This summer, much of Wyoming experienced maximum temperatures well into the 90s. While it is tempting to hunker down by the air conditioner or head for higher elevations until things cool down, these are the most critical times of the summer to ensure livestock are being properly managed to prevent heat stress and maintain productivity.

Heat stress onset

Heat stress occurs in livestock when heat inputs from the environment and from normal metabolic processes in the body exceed the capacity of the animal’s ability to dissipate the heat. On a cloudless day, radiation from the sun warms the air temperature and objects in the environment (i.e., ground surface, sheds, concrete pads), and this radiation also warms the animal itself.

Additional factors, including hide color and breed composition, can influence the tolerance or susceptibility of an individual to heat stress. Any time the air temperature exceeds 80 degrees Fahrenheit, it is wise to monitor livestock for signs of heat stress.

Measuring heat stress severity

A temperature-humidity index (THI) was created to identify and define situations in which beef cattle may experience heat exposure. A chart using the temperature and relative humidity to define the THI value for beef cattle can be viewed at bit.ly/unl-cattle-temp-humidity.

Categories for beef cattle are: normal (THI less than 75), alert (THI 75 to 78), danger (THI 79 to 83) and emergency (THI greater than 84). Sheep and goats are more tolerant of heat and do not enter the alert category until THI is greater than 82, which is extremely rare in Wyoming.

Animal response

An animal’s nervous system picks up signals indicating that the body’s core temperature is increasing and initiates behavioral and physiological cooling mechanisms. Ruminant livestock shift grazing to cooler periods of the day and spend less time lying down in direct contact with the warm ground.

Cattle begin sweating to release heat through the evaporation of moisture and will pant if sweating alone is not sufficient to provide cooling. Sheep and goats primarily utilize panting, but will also sweat from some portions of the body, including the ears and lower legs.

A lack of breeze and high relative humidity (both rare here in Wyoming) make heat stress worse by limiting evaporative cooling. Under mildly warm conditions, these coping measures are sufficient to reduce the core body temperature and restore balance in the system.

When extremely hot conditions are sustained for several days, animals may gradually lose their ability to self-regulate body temperature, which can lead to hyperthermia or death without immediate intervention.

Heat stress symptoms

Animals experiencing heat stress reduce their feed intake to limit the heat generated by digestion of feeds. This is even more evident in ruminant livestock since they rely on microbial fermentation of forages, which releases additional thermal energy. The consequence of limited intake is decreased rates of growth, and in severe cases, loss of condition.

Heat stress is also highly disruptive of reproductive processes and damages viability of sperm, oocyte, and expression of estrus. Breeding season heat waves negatively impact conception and pregnancy rates.

Effects of sustained heat stress in already-pregnant females may be transposed to their offspring, resulting in decreased muscle mass, impaired metabolic function, and ultimately, reduced market value.

Prevention

Management steps to prevent heat stress are simple.

First, ensure animals have access to cool, clean water at all times. This rule applies no matter what the weather conditions, but producers should be mindful that daily water intake will increase during hot spells. Always ensure watering systems have the capacity to keep up with livestock demand, or else plan to provide supplemental water.

Second, when possible, provide access to shade. Simply removing animals from direct exposure to solar radiation has been shown to alleviate the most severe effects of heat stress. Trees make excellent natural shades, and transpiration from the leaves can actually cool the microclimate beneath their canopies. Ensure sheds and other manufactured structures are well ventilated.

Third, avoid handling livestock during the hottest hours of the day. Morning work should be finished around 10 a.m. and evening work should not begin until after 4 p.m., as a general rule. When animal work or transport during this time cannot be avoided, use extreme caution to minimize handling stress when loading and unloading, and have water ready and available at the destination site.

Managing heat stress

Hot temperatures are hard on humans and livestock alike. Keeping a close eye on animals as summer conditions peak, ensuring access to water and shade, and limiting handling to cooler hours of the day will minimize the impact of heat stress on growth and performance. Managing for heat stress allows ranchers to continue making progress toward their production goals.

While summer humidity in Wyoming does not often reach the extreme conditions likely to cause emergency heat exposure situations, it is worth noting heat stress can occur in any animal experiencing conditions for which they are not acclimatized.

For example, a week of unusually warm temperatures following an April blizzard may induce mild heat stress. Likewise, bringing cattle down from the mountain as lingering summer conditions warm up the lower elevations may induce heat stress. As always, monitor animals for unusual behaviors during times of shifting weather patterns.

Additional resources

• Use Shade to Combat Heat Stress in Animals
• Heat Stress: Handling Cattle Through High Heat Humidity Indexes
• Heat stress in sheep and goats

Micah Most is a UW Extension agriculture and natural resources educator based in Johnson County. He can be reached at mmost@uwyo.edu.