Jack C. Whittier
Animal Sciences Department

During periods of high temperatures and humidity, livestock losses can occur from hot weather stress.

Hot weather stress is particularly hazardous to closely confined livestock (those in feedlots, sorting and holding pens, trucks and rail cars). High relative humidity when the temperature is at 80 degrees or more adds to the likelihood of profit-stealing losses if necessary precautions are not taken.

Missouri livestock producers can make their livestock handling and marketing plans flexible enough to take necessary precautions to reduce or eliminate livestock hot weather stress by following the Livestock Weather Hazard Guide.

Table 1
Livestock weather hazard guide

Alert livestock men will adjust ventilation and bedding to the prevailing temperatures. If livestock must be transported, vehicles should be bedded with sand, sawdust, shavings or a combination of these in the summer. Avoid the use of straw, particularly oats straw, in vehicles with solid sides or tight boxes during hot weather. Use "wet" bedding in "shirt sleeve" weather. Sprinkling animals in confined areas may be advisable when the temperature is above 80 degrees outside.

Using the Livestock Weather Guide

Obtain the current or expected temperature and relative humidity by listening to a weather forecast or read your own thermometer and hygrometer. Bear in mind what the temperature and humidity is likely to be by the time the animals reach their destination.

Locate the temperature (actual or forecast) in the column at the left of Table 1.

Follow the line to the right until you come to the "relative humidity" that is equal to or less than that reported or forecast. This number tells you what weather stress category (Alert, Danger or Emergency) you are in and how safe your confined livestock may be. This stress is closely related to the "discomfort index" for humans as developed by the Weather Bureau, originally termed the "temperature-humidity" index (THI).

Stress categories

ALERT (THI of 75-78)

A forecast of temperature and humidity conditions in this range at time of handling, loading or before animals reach their destination calls for an "alert."

Additional precautions may be needed to avoid excessive losses or to prepare for higher THI.

DANGER (THI of 79-83)

Temperature and humidity readings in this range are not only dangerous to confined livestock, but there is a need to adopt additional measures to avoid severe losses.

EMERGENCY (THI of 84 and higher)

A severe situation has developed. Consider changing livestock handling and shipping plans. If plans cannot be changed, these four suggestions at a minimum should be followed:

• All handling stress should be kept at a minimum.
• Keep animals in position for free circulation of air.
• Provide shade if at all possible.
• Make water readily available for drinking.

If water is to be used to cool the animals, avoid "shock" from cold water in too huge quantities. A continuous sprinkling or coarse mist will lower the temperature to a safe level with a minimum of danger to the animals. Loading rested hogs onto wet bedding will minimize the heat stress problem during transit.

The best solution is to plan your livestock handling and shipping activities for the periods when the THI reading is below 75. Moving livestock when the THI is above 75 should be considered risky at best.

Cattle suffering from tall fescue endophyte fungus

Cattle consuming tall fescue forage that is infected with the endophyte fungus are particularly susceptible to heat stress during handling. Because humidity relationships for endophyte-stressed cattle are not yet known, a safe rule of thumb is to increase the temperature within Table 1 by 5 to 8 degrees to determine what weather stress category you are in. Handling such cattle during high temperature and humidity periods should be avoided if at all possible. If it is necessary to confine or transport them, it should be done during the night when temperatures are cooler. Even then caution should be used so cattle do not overheat.

Adapted from University of Nebraska Guide G07357 by Allen C. Wellman, Extension Economist, Marketing.