A producer may be able to run two cow-calf pairs on well maintained, irrigated pasturage of a single acre........there are many places in the United States (think high desert areas, like Nevada, parts of Idaho, New Mexico, Arizona, etc), based on the forage that grows naturally (non-irrigated, native grasses/bushes), it often times takes 30 acres or more to run one cow-calf pair!!
Before folks jump the gun and suggest that the domestic livestock just get off the range.....please consider that their grazing enhances the growth of forage making it better for ALL animals! Additionally, it is possible to feed domesticated livestock grains with seeds which are passed thru their digestive system which, after passing thru and being deposited in a pile of natural fertilizer and awaiting the occasional rain/snow, will take root and increase forage for all animals.
While grazing is benificial in moderation, al ot of people working the public lands don't care about moderation. After all, they break it, they don't have to pay to fix it (largely correct, yes?). Solution: either force them to change their ways (difficult) or privatize the rangeland. Of course, that'ld go over well I'm sure...