how much of that was human edible callories? free grazing cattle will eat plant humans won't/can't. there is alot of land thats used for grazing that is not suitable for planting crops for a variety of reasons.
The vast majority of cattle are not grazing - instead they are fed farmed grain and soy in concentrated animal feeding operations (CAFO if you want to search it) where the density of animals is far greater than grazing would ever support.
Essentially most of that soy and corn in the American midwest and the clearcut Amazon are being raised to fatten cattle, not feed people - and it takes a lot of energy to do that.
Humans evolved to eat meat as a way to survive through winter. Early diets relied heavily on gathered foods.