Animal unit (AU) means one adult cow with unweaned calf by her side or equivalent thereof based on comparative forage consumption. Accepted conversion factors are: sheep and goats, one ewe, doe, buck or ram equals 0.25 A.U.; one sheep unit year long (SUYL) equals 0.25 Animal Unit year long; horses and mules, one horse, mule, donkey or burro equals 1.25 A.U.
25 CFR § 168.1
As used in this part, terms shall have the meanings set forth in this section.