From a recent email correspondence...
I came across your site and I was very thankful to read. I am wondering, do sheep typically eat the bark of fruit trees?
Yes
I have a flock of sheep and I would like to put them in the orchard to keep the grass down, but don't want them to damage the trees.
Unless the trees are quite large and well-established, with the trunks well protected, the sheep will definitely damage and probably kill the trees.
Is there something I could feed them in order to deter them from the trees in the case that sheep do eat tree bark?
Pasture the sheep in the orchard ONLY when the grass is very lush and thick, and move the sheep at first sign of the grass being depleted and/or the sheep start experimenting on the trees. Don't pasture rams with the trees. Resign yourself to losing the bottommost branches. Wrap the trunks with hardware cloth...NOT chicken wire or page wire. (Neither of those will do much good.)
Repellants (such as "Hinder") MIGHT work temporarily; but I wouldn't bet the farm on it.
Sorry I can't give you better news here; but as I look out at what was once our orchard, I can say all this from experience.