A grassy plain supports many fewer animals than the equivalent corn field. Otherwise there would be no economic incentive to grow corn instead of raising livestock on the same land. So you would need much more land to support the same amount of animals, land we just don't have.
Prior to Europeans arriving, it's estimated there were 60 million wild buffalo in the US. There are now 93 million cows in the US. And those cows are much fatter. Bred to produce more meat, grow faster. So consume much more food (and produce more per unit), so the comparison is even worse than that.
Prior to Europeans arriving, it's estimated there were 60 million wild buffalo in the US. There are now 93 million cows in the US. And those cows are much fatter. Bred to produce more meat, grow faster. So consume much more food (and produce more per unit), so the comparison is even worse than that.