I found this under the suggested veggie list:
IMPORTANT: Before introducing any fresh foods to a rabbit it is best if he has been eating grass hay for a minimum of 2 weeks. The grass hay will help to get his GI tract motility and flora in good working order so that he will be able to accept new foods more easily. When introducing new fresh foods to any rabbit’s diet it is best to go slowly to allow the gastrointestinal tract and all its important microorganisms to adjust. Introduce one new food every three days and keep a watch on the stools. It is rare for a rabbit that has been on a hay diet first, to have any problems using this method, but if you note softer stools that persist over a couple of days then you might want to remove that food from your bunny’s diet. Keep a list as you go of the foods that your rabbit has successfully eaten you will then have a handy shopping list when you go to the store!
Here’s the link:
rabbit.org/suggested-vegetables-and…bbit-diet/