Rabbits are small herbivores with specialized feeding needs and digestive systems. They are selective eaters and choose nutrient-rich leaves and new plant shoots over mature plant material that is higher in fiber. Rabbits are therefore considered concentrate selectors, because they naturally pick and choose foods higher in energy density, which predisposes them to obesity in captivity. Anatomically, rabbits are nonruminant herbivores with an enlarged hindgut. The large cecum supports a population of microorganisms that uses nutrients not digested in the small intestine. Most of the bacterial population in the cecum is made up of the gram-positive Bacteroides sp. This makes the rabbit very sensitive to oral antibiotics; administration of oral antibiotics can disturb the Bacteroides population and lead to fatal GI upsets. Separation of digesta on the basis of particle size occurs in the hindgut. Peristaltic action rapidly moves large particles (>0.5 mm), primarily lignocellulose, through the colon and excretes them as hard fecal pellets. This is the "indigestible fiber" component of the diet. The clinical importance of a diet high in long particle length is to maintain the motility of the cecum and colon. This is why these fibers are sometimes referred to as "scratch factor," because they mechanically stimulate GI motility. Antiperistaltic action moves smaller particles (<0.3 mm) and soluble material into the cecum, where they undergo fermentation. This component of the diet is known as "digestible" or "fermentable" fiber. At intervals, the cecal contents are expelled as “soft feces” or cecotrophs and consumed by the rabbit directly from the anus. Cecotroph ingestion is highest when rabbits are fed a diet high in nondigestible fiber. This reingested material provides microbial protein, vitamins (including all the B vitamins needed), and small quantities of volatile fatty acids, which are essential in rabbit nutrition. However, because amino acids obtained in this manner make only a minor contribution to the rabbits’ protein needs (particularly young, growing rabbits), the diet must supply the additional amino acids, although the requirements for essential amino acids in rabbits have not yet been defined.
Rabbits digest fiber poorly because of the selective separation and rapid excretion of large particles in the hindgut. A generous amount of dietary fiber (~15% crude fiber) is needed to promote intestinal motility and minimize intestinal disease. High-fiber intake can be provided by use of ad lib timothy hay (~30%–35% fiber). Fiber may also absorb bacterial toxins and eliminate them via the hard feces. Diets low in fiber promote an increased incidence of intestinal problems, eg, enterotoxemia. Carbohydrates will actually inhibit motilin release. Motilin is a polypeptide hormone secreted by cells of the duodenum and jejunum, which stimulates the GI smooth muscle. Excess starch can also be substrate for the proliferation of pathogenic bacteria such as Clostridium spiroforme, which produce a potent toxin. Cecum fermentation produces volatile fatty acids, which are responsible for 40% of the rabbits' calorie requirement. Volatile fatty acids also aid in the control of pathogenic organisms by helping to maintain the normal pH (6–7) in the cecum.
A dietary supply of vitamins A, D, and E is necessary. Bacteria in the gut synthesize B vitamins and vitamin K in adequate quantities; thus, dietary supplements are unnecessary. Disease and stress may increase the daily vitamin requirements. Feed preparation and storage must be done in a manner that will reduce losses from oxidation, which destroys vitamins A and E more readily than other vitamins. Diets containing ≥30% of alfalfa meal generally provide sufficient vitamin A. Levels of vitamin A in the diet must be >5,000 IU/kg and <75,000 IU/kg. Levels out of this range may cause abortion, resorbed litters, and fetal hydrocephalus. Vitamin E deficiency has been associated with infertility, muscular dystrophy, and fetal and neonatal death. Pet rabbit diets sold in pet stores or even in bulk at feed stores may not have adequate turnover, which may result in nutritional deficiency. Hay packaged for small mammals may have been sitting on the shelf for an extended period.
All the components of the basic diet (ie, protein, fiber, fat, and energy) should be managed in consideration of the life stage (growth, gestation, lactation, maintenance), breed, condition, and lifestyle of the rabbit. Ratios should meet the nutrient requirements of the National Research Council (see Table: Nutrient Requirements of Rabbits). Pelleted rabbit feeds provide good nutrition at reasonable cost. Fresh, clean water should always be available. Prolonged intake of typical commercial diets containing alfalfa meal by laboratory or pet rabbits kept for extended periods under maintenance conditions may lead to kidney damage and calcium carbonate deposits in the urinary tract. Ad lib timothy hay is usually recommended for the maintenance diet of adult rabbits. Reducing the calcium level to 0.4%–0.5% of the diet for nonlactating rabbits helps reduce these problems. This can be accomplished by feeding pelleted diets with a timothy hay base. Adult pet rabbits not intended for breeding should be fed a high-fiber pelleted diet, restricted to ¼ cup/5 lb body wt/day to prevent obesity and maintain GI health.
Nutrient Requirements of Rabbits
Rabbits are efficient converters of poorly digestible materials to meat. Therefore, it is easy to overfeed or underfeed does and growing, adolescent rabbits (fryers). The amount to feed depends on the age of the fryers or on the stage of pregnancy or lactation of the does. A general rule in feeding fryers is to feed all that can be consumed in 20 hr, with the feed hopper empty ~4 hr/day. Does are usually fed ad lib once they kindle. The general practice is to bring does from restricted to full feed slowly during the first week of lactation. Does bred to kindle five times during the year generally have their feed restricted between litters; those bred intensively should be on full feed continually once they begin the first lactation.