I'm dubious to the claim of "most". data I can find suggests about 60%> In the US the standard for grass fed is 50% of their diet to claim "grass fed".

There are days it's too wet to let your cows graze, they'll destroy the field. So you keep them in and have to feed them something. Then there are 3-4mo your fields don't produce grass, which means again, food comes from somewhere.

If the fields are growing enough grass to feed a meaningful amount of cattle, they can grow other crops.

My buddy is a dairy farmer though he calls himself a grass farmer. They only supplement feed probably 10-20% during the growing season as they can grow enough grass. He cannot produce enough extra and has to buy feed for the 3-4mo he can't pasture them.

I'm not against meat. The truth is it takes more everything (water, fertilizer, land, blah) to cultivate rather than plants. I'm all for reducing our meat consumption. It will be interesting to see how lab grown meat changes the equation.