AUM and AU seem to confuse people. rem_243 definition is about the simpliest way to explain it. Best way is to stock appropriatly for your area. It varies so much depending on how intense your grazing it and depending on rotation. Got one landlord that follows AUMs and dont care cow size and others that just tell me x amount of acres per pair for x months. You have to break it down to daily or monthly cost/pair to compare. No matter what my landlords say an aum is or the stocking rate i usually shake my head yes and do what they want if i am not overgrazing and the price is right.