Fishin'Rod we use the stinky spray product called Liquid Fence. It has a putrid smell of rotten eggs that can protect from deer and rabbit munching for up to a month. We buy a quart of the concentrate and mix it with water, then fill a spray bottle. Dousing the perimeter of an area seems to keep the deer and rabbits out of the area; spraying the individual plants also works well.
I'd imagine regular use of this will help quite a bit. Then again, you might have to use several other techniques, too, like the castor oil. I can't imagine dousing the trees with castor oil solution will damage the trees at all. Some people have had success keeping the gnawing mammals away by placing human hair in their burrows.
Good luck. Let us know what works and what doesn't.
