It has been suggested (by yours truly, among others) that the trees need to be petted, watered and harvested occasionally for the pigs to eat off them. However, there is, as far as I know, no definitive statement one way or the other on this. Also, while piggies can eat from feeders that they can not physically reach, I suspect they need to get to the trees to feed off them. To be safe I'd recommend leaving a pile of grain on the ground where they can reach it.