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Understanding AUMs (Animal Unit Months)
By Karla Jenkins, Ph.D., PAS
Cow/calf and range management specialist
UNL Panhandle Research and Extension Center
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.
Dr. Jerry Volesky, 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 http://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 http://www.ianrpubs.unl.edu/epublic/live/ec158/build/ec158.pdf
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 animal is considered 1 AU, making a 600-pound animal 0.6 AU and a 1,200-pound animal 1.2 AU. Therefore, a 1,200-pound cow and her 300-pound calf would be considered 1.5 AU.
Furthermore, 780 pounds of air-dried grass is considered 1 AUM. This means a 1000-pound animal would utilize 780 pounds 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-pound steer grazing on a pasture rated at 0.3 AUM/acre would need 2 acres for one month of grazing (0.6/0.3) while a 1,200-pound 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/.