates are highly correlated to body temperature. Count respiration rate by watching the movement of the flanks or the nostrils. If cows are breathing with an open mouth, observe the movement of the cheeks. Check at least eight to 10 cows in a pen by counting for 20 seconds and multiplying by three to getrespirations/minute. Check a few cows before and after they go to the holding pen to see if the respiration rate has changed.

Heat stress usually peaks in mid to late afternoon. However, humidity is highest in the late evening and early morning. Cows have difficulty dissipating heat at this time due to a “sauna” effect. Early summer mornings in Florida and other humid climates are often the least comfortable time for a cow.

Use the following graph developed by Brouk to extrapolate body temperature from the respiration rate that you counted. Normal body temperature is 101.5 F and normal respiration is about 45 breaths per minute. From the graph, a quick reference is 60, 80 and 100 breaths/minute equates to a little over 102, 103 and 104 F.

Cows are most comfortable at temperatures between 40 F and 60 F. Add in humidity, and a temperature-humidity index (THI) above 65 can affect reproduction. Besides fans and sprinklers or misters, the most important nutritional consideration to prevent heat stress in your herd is an ample source of clean, easily accessible drinking water.