Managing Heat Stress in Feedlot Cattle

Summer is here. With summer comes the dreaded heat and concern about heat stress in livestock. Heat stress is responsible for the loss of hundreds of millions of dollars in the livestock industry annually. Animals confined in a feedlot setting are more susceptible to heat stress than pastured animals. Reasons for this include the fact that they are not able to find natural shade and/or water, and radiant heat of open soil is much higher than that of grass. Therefore, special precautions should be taken to mitigate the effects of heat stress for these animals.

Environmental Factors
While the Temperature Humidity Index (THI) is often considered a standard measure for the intensity of heat stress, the THI accounts for just as it sounds—temperature and humidity. The Livestock Weather Safety Index (LWSI) was developed to assign various categories of danger associated with corresponding THI values. These can be seen in the Livestock Weather Safety Index table. Although the THI and LWSI have been effectively used as an indicator of heat stress, it is critically important that other factors including wind speed and solar radiation be taken into consideration as well. Coupling high temperatures and relative humidity with increased solar radiation and low wind speed will create a recipe for disaster for 1200 lb animals wearing a leather coat. Fortunately, there are a variety of strategies that can be implemented to help mitigate the effects of heat stress.

Feeding 
Decreasing caloric intake, for instance, moving cattle from a 58 Mcal to a 52 Mcal/cwt ration has been shown to decrease heat stress in feedlot cattle. However, this is not the best option for cattle close to finish as it can disrupt their feeding pattern. Another strategy is to decrease ad-libitum intake during heat events, by limiting the amount of feed the animals receive each day. This decreases the amount of heat generated by fermentation in the rumen, which decreases the heat load on the animal. Changing feeding time to late afternoon would allow peak fermentation to occur during the evening hours when air temperatures are cooler. It takes six hours for heat to dissipate, so allowing this to occur overnight can be very beneficial. Capsaicin is a natural feed additive that has been shown to aid in heat stress abatement. It acts as a vasodilator, which increases blood flow and stimulates water intake. Consistent water intake then helps to drive feed intake—an important performance parameter. Many companies offer products containing capsaicin. Be sure to ask a Sioux Nation Ag Center field marketer or nutritionist about our Polar Power product.

Water
Allowing adequate water space (3 in2/animal) during summer months is critical for both animal health and comfort. A 1000 lb animal will consume 1.5 gal/hr during heat events. Following this formula, a pen containing 100 hd should have the water capacity needed to provide 150 gal/hr. Cattle are not able to dissipate heat through sweat as humans do. Spraying water directly on the animal may help reduce heat load by increasing the amount of heat removed through the evaporation process. Spraying the pen surface is often more helpful. Water improves the thermal conductivity of the soil on the pen surface, reducing the ground temperature. When ground temperature is less than animal body temperature, heat can escape and reduce heat load on the animal.

Shade
Shade will have the same effect as water on cooling ground temperature. For maximum results, allow 20-40 ft per head. A variety of commercial shades are available. Another option is access to building shade; however, it is critically important that shade provisions not impede air flow.

Color
Color matters. In a study conducted by Arp et al. (1983), the radiant heat off a black-hided animal was 42°F higher than that of a red-hided animal and 53°F higher than white-hided animals. This does not mean you have to feed only Charolais cattle in the summer; however, it does mean that special provisions should be made considering the type of cattle on feed during summer months.

DO NOT WORK CATTLE ON HOT DAYS!
If it must be done, working should be done in the early morning when the temperature is low to allow the stress to dissipate before temperatures rise.

Livestock Weather Safety Index
Relative Humidity (%)
Temperature (°F)303540455055606570758085
76697070717172727373747275
78707171727373747475767676
80727273737475757676777878
82737374757576777778797980
84747575767778787980808182
86757677787879808181828384
88767778798081818283848586
90787979808182838485868687
95798081828384858586878889
94808182838485868788899091
96818283858687888990919293
98838485868788899091939495
100848586878890919293949597
Normal < 75No stress
Alert 75-78Expect ≥ 10% production loss
Danger 79-83Expect ≥ 25% production loss
Emergency >83Expect ≥ 45% production loss