Even if the horses get fed 4x a day they are still without food in between because they wolf those feedings then are bored inbetween. My horses were eating our oaks... not good. We put chicken wire around them and they pushed it up to eat around the bases... not good. Then we made some restricted free choice feeders where the horses have to work to get the food through 2x2 welded wire grids. The result is that the forage takes much longer to eat, fat horses lost weight, thin horses gained weight, they aren't bored and voila no more eating the trees. You can go to Home of Easy Feeding
for some ideas on how to make these feeders.