I remember this question from when we had a pet rabbit. They require such a large amount of fiber in their diet that eating such calorie dense foods is something that
should only be done in moderation.
I don't remember if the issue is excessive weight gain or if the rabbits become satiated and then don't eat
enough fiber to keep their digestive system in good health. Without enough fiber they are prone to life threatening blockages of their intestines. I think there's also issues with not wearing down their teeth enough, but you're probably already covering that with the tree trimmings.
I found this information on a forum about pet rabbits. They recommended grains be fed as a treat rather than a
staple. Incidentally, they had the same feeding recommendations as for
root vegetables. It is possible, if they only problem was weight gain, that it might actually be good for raising meat animals.