This is usually recommended when the rabbit hasbeen raised on a pelleted diet. For rabbits that have alwaysbeen raised on greens, they can have them at a very youngage. For bunnies that have never had green feeds, a suddenchange in diet can be fatal.
My friend feeds her Angora young babies parsley and carrots (along witha well balanced diet). They are used to getting these feeds,and the does have always been fed green feeds.
Always make diet changes very slowly so the gut flora has a chance to balance with the new demands of a different food.
Pam