You can’t manage the weather, but you can prepare to manage heat stress in cattle.  Fortunately, there is a USDA Agricultural Research Service website that has detailed heat stress forecasts for the north central U.S. This can be accessed at http://www.ars.usda.gov/Main/docs.htm?docid=20426. The forecasts consider several factors – temperature, humidity, wind speed and cloud cover – and project cattle heat stress for seven days. As expected, high daily air temperatures, little or no wind, high humidity and little cloud cover create heat stress, but there is also the potential for heat stress on nights where overnight lows greater than 70?F do not allow cattle to cool down.

Besides monitoring weather, it’s important to monitor the cattle for signs of heat stress, such as breathing rate, standing, restlessness, drooling, and grouping together. With increasing heat stress, most of these symptoms increase. However, in cases of more severe heat stress, there may be open mouth breathing with the tongue projected and not necessarily drooling.  Respiration rate is an excellent indication of heat stress and is easy to monitor. An animal with <90 breaths per minute is considered normal.  Rates of 90-110, 110-130, and >130 breaths per minute would be in the alert, danger and emergency stages of heat stress, respectively.

Some cattle are more susceptible to heat stress than others depending on their genetics, health, production status and previous exposure. Bos Taurus, black or dark red colored, and excitable cattle are at a higher risk for heat stress. Cattle with poor current health, past cases of pneumonia or previous health issues are also more susceptible. Market-ready finished cattle, newly arrived cattle and those in poor condition are more prone to heat stress. Also, cattle in the late spring to early summer or cattle transferred from a more northern location may not yet be acclimated to warmer summer days.   

A producer can manipulate three factors to reduce heat stress – facilities, nutrition and cattle handling. Air movement, adequate shade and water spraying are important considerations for the feedlot facilities. However, the effects of shade and sprinklers are not additive so use either shade or sprinklers, but not both. If using a sprinkler to cool the animal, the animal must be intermittently wet to the hide (5 minutes out of 30) and there must be air flow to evaporate the water and cool the animal. Avoid a fine mist and remove any barriers to air movement. 

Nutrition includes both water and feed. Cattle should have access to water that is less than 80 degrees F. Check to make sure that there is adequate water tank space and flow rate. The water supply should be able to deliver 1.1% of body weight of the cattle per hour. With feed, consider shifting the daily feed delivery schedule toward evening feeding. Rations may be reformulated to lower the energy content by 5 to 7 percent or total feed intake may be lowered to minimize the metabolic heat load of the animal. Busby and Loy noted heifers fed MGA had lower death loss due to heat stress compared to those not fed MGA.

If possible, do not work or move cattle during periods of heat stress. In hot weather, schedule cattle handling between midnight and 8 a.m. Never handle cattle after 10 a.m. Be aware that any kind of stress, such as heat stress can increase the incidence of dark cutter carcasses.

For detailed information about heat stress, check out the Iowa Beef Center website at http://www.iowabeefcenter.org/heatresources.html.

Source: Beth Doran, Iowa State University Extension beef program specialist