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April 23, 2012

Understanding AUMs

As producers get ready to take cattle to summer pasture, questions often arise about the relationship between range management and stocking rate.

Clearly, if the range resource is underutilized, then profitability will be decreased as more animals could have been using the range. Conversely, and more commonly, the tendency is to overgraze the range and put the sustainability of the range in jeopardy.

Developing a grazing plan to utilize pastures can help producers effectively manage their forage resources.

Jerry Volesky, University of Nebraska–Lincoln (UNL) forage and range specialist, has developed a Microsoft Excel worksheet to help producers track grazing days. The
Grazing Records Template (Extension Circular 165) is available on the World Wide Web at www.ianrpubs.unl.edu/epublic/live/ec165/build/ec165.pdf. Additional information also is available from another Extension Circular, Grazing Strategies for Semi-Arid Rangelands (EC 158), at www.ianrpubs.unl.edu/epublic/live/ec158/.

Often grazing plans and recommendations use terminology such as animal unit months (AUMs) to describe the carrying capacity of a given forage or pasture. This is simply a system used to standardize the forage needs of cattle and the forage available. In this system, a 1,000-pound (lb.) animal is considered 1 AU, making a 600-lb. animal 0.6 AU and a 1,200-lb. animal 1.2 AU. Therefore, a 1,200-lb. cow and her 300-lb. calf would be considered 1.5 AU.

Furthermore, 780 lb. of air-dried grass is considered 1 AUM. This means a 1,000-lb. animal would utilize 780 lb. of forage dry matter (all the moisture removed) in one month's time.

The state of Nebraska is divided into four vegetative zones based on rainfall and forage species composition. Because of the differences across the state, each zone has a little different carrying capacity for each range site. Most typically in the Nebraska Panhandle (Vegetative Zone 1), common upland range sites in good condition will have a carrying capacity of 0.3 to 0.5 AUMs per acre. Therefore a 600-lb. steer grazing on a pasture rated at 0.3 AUM per acre would need 2 acres for one month of grazing (0.6/0.3) while a 1,200-lb. cow would need 4 acres for the month (1.2/0.3).

These are estimates and can change. For example, research at UNL has indicated that physiological state (lactation, pregnancy, growth) influences forage intake. Additionally, the carrying capacity of the range can be reduced by drought, hail, fire and insects. Therefore, producers should monitor pastures closely to avoid improper utilization.

Producers wanting more accurate estimates of their pastures' forage production can contact their local extension office or the Natural Resource Conservation Service for information on sampling and calculating carrying capacity.

For more information on beef and forage management, go to http://beef.unl.edu/.


The Panhandle Research and Extension Center is on the World Wide Web at panhandle.unl.edu.