For rabbits and rodents, the nutritional components of hay primarily involve complex carbohydrates, along with adequate levels of calcium and a balanced calcium: phosphorus ratio. In contrast, fruits, nonleafy vegetables, and grains contain higher levels of digestible energy than is present in the natural diets of rabbits and rodents. Hay also provides forage that allows normal tooth wear, results in increased water consumption (protecting against urolith formation), lowers the chance of overeating, and helps maintain normal gut flora. For a hay-only diet to be nutritionally adequate, however, a variety of hay from different locations should be offered. Because owners often obtain hay from a single source, supplying 1 tbsp/kg of high-fiber pelleted feed with a balanced vitamin and mineral content is often recommended. Hay should be changed daily and, while it does not necessarily need to have high nutritive quality, it should be of high hygienic quality. The 2 most common hays are Lucerne–alfalfa and grass hay. Because the former is higher in protein and calcium and often higher in energy, it is most suited for growing animals.

Commentary
Rabbits, guinea pigs, chinchillas, and degus are hindgut fermenters with GI tracts that efficiently digest fibrous diets. Despite this, veterinarians still encounter owners who believe that feeding hay as the primary dietary component is inappropriate or boring. This article supports the beneficial aspects of feeding hay mixtures. Unfortunately, most owners feed single varieties of hay while giving a proportionately larger amount of pellets than recommended. Two handy tables are provided to help clinicians guide owners in evaluating the nutritional and hygienic qualities of batches of hay.—Dominique Keller, DVM, PhD

Source
Feeding hay to rabbits and rodents. Clauss M. J EXOTIC PETMED 21:80-86, 2012.