There were herds of bison on the American plains during the early 1800s that were so large it would take a week for the herd to walk past a campsite. I was reading the memoirs of Ulysses S Grant a couple years ago and he reports that when he was on a military expedition in Texas a couple of decades before the American Civil War, they came across a herd of wild horses that was so large that they could climb to the top of a nearby hill and see the herd stretching from horizon to horizon on either side.

My question for you is: what were all these animals eating, since we were not cultivating food for them?

Also, why do you think cows don't graze in open fields? Maybe you should take a drive through Lancaster County, PA, sometime. I hear there are cows out west, too.