A 1,200 lb lactating beef animal needs around 3% of it’s body mass every day. So around 35lbs of dry matter forage per day. Works out to around 6.4 tons DM/year.
Under irrigation, In areas without freezing temps, 25tons DM/acre is possible (not easy) or 4 cows. In areas with freezing temps 12-15 tons DM/acre can be accomplished or 2 cows (1 cow if the growing season is short)
10-15" rainfall zone produces around 600lbs DM/acre of which around 50% is available (timing issue) this is around 0.15 tons DM/acre. 6.4 tons DM for one cow is around 43 acres.
In a 5-10" rainfall zone it reduces to under 200lbs DM/acre total. Or 0.05 tons DM/acre or around 128 acres per cow. With that much walking their energy needs increase by as much as 50%. Or around 200 acres/cow.
Guess who grew up on a ranch with BLM grazing ground :-) My grandfather decided going bankrupt was a better than listening to a younger more hotheaded me.