How to Use the Model

Heat Stress:
Heat stress can cause reduced productivity in beef and dairy cattle herds. The effects of severe heat stress are often seen in the form of reduced reproductive performance, reduced daily weight gain of growing cattle and reduced milk production. Cattle are more sensitive to heat stress than humans, although cattle do seem to have a wide range of heat tolerance. From an environmental perspective, heat stress is a combination of temperature, relative humidity, and wind speed. However, animal factors, such as age, hair coat length, hair coat color, and nutrition status, interact with these environmental factors to determine the severity of heat stress.

The Mesonet Cattle Stress Index was designed to alert producers when cattle stress occurs from either high or low temperatures. The Cattle Heat Stress Index is a calculated value that is unique for livestock. The calculated value is not the same as the air temperature or human heat index.

When the Cattle Heat Stress Index is at 71 or below, cattle are in the thermal neutral zone, meaning that they are comfortable and production will not be sacrificed due to severe environmental conditions. Values ranging from 72-79 indicate “Mild Stress.” This is often when cattle will move to shade to cool themselves. Livestock managers should monitor the weather and prepare to take action if conditions worsen. Heat Stress Index values ranging from 80-89 indicate a “Moderate Stress.” This is the time to implement management strategies to help reduce cattle stress. Values above 90 indicate “Severe Stress.“

What can you do in a heat stress situation?

Provide ample water.

On days when the index is 72 or higher the cattle may need more than 2 gallons of water per 100 pounds of body weight. Provide enough tanks for cattle to be able to get the water they need. If possible, water should be cooled. Tanks should be cleaned weekly to encourage water consumption. Making water available under a shaded area will increase cattle water consumption.

Avoid handling cattle:

Handling cattle can elevate their body temperature by as much as 3.5∞F. If cattle must be worked on days when the Cattle Stress Index is likely to go over 80, try to do the work before 8:00 AM and keep the maximum time in the holding facilities to no more than 30 minutes. On days when the index will be 80 or above, do not work cattle after 10:00 AM. The 60-hour forecast component of the cattle stress index, will allow you to schedule management practices to best maintain cattle health.

Change feeding patterns:

Shift the feeding schedule toward evening on days when the Cattle Stress Index is above 72. Try to deliver 70% of the daily scheduled feed two to four hours after the peak air temperature. Providing only small amounts of feed during the heat of the day, will decrease the metabolic heat of digestion.

Provide shade:

A shade tree is just as welcome a relief for cattle as humans on a hot summer day. Shade can also be constructed. Shade height should be 8-14 feet tall and should be large enough to provide 20-40 square feet per animal. The most effective shade is a solid reflective roof constructed of white colored, galvanized, or aluminum materials. Shading with wooden slats, plastic fencing, or other materials that allow flecks of sunlight to hit the animals are less effective. If possible two shaded areas are recommended, one over the feed area to increase feeding time, and another away from the feed area to encourage the cattle to rest. Water should be made available under both shaded areas, to increase the water consumption during heat stress period. If the structure is left up year-round, construct a frame adequate for snow load. Shade is insurance against mortality loss. Any performance benefits are a bonus.

Improve airflow:

Consider where the cattle are located and if there is any air restriction. Buildings, high fences, or vegetation can block airflow. A 6-foot high windbreak can obstruct airflow for 60 feet downwind.

Provide water mist:

Providing a spray of water will help to cool the animals down. However it is important to place misters over a clean, concrete area. Running misters over dirt creates mud and increases the potential for mastitis or other bacterial diseases. A timer should be used to run the mister long enough to cool, but not wet the cattle. Do not allow mist to wet nearby feed. Wet feed spoils rapidly with Oklahoma’s summer heat.

Control biting flies:

Stable flies cause cattle to bunch and disrupt cooling. Monitor the situation and control the flies as needed. Eliminate any shallow pools or muddy areas nearby, since they are common breeding areas for flies.

Reference:

• Managing Feedlot Heat Stress, Nebraska Extension NebGuide G00-1409-A, August 2000.

Cold Stress:

Experienced livestock producers are well aware of the toll severe winter weather can have on animal health and performance. The Oklahoma Mesonet Cattle Cold Stress Index alerts livestock producers to cold stress conditions. Research indicates that the effects of cold, wind, wet hair coat and muddy pastures and pens are additive. These stresses can be managed to a limited degree. Beef cattle can be comfortable within a wide range of temperatures; from 20 to 70° F, depending largely on hair coat length and hair coat condition (dry, wet, muddy etc.). The lower critical temperature is defined as the effective ambient temperature at which energy intake must increase in order to minimize reduction in weight gain, in the case of growing cattle, or to prevent weight loss in mature cattle. The following table suggests guidelines for lower critical temperature for various hair coat conditions.

The combined effects of temperature and wind are often expressed as a wind chill index. The wind chill index, rather than ambient temperature, is used to estimate effective temperature when considering the severity of cold stress. For example, when the temperature is 20°F with no wind, the wind chill index is 20°. At the same ambient temperature, 5, 15 and 25 mph winds would result in a wind chill index (or effective temperature) of 13°, 4° and -7° F, respectively. Obviously, anything that can be done to reduce exposure to wind will dramatically reduce cold stress.

In general, a cow’s energy requirements increase 1% for each degree the wind chill is below 32°F. For a wet cow, the increased energy requirement begins at 59°F and increases 2% for each degree drop.

In cold wet conditions, this increased energy need is often virtually impossible to accomplish with feedstuffs available on ranches. In addition, this amount of energy change in the diet of cows accustomed to a high roughage diet, must be made very gradually to avoid severe digestive disorders. Therefore, the more common-sense approach is a smaller increase in energy fed during wet cold weather, and extending the increase into more pleasant weather to help regain energy lost during the storm.

For example, a cow consuming 16 pounds of grass hay per day and 5 pounds of 20% range cubes under mild weather, could have its feed in increased to 20 pounds of grass hay per day (also possibly offering a better quality hay) plus 6 to 7 pounds of range cubes during a severe weather event. This is not a doubling of the energy intake but extending this amount for a day or two after a storm may help overcome the energy loss during the storm and is done in a manner that does not cause digestive disorders.

A second approach that is often used is to reserve the highest quality hay for feeding during stressful weather periods.

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.