Ditto "cheri". My horses have all sorts of tree leaves available to them.
We have plenty of Oak, Walnut, Birch, Cedar, Pine; I don't know what else is in the pasture and in three neighbors' woods that hang over the fence boundary.
My horses are on 22 acres with plenty of pasture and the only thing I ever see a tree get used for is a rubbing post.
That being said, when the Fall leaves get thick in the yard, I will mow/mulch everything away so they can eat grass without coming up with a mouthful of dying leaves (those are toxic). If I can't get the leaves taken up, they can't come out and play in the yard.
If you don't have a lot of pasture, it's the dying leaves that I would keep picked up.
As long as you have ample hay and some pasture, and they aren't bored or hungry, they most likely will leave the green tree leaves alone:)