Model Description

Oklahoma’s weather extremes can have a direct and dramatic impact on grazing livestock. When weather conditions are ideal and livestock are “comfortable”, their performance and nutritional requirements are not affected. However, extreme weather conditions can dramatically alter feed intake, reduce daily weight gain, and increase nutritional requirements. The Oklahoma Mesonet Cattle Stress Index is a tool to assist cattle producers in identifying stress periods, caused by extreme weather conditions. The index provides an alert and an indication of the level of stress outdoor cattle may experience from either heat or cold temperatures. This measure of cattle stress allows producers to know when to take appropriate action to reduce cattle stress. Both cold and heat stress indexes are run on a continuous basis.

When ambient temperature and thermal radiation exceed the temperature of the animal’s skin surface, the animal’s body gains heat. Cattle shed heat primarily through evaporation from the skin and through respiration (breathing). As relative humidity increases, the effectiveness of evaporative heat loss diminishes. In fact, when relative humidity reaches 100%, evaporative heat loss is totally ineffective. The heat stress index is based on air temperature and relative humidity. The cattle heat stress index values are not the same as the human heat index or air temperature. The cattle heat stress index numbers are unique to outdoor cattle and cannot be used without additional interpretation for dairy cows or other livestock.

The cattle heat stress index formula is –

THI = tair – [0.55-(0.55*relh/100)]*(tairf-58.8)

The cold stress experienced by outdoor cattle is based on air temperature, wind speed, and the presence of rain or snow. The Cattle Cold Stress Index numbers are based on human wind chill calculations developed using the 1945 Siple and Passel Index. As of November 2001, the National Weather Service is using a new human wind chill formula that will undergo additional refinement in 2002. The new human wind chill values are warmer than the values based on the 1945 Siple and Passel Index. Thus, the Cattle Cold Stress Index numbers are lower than the current human wind chill values. Since cattle management recommendations are based on the older wind chill formula, it will continue to be used until a more accurate formula is created, based on new cattle research.

Calculation of cattle stress levels is complicated by cattle coat changes as they are exposed to seasonal temperature variations. As temperatures cool down in the fall cattle coat hair thickens to offer the animal more protection.

Another factor that contributes to animal stress is rainfall. A wet cattle coat loses its insulative properties. In terms of stress a wet coat is the same as a summer coat.

The following is the basic formula used to calculate the Cattle Cold Stress Index when temperatures fall below 45°F.

WCT = 0.0817*[(3.71*wind0.5)+(5.81-0.25 wind)]*[(tair-91.4)+91.4]

When temperatures are between 59°F and 46°F, the following formula is used.

CSI = [(tair-45F/14)] x tair + [(59F – tair)/14] x WCT

The following table shows the Wind Chill Temperature ranges in farenheit where “Mild, Moderate, and Severe” cold stress is likely. Actual cattle stress will vary with location, cattle breed, stage of hair growth, and wind exposure.

Cattle Coat Impact on Wind Chill Temperature Stress Levels

Whenever 0.1 of an inch of rain occurs in the last hour, the calculated cold stress is the same as if the animal had a summer dry coat.

The forecast model will indicate an alert if 0.1 of an inch of rain is forecast during the 6-hour period covered by the forecast model.

This model has been developed by Oklahoma State University in conjunction with the Intermountain Fire Sciences Lab of the US Forest Service in Missoula, MT.