What a rabbit wants to eat and what a rabbit should eat are two different things but since a rabbit's digestive system is so sensitive it is important to know what your rabbit needs to consume.
What to Feed Rabbits
Fiber is vital to the normal function of the digestive system in rabbits. Fresh grass hay and vegetables should make up the bulk of the diet for house rabbits. Feeding a diet consisting mainly of pellets may result in obesity and increase the likelihood of digestive problems. While there is some fiber in pellets, it is finely ground and does not appear to stimulate intestinal function as well as the fiber found in grass hays. Roughage, such as hay, also aids in the prevention of hairballs and keeps teeth trimmed. The addition of some pellets does add some balance to the diet, however, if your rabbit is a picky eater.
Anything other than hay, vegetables, and pellets is considered a treat and should be feed in strict moderation. The digestive system of a rabbit is very susceptible to serious upsets ( ileus) if the diet is inappropriate. The number of pellets should be restricted, especially in overweight rabbits, but any reduction in pellets should be made up with a variety of fresh vegetables and unlimited access to hay.
Feeding Rabbits Hay
Hay (grass hays such as timothy or oat hay) should be available at all times to your rabbit. Some rabbits may not eat much hay at first but by adding fresh hay a couple of times a day and reducing the number of pellets you offer, your rabbit will likely become hungry enough to eat the hay. The House Rabbit Society recommends starting baby bunnies on alfalfa hay and introducing grass hays by 6 to 7 months, gradually decreasing the alfalfa until the rabbit is solely on grass hays by 1 year of age. Alfalfa hay is higher in calcium and protein and lower in fiber than the grass hays (and can cause issues in adult rabbits), although many owners find their rabbits prefer alfalfa hays. If your adult rabbit is used to alfalfa hay, try mixing alfalfa with a grass hay to start and gradually reduce the amount of alfalfa you have to offer.
Vegetables for Rabbits
Vegetables should make up a large portion of your rabbit's diet. Depending on the size of the rabbit, 2 to 4 cups of fresh veggies should be given per day. A variety must be fed daily to ensure a balanced diet. If a rabbit is used to eating mainly pellets, the change must be made gradually to allow the rabbit's digestive system time to adjust. Only add one new vegetable to the diet at a time so if the rabbit has diarrhea or other problems it will be possible to tell which vegetable is the culprit.
Suggested vegetables to feed include carrots, carrot tops, parsley, broccoli, collard greens, mustard greens, dandelion greens, turnip greens, endive, romaine lettuce, kale, and spinach. However, kale, spinach, and mustard greens are high in oxalates so their feeding should be limited. Beans, cauliflower, cabbage, and potatoes may cause problems and should be avoided. Iceberg lettuce has almost no nutritional value and can cause diarrhea so it should be avoided. Rhubarb should also be avoided since it is toxic to rabbits. Wash vegetables well and only feed dandelions and other plants that are known to be pesticide free if you are taking them from a yard.
Vegetables should be introduced to bunnies around 12 weeks of age, in small quantities, and one at a time. As more vegetables are added watch for diarrhea and discontinue the most recently added vegetable if this occurs.
Feeding Rabbit Pellets
Pellets are basically designed for commercial rabbit production and are quite high in calories. As a result, house rabbits fed unlimited pellets may end up with obesity and related health problems, as well as an excess of other nutrients. Pellets do have a place in rabbit nutrition, as they are rich and balanced in nutrients. However, experts recommend restricting the number of pellets fed and instead feed more fresh vegetables and grass hays.
Choose a fresh, good quality pellet. The House Rabbit Society recommends a minimum of 20-25% fiber, around 14% protein (with no animal protein), and less than 1% calcium for most house rabbits (spayed/neutered). For adults, the amount should be carefully regulated depending on the size (weight) of the rabbit. As a rule, give about 1/4 cup of pellets to rabbits 5 to 7 lbs., 1/2 cup for 8 to 10 lb. rabbits, and 3/4 cup for 11 to 15 lb. rabbits. Baby rabbits can be fed pellets free choice (available at all times) and then the amount can be decreased to 1/2 cup per 6 lb. of body weight by around 6 months of age.
Treats for Rabbits
The House Rabbit Society recommends 1 to 2 tablespoons of fresh fruits be given daily as a treat. Treats sold in pet stores marketed for rabbits are generally unnecessary and in some cases could cause digestive problems due to their high carbohydrate or sugar content. Also, as an alternative to food treats, consider offering twigs from apple or willow trees (pesticide-free only) or other rabbit safe woods.