I feed collards to my pair and do not seem to have problems. However, I do not believe in making any particular green or food source a staple necessarily. I feed mine collards, turnip greens, mixed spring greens, mustard greens, and romaine lettuce. Every couple of weeks I give them Mazuri tort chow as well. I usually give them veggies if available, and may give them banana or apple very, very infrequently, like maybe once every other month or so.