Lakota would feed young cottonwood branches to their horses to get the through the winter. The Europeans used to do "Pollarding" where they cut the tops off of a lot of trees and fed those, then next year they cut them again. They cut them about four feet high or so. There is the issue of whether or not things are poisonous. I know maple leaves are poisonous to horses.
This is really the kind of thing I'm talking about. And the goats are already eating all of this stuff in their pastures, so there's no real problem with poisonous trees. We have a section of the property that is not easily accessible and is extremely overgrown, and my sister is planning on utilizing this area for producing tree hay, since we can't put the goats directly into the forest here.
There's actually not a lot of reliable literature about what is and is not poisonous to goats, but adult goats are pretty good about not eating something that is bad for them. They don't really like maple, so perhaps it is somewhat toxic for them too, but my sister obviously wouldn't do this with any tree species that they don't love to eat.