Grazing livestock can either be described as an art, as a science, or more commonly as both. Industry research and science-based efforts support the feasibility of running a successful livestock operation, as well as inform land management agencies when creating policy or developing grazing regulations. But long-time ranchers and livestock managers are also able to add their own personal touch of success, their own finesse, to an operation to ensure it continues to operate smoothly and effectively, even when faced with adverse conditions. This can only be learned through a lifetime of experience and try.

Every topic studied in science, has an associated unit of measure. The science of sustainable grazing is no different. The standard unit used to determine the stocking rate of an area is the “Animal Unit Month,” abbreviated as the AUM. An “Animal Unit Month” is defined as the amount of dry forage a 1,000-pound cow and her unweaned calf will eat in one month. This number can be further equated to roughly 750 pounds of forage intake per month for the cow-calf pair, based on the numbers of a 1,000-pound cow, a cow will consume about 2.5% of her body weight daily, and 30 days on average in a month (1,000 x 2.5% x 30 = 750).

However, every cattle breed is not built the same. Even individual cows within the same breed will grow and mature at different weights. And what about ranchers and livestock producers that raise something other than beef cattle? Like the sheep, goat, bison, elk, or miniature cattle breeders?! This is where the “Animal Unit Equivalent” enters the stocking rate equation. An “Animal Unit Equivalent” or “AUE” is simply the comparison or proportion of forage a different species or size of animal will eat compared to the standard 1,000-pound cow and the “Animal Unit Month” measurement. A 1,000-pound cow and calf equals one AUM and also equals one AUE. But a 1,200-pound cow is proportionally bigger (1,200 versus 1,000 or 1.2:1) than the standard 1,000-pound example. Therefore, a 1,200-pound cow and calf equals 1.2 AUE. Whereas a smaller animal like a sheep would eat 20% of what a 1,000-pound cow would, equaling 0.2 AUE or five sheep per one AUE.

“Animal Unit Equivalents” can be factored for any type of grazing species, both big and small. It is just a measurement of how much forage an animal eats in a month. A paper released by the University of Wyoming Extension lists several examples of different livestock and wildlife species and their corresponding “Animal Unit Equivalents;” such as a mature elk is 0.6 AUE, a mature horse is 1.25 AUE, a dry (calf-less) 1,000-pound cow is 0.92 AUE, and a mature domestic sheep (ewe with lamb) is 0.2 AUE. A paper between the Natural Resources Conservation Service (NRCS) and North Dakota State University (NDSU) takes it one step farther. Their examples include a mature bison bull is 1.5 AUE, yearling cattle (600 lbs. – 800 lbs.) are around 0.7 AUE, a mature white-tail deer is 0.15 AUE, a mature mule deer is 0.2 AUE, and a mature antelope is 0.2 AUE. They also list a white-tailed jackrabbit as 0.02 AUE, meaning 50 jackrabbits eat as much in one month as one average -sized cow (1 AUE 0.02 AUE = 50). And a prairie dog equals 0.004 AUE or 250 prairie dogs per one “Animal Unit Equivalent” (1 AUE 0.004 AUE = 250).

While “Animal Unit Months” and “Animal Unit Equivalents” are an enlightening concept, and it is amusing to mathematically figure that 250 prairie dogs eat as much as a one cow in a single month, AUM’s/ AUE’s serve a very practical purpose in the grazing world. The AUE number is instrumental in calculating the appropriate stocking rate or carrying capacity of livestock for a set timeframe and set acreage amount. Inversely, using the AUE, the utilization rate, and forage consumption, one can calculate the total forage of the area and the available AUM’s for a given pasture. “Animal Unit Months” (AUM’s) are an important part of grazing management and provide a set calculation to aid in land management.

References:
North Dakota State University and Natural Resources Conservation Service https://www.nrcs.usda.gov/sites/default/files/2022-10/Determining%20Carry%20Capacity%20and%20Stocking%20Rates%20_ND.pdf

University of Wyoming Extension https://wyoextension.org/publications/html/B1320/

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