What kind of goats are they and what is their milk
In general, with the modern dairy
animals, feeding grain or any other concentrated high protein feed it’s because they make so much milk, they can’t really get all their nutritional needs from just pasture or browsing, no matter how great those are. They will “milk off their back” if all those nutritional needs are not met.
If your goats are the high producing kind, you may never be able to keep them just on pasture or browsing. You might be able to get to a point where you don’t need to supplement their pasture/browsing if you selectively breed several generations for that purpose and cull mercilessly everything that doesn’t do well for your goal.
If, on the other hand, you do have low producing goats, the best feed for them would be a mixture of pasture that has lots of grasses and forbes and browsing of small bushes, branches of trees
and the like.
Also very important, goats have very high and specific mineral needs, and that needs to be addressed depending on your region and how the soils are in that area.