That's a great question. It's actually hard to get this type of data. I once tried to calculate something adjacent by looking at feedstock production, but actually a lot of feedstock is used for 'biofuel'.
My knowledge comes from exposure to the agricultural industry. You might be able to find some animals grazing on a grassy hill, but that doesn't scale. In addition, most NA climates don't support year long grazing.
It seems to me that there are many grassy hills that cannot be farmed using tractors and to farm them using hand tools is too labor-intensive.