Rabbits are found in virtually every country in the world providing protein, fiber, research models and companionship (they are third in number to dogs and cats as companion animals in the United States). Almost everyone has had a rabbit in the back yard, but have you ever taken the time to actually consider how to feed this unique beast? As a child I was instructed to feed rabbits alfalfa hay and an ear of corn - in the summer, I was "lucky" enough to have the chore of pulling grass for them. Because of the rabbit's ability to utilize low grain and high roughage diets, they may become future key players in providing a major animal protein source for developing countries. The rabbit is not, however, able to survive solely on poor quality, low energy forages due to its small size and high metabolic rate. This is illustrated by rabbit's ability to select only the most nutritious part of plant they can in natural habitats. In addition to its ability to perform on high roughage diets, the rabbit has a much "quicker" generation interval than many other animal species and is able to breed year-round.
The rabbit is classified as an herbivorous non-ruminant. It has a simple, non-compartmentalized stomach along with an enlarged cecum and colon inhabited by a microbial population (primarily Bacteroides). Due to their small size they do not have sufficient "gut space" for microbial fermentation, and the rabbits have adapted the strategy of high feed intake (65-80 g kg -1 BW) and fast feed transit time (19 hr). This strategy allows rabbits to compensate for some lower quality forages and still meet nutritional requirements. Most problems seen in rabbit production involves the gastrointestinal tract, primarily enteritis often resulting in diarrhea. By understanding the idiosyncrasies of the rabbit's gut and what causes enteritis will provide insight to elicit maximum production when feeding these animals.
Rabbits have a symbiotic microbe population that ferments cellulose (14% in rabbits vs 44% in cattle) in the hindgut. Even though rabbits have a low fiber fermentation extent, a level of 20-25% fiber is essential to maintain gut health, stimulate gut motility (insoluble fiber only), reduce fur chewing and to prevent enteritis. Fiber fermentation of the rabbit is enhanced by coprophagy. Low fiber diets result in gut hypomotility, reduced cecotrope formation and prolonged retention time in the hindgut. Acetate is the primary volatile fatty acid (VFA) produced by microbes, with more butyrate than propionate being formed. Unlike ruminants, more VFA are produced on starch than forage diets and VFA actually provide limited energy for maintenance. Gut microflora of rabbits are sensitive to antibiotics and if antibiotics are fed, the microbe population is altered and E. coli and Clostridia organisms produce toxins that harm the gut lining, causing diarrhea and enterotoxemia. Virginiamycin seems to be an exception and is used as a growth promotant. On the other hand, if the ionophore monensin is fed, it is toxic and the animals will die.
In ruminants, microbial protein satisfies major amino acid requirements for the animal, however, this is not true for the rabbit. As a result, synthetic amino acids are often added to commercial rabbit diets to completely meet amino acid needs. Lysine and methionine may be limiting amino acids in traditional alfalfa-corn diets. Urea is not well utilized by microbes in the rabbit and prolonged feeding of 0.5% urea may result in liver or kidney lesions.
Low levels of dietary protein fed to rabbits increase cecotrope consumption and high levels of protein decrease consumption - an obvious protein sparing mechanism. Coprophagy has been found to increase protein digestibility (50% vs 75-80% for alfalfa) of forages in the rabbit. Feces are excreted according to a circadian rhythm and data indicate that the internal cycle differs when shifting from ad libitum to restricted feeding - compromising growth. Care should be taken when feeding high levels of dietary protein, as excess protein may increase cecal ammonia levels causing an increase in cecal pH. This increase in pH allows pathogens to flourish and again resulting in enteritis.
High starch diets are often incompletely digested in the small intestine of the rabbit due to rapid transit times. Incomplete chemical digestion of the starch also results in enteritis due to bacterial fermentation of that same substrate. Higher levels of starch stimulate bacterial growth, thus low energy grains like oats are preferred over corn or wheat. If grains are finely processed, this too can lead to enterotoxemia so a coarse grind is recommended. Rabbits seem to perform better when fed pellets compared to mixed grains or textured feeds, because the animals are not able to sort out preferred items. A feed pellet that is solid and firm, 1/4 inch in length and 3/16 inch or less in diameter is optimum for rabbits.
Rabbits have an unusual calcium metabolism, absorbing calcium without vitamin D facilitation - excess calcium being excreted in the urine (parathyroid hormone and calcitonin regulate serum Ca levels). Diets high in calcium (alfalfa based) may result in kidney damage for animals at maintenance, as homeostatic mechanisms are not as effective as in other species. Prolonged high dietary calcium will result in calcification of soft tissues like the aorta and kidney. This calcification is intensified if the rabbit is supplemented with vitamin D as is often found in commercial rabbit pellets.
So - feeding the traditional alfalfa and corn diet may not be the key to "feeding" and or managing the rabbit gut to result in optimal rabbit production. Traditional alfalfa is high in protein and calcium, both of which can be a concern with rabbits. Corn is high in starch - high levels of starch can result in enteritis. Although alfalfa is recommended for growing rabbits, it is advised to feed mature animals a diet of grass hay lower in protein and calcium along with high fiber pellets (1/4 cup high fiber pellets per 5 pounds of body weight for maintenance). Amounts of the high fiber pellet can be increased as energy demands of gestation and lactation demand, but ultimately higher fiber levels, low starch, moderate protein and calcium levels are the keys to correct management of the gastrointestinal tract of the rabbit - fitting in with potential feed sources found in third world countries.