Interesting, I didn't know that. I grow yarrow in many places but never have offered it to the chickens. Well, skip that one, I guess.
As mentioned above, just about any low growing perennial herb would be excellent for orchards. Be creative and don't limit yourself to the suggestions but look for anything you can find. In our orchards we also grow mints, lemon balm, onions, chives, even daffodils and daylilies. The list goes on and on. Each plant brings its own interesting and useful characteristics. Variety is the spice of life.