While some hay is desirable in a rabbit’s diet, complete diets based on a high level of hay will not work. Rabbits need some fiber (which hay is high in) to keep their digestive system working properly; without fiber, they will become very ill. However, rabbits have fairly small digestive systems, which means they cannot hold much food, and they are surprisingly inefficient at digesting high-fiber foods, so they don’t get much energy from them. If fed a diet high in a bulky fiber such as hay, they simply will not be able to eat enough or digest it thoroughly enough to obtain enough energy to maintain their bodies, much less be productive (such as having babies, lactating, etc.).
In fact, if you feed alfalfa to a rabbit, you will notice that they tend to avoid the stemmy portion and pick off the little leaves. A fine grass hay is good to have for you rabbit to nibble on, but it will need a complete pelleted feed to meet all its nutritional needs. Our different Rabbit Chow® diets range in fiber from about 17% to 24%, to meet the different production goals of the owners (pet, breeder, show, meat production); they have alfalfa in them, but they also have other ingredients which provide protein, energy, vitamins and minerals rabbits need to thrive.