No, AUM stands for “animal unit month” – a standard baseline value for stocking rates, grazing fees, reference books and all sorts of calculations involving grazing and land use. AUM comes from the term “animal unit” (AU), which is the standard livestock reference value that allows us to equate different livestock species and weights.
But there’s a slight problem with animal unit: Its universal reference definition is not exactly universal. As we’re in the grazing season now, grazing calculations are on our mind, so let’s look under the hood.
There’s an old joke about “How many Xs does it take to do Y?” Well, our grazing equivalent is, “How many sheep does it take to equal a cow?” Reference books and government agencies routinely use the conversion rate of five ewes to one cow.
This formula has been in place since the early part of the 20th century, when the federal government first designated vast swaths of public land as forest reserves and needed some way of establishing grazing fees and stocking rates. By 1907, federal grazing inspectors started using the 5-1 formula, and this has been in place ever since.
But the 5-1 ratio is not based on bodyweight. Even in 1900, before computers, scientists and ranchers knew that five ewes weighed considerably less than one cow. At that time, beef cows were generally Herefords or Hereford crosses that tended to weigh around 1,000 pounds. Commercial ewes generally weighed 130 to 170 pounds. If we accept an average ewe weight of 150 pounds, it’s clear that 5 multiplied by 150 equals 750, not 1,000.
But in 1900, those folks were good stockmen. Their real goals were to estimate carrying capacity and allocate public land forage. Sheep are much lighter than cattle, but pound for pound, they eat more than cattle. Ewes typically consume feed (dry matter intake or DMI) at 2.5 to 3.5 percent of their bodyweight (or more), while cows generally consume feed at around 2 percent bodyweight.
You can make the same calculations those folks did. Federal land managers judged that five ewes ate as much forage as one cow, and that conversion rate is still being used today.
This is interesting background, but underlying these calculations is one critical question: “What exactly constitutes a cow?” This is where things get a bit muddy.
If we search current reference books and scientific papers, we can find a surprising amount of variation. Most results, however, fall into three camps:
Camp No. 1: The big AU
An important 2011 monograph by Vivian Allen and her colleagues with the academic title “An International Terminology for Grazing Lands and Grazing Animals” gives a very precise definition. Definition No. 4.5.1.1 defines AU as a mature, nonlactating 500-kilogram cow in the middle-third of gestation fed at maintenance with zero bodyweight gain. Whew! Don’t let the metric system throw you; 500 kilograms equals 1,100 pounds.
The translation of this definition is a 1,100-pound pregnant cow fed at maintenance. We’ll come back to some of these details later.
Other major reference books similarly define AU as a 1,100-pound cow. Two of these are the classic textbook Forages that is currently used in many university courses, and also Jim Gerrish’s 2004 book Management-Intensive Grazing.
Camp No. 2: The small AU
One alternative AU definition is also very common. The Congressional Research Service, in a formal 2005 Glossary that it presented to Congress, defined AU as a 1,000-pound beef cow that ate 26 pounds of feed per day. I found the same definition echoed in documents from such diverse places as Alberta Agriculture, Delaware, Illinois, Utah, the NRCS and even that ultimate source of internet information – Wikipedia.
This congressional document also formally defines a sheep as 0.2 AU – which simply describes the 5-1 conversion ratio in a different way.
An interesting variation of this AU definition comes from the Society for Range Management (SRM). These are the scientists and others who specialize in grazing on public rangelands, primarily in the West. In a 1989 document, the SRM defined AU as a mature cow of 1,000 pounds, with or without a calf by her side. Apparently these folks didn’t want to commit, so they left their options open.
Camp No. 3: Everything else
Other folks simply don’t conform – like the Environmental Protection Agency (EPA). For many years, the EPA used AU in its regulations about emissions and pollutants from confined animal feeding operations (CAFOs), but now it has dropped AU in favor of … well, counting animals. Current CAFO regulations are based on the actual number of animals, listed in various categories, and not their feed intakes.
Actually, this makes sense, kind of, because the AU system really describes what goes into the front end of an animal, whereas the EPA is more interested in what comes out of the rear of the animal. (Although, rumen gases notoriously leave ruminants from front-end mouths, but we won’t quibble.)
Let’s return to that first definition of an AU as a mature, nonlactating 1,100-kilogram cow in the middle-third of gestation fed at maintenance with zero bodyweight gain. Consider this for a moment: Cows have a gestation length of nearly nine-and- a-half months. Weaning routinely occurs at seven months.
If we do the math, we see something intriguing. If a mature beef cow has a calf every 12 months, this means the middle-third of her gestation must occur during the last part of her lactation (just before the previous calf is weaned). In practice, cows should be gaining weight during midgestation, not eating at a maintenance level of intake. Putting all of this together means the first AU definition is for a cow that doesn’t really exist. To follow that logic, what is the correct ratio of sheep-to-cattle if the cattle don’t exist?
One additional point: The folks who designed this AU definition also listed the forage intake of their imaginary cow at 8.8 kilograms per day (19.4 pounds) which equates to 1.8 percent bodyweight (19.4 divided by 1,100 as a percent). They called this amount of feed a “forage intake unit” (FIU).
Actually, this logic makes good sense in terms of grazing. A land-use system based on the amount of forage intake dovetails nicely with good forage management, because all the calculations directly focus on the amount of forage available or consumed.
With FIUs, livestock conversions become less arbitrary and more accurate. All it takes is some ratio arithmetic. For example, a 1,000-pound Angus cow that eats 19.4 pounds of forage would be exactly 1.0 FIU. A 1,500-pound Simmental cow that eats 30 pounds of forage (2 percent bodyweight) would be 1.55 FIU. A 150-pound ewe that eats 5 pounds of forage (3.3 percent bodyweight) would be 0.26 FIU.
A 100-pound lamb that eats 3 pounds of forage (3 percent bodyweight) would be 0.15 FIU. And a ravenous 80-pound lamb that eats 3 pounds forage (3.75 percent bodyweight) would still be 0.15 FIU, because 3 pounds of forage is still 3 pounds of forage, no matter who or what is grazing it.
We’ve come full circle. We began by trying to determine how many animals can graze on an acreage, and we ended by equating their intake to the amount of forage. It always comes back to the amount of forage.
There are a lot of acronyms in this article, so let me summarize. If the DMI of an AU is an FIU, and the EPA defines CAFOs in a way that is different than the NRCS and SRM, and after translating kilograms to pounds, some AUs are 1,100 pounds and others are 1,000 pounds, how many sheep does it take to make a cow? I know it’s an old joke, but someone has to say it … 
Woody Lane, Ph.D., is a livestock nutritionist and forage specialist in Roseburg, Oregon. He operates an independent consulting business and teaches workshops across the U.S. and Canada. His book, From The Feed Trough: Essays and Insights on Livestock Nutrition in a Complex World, is available through Woody Lane.
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Woody Lane
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- Roseburg, Oregon
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