If it's good prairie that is seeded out, then possibly. But because I've grown up FFA and 4-H, I know our dairy's have to have more supplement to improve weight and milk. Especially since they are producing, calf manna, etc also needs to be added in. At our fair this year, the best goats were fed nearly all of their dietary needs by grains. Grasses were supplemental, and inconsequential since we are in drought. Even our project meat goats are the same way-- grain fed. Oh, and these aren't mine-- I'm not saying "me" and trying to make it sound like I'm doing this personally. Our club does both market/meat goats and dairy and sheep. As a project, we go to their homes, and we have talks about what to feed them, how much, what kinds of grains, how to yield better meat, more milk, etc. It's a fantastic opportunity to learn, and then of course, I sit in on meetings with the county market group that does market animals like swine, bucket calves, meat goats, etc.. It's really fun.
But they can not live on grass alone, no. Or I suppose they could, but they would be terrible producers and it would be a terrible thing to do to them. It can also have a lot to do with the area of the country you are in, and what your fields will yield during the year, too and if it could really sustain them. I have a pretty good understanding since I'll be taking over my dad's ag farm at some point. We have both pasture land for cattle and ag land where we grow grains, beans and corn.
