I give mine cabbage - white, savoy, and tenderheart, with no probs but think like othrs said every bun is different so need 2 add anythin new carefully.
Mine get spring greens, cabbage, peas in pods, peppers, cauliflower (leaves & florets), broccoli, celery, brussel sprouts, swede, carrots, kale, parsley, coriander, basil, apple, pear, cherry tomatoes. They turn their nose up at parsnips (but thy'll eat the peelings if I'm having them), asparagus, baby corn, butternut squash, banana, and green beans. Occassionally thy get a strawberry or a few grapes or blueberries each as a treat. In the summer I gave them half a small netarine each (with the stone removed) as well as a treat and they loved it.