The most important thing of a rabbit’s diet should be unlimited quantities of hay, which is dried grass. Its high fiber content is the primary component for maintaining good dental and intestinal health. Without fiber, the rabbit’s digestive system cannot move food through the gut and their ever growing teeth will not wear down and could grow painfully long.
If they don't currently eat grass, wean them onto it slowly to avoid upset stomachs. Initially, limit the time spent grazing or the amount of grass you feed. You can build up the amount you feed until you rabbit has free access to as much as they want to eat. Don't feed them grass from lawn mowers because the way it's chopped causes it to ferment, which is bad for the bunnies’ tummies, and only feed grass that hasn't been near traffic fumes.
Grass AND Hay
You can safely give your rabbit grass but only in addition to hay, not instead of it. The differences between grass and hay are minor but if misunderstood, could affect your rabbit's health. Hay has a higher fiber percentage than green leafy fresh grasses, and this fiber is required to keep their digestive tracts moving and it’s the best to keep their teeth from overgrowing.
Provide a constant supply of good quality fresh grass and grass hay. Types that most rabbits like are Timothy, Oaten, Wheaten, Pasture, Paddock, Meadow or Ryegrass hays. Adult rabbits should not be fed Lucerne (alfalfa) or Clover hays as they are too high in protein and calcium. Alfalfa hay has more protein than grass hay, making it too fattening to feed as the main diet for the average adult rabbit. It can be good for growing youngsters or putting weight on an underweight rabbit.
Most experts suggest that a rabbit’s diet should be around 80% hay or more so while making sure your bunny has the kind of hay it likes and needs, you can give them grass or let them graze. You’ll likely find that they have different tastes, likes, and preferences so when you find what they like, keep it coming and if introducing new grasses start with tiny portions to avoid upsetting your bunny’s digestive system.