A rabbit is designed to eat nothing but grass, with a few extra leaves which it browses on the side. It will normally feed at dusk and dawn for 30 to 120 minutes each time. Because it is so selective in the wild state, the rabbit has an extremely poor capacity to cope with a modified diet. Incorrect diets can lead to life-threatening problems associated with overgrown teeth, digestive disturbances and obesity.
The correct calcium levels in grass are vital. Also the chewing action is essential to strengthen the jaw bones and wear down the teeth evenly [which grow throughout life]. Sunlight is important for Vitamin D production, which is essential for calcium absorption from the intestine, so allow your rabbit to run outside in daylight if possible.
Good quality hay [dried grass] is an excellent substitute when plentiful grass is unavailable. Other fresh foods such as carrots, broccoli stalks, cauliflower leaves, cabbage, spring greens, parsley, apples, sprout peelings, celery, pea pods, radish, carrot tops, cabbage and dandelion leaves can be fed in moderation. Turnips, spinach and kale can be fed occasionally. Dry rabbit mixes should be considered a "treat" food only. Light green foods such as lettuce and cucumber have little nutritional value and should not be offered. Foods such as biscuits, chocolate and cake should never be fed. The great bulk of the diet should be grass and more grass! Vitamin supplements are not required.
Ideally, place the rabbit's run on the lawn where he can select his favourite species of grass. Alternatively, freshly cut grass can be offered, but mower clippings are generally too crushed and should be avoided. If you have been feeding your rabbit a high proportion of dry mix, make the change to grass or hay over about 2 weeks, to allow the gut to adapt to the change.