If you’re new to the wild horse world, you may not know the difference between an AUM and AML but you need to get up to speed on this stuff as soon as possible. These are not the stated definitions, they reflect current management practices.
AML (Appropriate Management Level) – The number of horses or burros an area can support after diverting most of the resources to privately owned livestock.
HA (Herd Area) – An area identified for wild horses or burros in 1971 but no longer managed for them (AML is zero). Now managed almost exclusively for livestock.
HMA (Herd Management Area) – Usually a subset of an HA, an area identified for wild horses or burros in 1971 but now managed primarily for livestock. Referred to as a Wild Horse Territory (WHT) or Wild Burro Territory (WBT) on Forest Service lands.
WHR (Wild Horse Range) – An area managed principally for wild horses, per the original statute, now accounting for less than two percent of areas where wild horses are still allowed by plan.
AUM (Animal Unit Month) – The amount of forage consumed by one cow/calf pair, one wild horse, two wild burros or five domestic sheep in one month.
RMP (Resource Management Plan) – A document that specifies management priorities on public lands and allocates resources accordingly. AMLs derive from these plans.
EA (Environmental Assessment) – A review of the expected consequences of a proposed action, along with those of one or more alternatives, usually carried out before anything happens in the field. Roundups enforce the resource allocations of RMPs—they cannot not change them—so comments involving forage distribution on EAs for roundups are usually rejected.
EIS (Environmental Impact Statement) – A more in-depth analysis of a proposed action and its alternatives, usually reserved for major changes to the way public lands are managed.
The number of wild horses or burros an area can support, after taking back the resources assigned to privately owned livestock, is usually referred to as the True AML on these pages and represents the sum of the animals allowed plan (AML) and the animals displaced from their home range by domestic livestock, frequently climbing into the thousands.