In it's natural state bison would have been grazing on it. That also doesn't solve the gripe that vegans have which is that land could be used for crops, which really destroys the biodiversity of land. At least with cattle, you just let them eat anything that grows. Horses are usually terrible for biodiversity because people mow the land and want nice lush fields, were as cattle farmers don't, they let the cows eat roughage which is actually healthier for them. They also rotate pastures a lot more than most horse people do.