As we move into summer, it is important to recognize the danger high heat and humidity pose to cattle. Cattle are more sensitive to heat than humans. They reach their “critical” temperature (the temperature at which negative effects start to occur) when the temperature-humidity index reaches the high 80s.
Temperature, humidity and the amount of direct sunlight are the primary factors that determine an animal’s body temperature. However, other factors — such as precipitation, wind, amount of night cooling and exposure to fescue endophyte— are important.
Precipitation can cause problems because high humidity reduces the ability of cattle to use evaporation to dissipate heat. Evaporative cooling occurs when sweat or moisture evaporates from the respiratory tract or skin. Evaporation is the primary means by which cattle cool themselves at temperatures higher than 70° F. The effects of wind and nighttime temperatures are also important. If winds are calm or if cattle congregate behind a windbreak, the animals’ ability to be cooled is reduced.Night temperatures that remain above 70° F increase the danger of heat stress because of little or no night cooling.
Cattle that are not acclimated to hot weather are also at greater risk if weather changes rapidly or if the cattle are moved to an environment with greater heat stress. Cattle that have eaten endophyte-infected fescue may have increased body temperatures and may be predisposed to heat stress.
Another factor that plays a role in heat stress is hide color,with black-hided cattle at greater risk than cattle with light-colored hides. Breed plays a role in that Bos indicus breeds (Brahman and others) handle heat better than do Bos taurus (European) breeds. For cattle in a feedlot situation, special attention should be given to newly arrived cattle and heavier cattle approaching finished weights. Iowa researchers found that unshaded lots facing south, southwest or west had higher death losses than lots facing east or southeast during a period of severe heat stress.
To read the entire article, link to the Angus Journal.