How to feed the rabbit (Oryctolagus cuniculus) gastrointestinal tract

Abstract

Rabbits are found in virtually every country in the world, providing protein, fiber, animal research, and companionship (third to dogs and cats). Because of an ability to utilize low-grain and high- roughage diets, they have the potential to be a future protein source. Classified as an herbivorous nonrumi- nant, rabbits have a simple, noncompartmentalized stomach along with an enlarged cecum and colon inhab- ited by a microbial population (primarily Bacteroides). Rabbits practice coprophagy, which enhances strate- gies of high feed intake (65 to 80 g/kg BW) and fast feed transit time (19 h), allowing rabbits to meet nutritional requirements. Coprophagy also increases protein di- gestibility (50 vs 75-80% for alfalfa). Feces are excreted on a circadian rhythm, and data indicate that the inter- nal cycle differs when shifting from ad libitum to re- stricted feeding. Microbes digest cellulose (14% in rab- bits vs 44% in cattle) in the hindgut of the rabbit, but the contribution of amino acids from microbial protein is thought to be minimal. Lysine and methionine may be limited in traditional diets, and urea is not utilized. Acetate is the primary microbial VFA, with more buty- rate than propionate. Unlike ruminants, more VFA are

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N. A. Irlbeck
How to feed the rabbit ( Oryctolagus cuniculus ) gastrointestinal tract
2001, 79:E343-E346.J ANIM SCI
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How to feed the rabbit (Oryctolagus cuniculus) gastrointestinal tract
N. A. Irlbeck
1
Department of Animal Sciences, Colorado State University, Fort Collins 80523-1171
ABSTRACT: Rabbits are found in virtually every
country in the world, providing protein, fiber, animal
research, and companionship (third to dogs and cats).
Because of an ability to utilize low-grain and high-
roughage diets, they have the potential to be a future
protein source. Classified as an herbivorous nonrumi-
nant, rabbits have a simple, noncompartmentalized
stomach along with an enlarged cecum and colon inhab-
ited by a microbial population (primarily Bacteroides).
Rabbits practice coprophagy, which enhances strate-
gies of high feed intake (65 to 80 g/kg BW) and fast feed
transit time (19 h), allowing rabbits to meet nutritional
requirements. Coprophagy also increases protein di-
gestibility (50 vs 75–80% for alfalfa). Feces are excreted
on a circadian rhythm, and data indicate that the inter-
nal cycle differs when shifting from ad libitum to re-
stricted feeding. Microbes digest cellulose (14% in rab-
bits vs 44% in cattle) in the hindgut of the rabbit, but
the contribution of amino acids from microbial protein
is thought to be minimal. Lysine and methionine may
be limited in traditional diets, and urea is not utilized.
Acetate is the primary microbial VFA, with more buty-
rate than propionate. Unlike ruminants, more VFA are
Key Words: Animal Nutrition, Digestive Tract, Feeding, Management, Rabbits
2001 American Society of Animal Science. All rights reserved. J. Anim. Sci. 79(E. Suppl.):343–346
Introduction
Domestic rabbits (Oryctolagus cuniculus) are found in
virtually every country in the world, providing protein,
fiber, research models, and companionship; they rank
third in number to dogs and cats as companion animals
in the United States. Almost everyone has had a rabbit
in the backyard at one time or another. But have you
ever taken the time to actually consider the physiological
idiosyncrasies of the rabbit and how they apply to feed-
ing? As a child, I was instructed to feed my rabbits alfalfa
hay and an ear of corn. In the summer, I was “lucky”
1
Correspondence: phone: (970) 491-0668; fax: (970) 491-5326; E-
mail: nirlbeck@ceres.agsci.colostate.edu.
Received March 13, 2001.
Accepted July 31, 2001.
E343
produced on starch than on forage diets; however, VFA
provide limited energy for maintenance. Fiber is essen-
tial to maintain gut health, stimulate gut motility (in-
soluble fiber only), and reduce fur chewing. Low-fiber
diets result in gut hypomotility, reduced cecotrope for-
mation, and prolonged retention time in the hindgut.
High-starch diets may be incompletely digested in the
small intestine due to rapid transit times, resulting in
enteritis. Low-energy grains like oats are preferred.
Low-protein concentrations increase cecotrope con-
sumption and high levels decrease it. Finely ground
feeds lead to enteritis, so a coarse grind is recom-
mended. Rabbits have an unusual calcium metabolism,
absorbing Ca without vitamin D facilitation; the excess
calcium is excreted in the urine (parathyroid hormone
and calcitonin regulate serum Ca levels). Diets high in
Ca (alfalfa based) may result in kidney damage for
animals at maintenance. Correct feeding management
based on the idiosyncrasies of the rabbit gastrointesti-
nal tract will maximize production for future meat pro-
duction. A balanced diet containing adequate fiber (20
to 25%), minimal starch, and optimum protein concen-
tration is the key to preventing gastrointestinal
distress.
enough to have the chore of pulling grass for them. To-
day, pet owners are advised to feed a complete pelleted
diet, supplemented with grass hay and small amounts of
produce. Commercial rabbitries throughout the country
feed a complete pelleted diet (alfalfa-based) with various
supplemental feeds. Rabbits can utilize low-grain and
high-roughage diets (McNitt et al., 1996). Because rab-
bits are able to utilize this type of diet, are able to breed
year-round, and have a “quick” generation interval, they
are uniquely poised to provide animal protein for devel-
oping countries, where grain can only be justified for
human use. It must be clarified, however, that rabbits
are not able to survive solely on poor-quality, low-energy
forages. Due to their small size and high metabolic rate,
a high-quality forage is needed. So, what is the correct
way to feed rabbits? An in-depth evaluation of the diges-
tive tract of adult rabbits may help to understand poten-
tial complexities when feeding this unique animal.
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IrlbeckE344
Classification and Description of the Gut
Rabbits are herbivores, are concentrate selectors, and
are classified as hindgut (cecum and colon) fermentors
(Cheeke, 1987; McNitt et al., 1996). Because there are
no mammalian enzymes to break down the cellulose
components of their plant-based diets, rabbits as well
as other herbivores have a symbiotic microbe population
(primarily Bacteroides). Ruminants have a voluminous
area (rumen) within the gastrointestinal tract where
fiber fermentation occurs. The omasum limits the re-
moval of fiber from the rumen until fermentation is com-
plete. In horses, also a hindgut fermentor, the colon is
the major site for the microbial population and fiber
fermentation. Horses compensate for reduced fiber di-
gestion by consuming more forage and increasing its
passage rate through the gastrointestinal tract. Because
of a smaller body size and higher metabolic rate than
horses, rabbits rely on other adaptations for forage utili-
zation (Cheeke, 1987).
In rabbits, the microbial population is found in the
cecum. The rabbit cecum is very large, compared with
the rest of the gut (Stevens and Hume, 1995) and forms
a spiral that fills the abdominal cavity. The cecum has
a capacity 10 times that of the rabbit’s stomach, about
40% of the gastrointestinal tract (Jenkins, 1999). Instead
of completely fermenting fiber, rabbits utilize a mecha-
nism to sort out indigestible fiber and expel it from the
body, a process that is a specialized feeding strategy that
overcomes poor-quality protein (Carabano and Piquer,
1998; Jenkins, 1999). This sorting mechanism occurs as
digesta enter the rabbit large intestine and muscular
contractions facilitate the separation of fiber and nonfi-
ber (protein, soluble carbohydrates, etc.) fractions. A se-
ries of peristaltic (move fiber through colon) and antiperi-
staltic waves (move fluid and nonfiber components to
cecum for fermentation) separate out nonfiber fractions
for further fermentation in the cecum (Cheeke, 1987;
Carabano and Piquer, 1998); particle size and density
aid separation (Cheeke, 1994). The fiber components are
voided from the body (day, or hard, feces) about 4 h after
consumption of the diet (Cheeke, 1994). After fermenta-
tion of the nonfiber components in the cecum, a pellet
is formed (called a cecotrope, also soft, or night, feces)
that is voided from the body approximately 8 h after
consumption of the diet (Cheeke, 1994). A neural re-
sponse (Jenkins, 1999) or the strong odor of VFA (Ste-
vens and Hume, 1995) in the cecotrope seem to stimulate
its consumption directly from the anus. This practice of
consuming cecotropes is called copraphagy,orcecotrophy
(Cheek, 1987). In natural settings, copraphagy usually
occurs during the day, opposite of feed intake and the
voiding of hard feces, in a circadian rhythmic pattern
(Carabano and Piquer, 1998; Jenkins, 1999), and is an
integral part of the rabbit’s digestion process (Cheeke,
1994). If a rabbit is equipped with a collar preventing
copraphagy, the digestion of the diet is significantly re-
duced, even when a highly digestible diet is fed.
Because of their small body size, if allowed to consume
a diet ad libitum, rabbits will daily eat an amount that
approximates 5% of their body weight in dry matter and
drink about 10% of their body weight in water (Okerman,
1994). Even at this intake, if a rabbit were to consume
only low-quality forages, there would be insufficient en-
ergy and nutrients to meet its metabolic requirements.
However, if rabbits at maintenance are fed a high-qual-
ity pelleted diet for ad libitum consumption, they will
become obese (Cheeke, 1994; Brooks, 1997). A recom-
mended amount of 26 g of high-fiber (25% crude fiber)
pellet per kilogram of BW is recommended to maintain
body condition (Jenkins, 1999). Rabbits require a diet of
2,200 kcal/kg of diet (as cited by Cheeke, 1994), or 2.2
kcal/g of diet. If a 3.64-kg rabbit is fed according to Jen-
kins’s recommendation (1999) of 26 g/kg BW, the animal
will be consuming 208 kcal of energy (94.64 g ×2.2 kcal/
g=208 kcal). If a rabbit is allowed ad libitum consump-
tion of the pelleted diet, 5% of BW, it will almost double
Jenkins’s (1999) recommended allowance, resulting in a
higher energy intake and ultimately obesity.
When allowed to select their own diet in a natural
setting, rabbits will select the most tender, succulent
plant parts or the plant parts that are most nutrient-
dense and lowest in available cell walls. Some research-
ers call animals that practice this type of eating behavior
concentrate selectors, a practice that allows the animal
to meet the dietary requirements for their high metabolic
rate (Cheeke, 1994). Their chosen selections are low in
fiber and high in carbohydrate and protein; therefore, a
larger gut volume is not needed. Rabbits simply elimi-
nate fiber as quickly as possible from their gastrointesti-
nal tract. Rabbits have high feed intake (65 to 80 g/kg
BW) and fast feed transit time (19 h), which enable
them to consume lower-quality forages and still meet
nutritional requirements (Carabano and Piquer, 1998).
Most problems seen in rabbit production (commercial
and companion animal) involve the gastrointestinal
tract. Enteritis is the primary gastrointestinal disorder,
and it often results in diarrhea (Cheeke, 1994). This
disorder is often a result of an imbalance in normal
microflora in the gut, whether that imbalance is due to
insufficient fiber, too much starch, or the addition of
antibiotics to the diet. Understanding the idiosyncrasies
of the rabbit gut and how dietary components affect that
microbe population is a key to proper feeding manage-
ment. These issues are discussed next.
Gut Microbes and Utilization of Fiber
As stated earlier, rabbits have a symbiotic microbe
population found in the hindgut responsible for fiber
fermentation. When compared with other herbivores,
actual fiber digestion capability for rabbits is relatively
low (14% for alfalfa hay in rabbits compared with 44%
in cattle, 41% in horses, and 22% in hogs) (McNitt et
al., 1996). The actual crude fiber component of most
forages fed is only 20 to 25% (McNitt et al., 1996), de-
pending on forage maturity. Obviously, the more mature
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Feeding the rabbit gut E345
a forage, the higher the crude fiber. Examples of crude
fiber in forages from the United States-Canadian Tables
of Feed Composition (NRC, 1982) are 23% in early bloom
alfalfa (31% ADF, 40% NDF), 38% in mature alfalfa
(44% ADF, 57% NDF), 28% in early bloom timothy (32%
ADF, 61% NDF), and 32% in full-bloom bluegrass (38%
ADF, 68% NDF). This is comparable to the range of 12
(low fiber) to 25% (high fiber) crude fiber found in rabbit
pellets (Brooks, 1997). Other nonfiber fractions of forage,
protein, and soluble carbohydrates are easily digested
by rabbits.
In rabbits, dietary fiber has a critical role in main-
taining gut health, stimulating gut motility (insoluble
fiber only), reducing fur chewing, and preventing enteri-
tis (McNitt, et al., 1996; Brooks, 1997). Rabbits need a
minimum dietary fiber level of 20 to 25% to maintain
gut health. Diets less than 20 to 25% fiber result in gut
hypomotility, reduced cecotrope formation, prolonged re-
tention time in the hindgut, and often enteritis (Cheeke,
1994; Jenkins, 1999). Composition of the hard feces and
the cecotrope is influenced by the diet. If dietary fiber
concentration increases, the fiber composition of the fecal
pellets also increases. Thus, high-quality fiber is essen-
tial for gut health in rabbits (McNitt et al., 1996; Stein
and Walshaw, 1996). Fiber fermentation in rabbits does
not seem to be enhanced by coprophagy (as cited by
Cheeke, 1994).
Microbes in rabbit gut produce VFA, as do microbes
in the rumen of a cow. In rabbits fed a traditional alfalfa/
corn diet, acetate is the primary volatile fatty acid pro-
duced by microbes, with more butyrate than propionate
being formed. Butyrate is the preferred energy source
for the hindgut (Stevens and Hume, 1995; Gidenne et
al., 1998; Jenkins, 1999). Microbes in rabbits produce
more VFA on starch-based diets than on forage diets
(Cheeke, 1994). Stevens and Hume (1995) indicate that
VFA provide a major energy source in rabbit colon.
Gut microflora of rabbits are sensitive to most antibiot-
ics (McNitt et al., 1996). If antibiotics are fed, the microbe
population is altered, favoring E. coli and Clostridia or-
ganisms that produce toxins harming the gut lining,
causing diarrhea and enterotoxemia (Cheeke, 1994;
Stein and Walshaw, 1996; Brooks, 1997). Antibiotics
that may cause this effect include lincomycin, ampicillin,
amoxicillin, procaine penicillin, cephalexin, erythromy-
cin, clindamycin, tylosin, and metronidazole. The actual
effect from each of the drugs will differ between animals
(Stein and Walshaw, 1996). Oxytetracycline, virgin-
iamycin (Cheeke, 1994), or tetracycline (Brooks, 1997)
are exceptions and are used as growth promotants, and
sulfaquinoxaline is to control coccidia (Brooks, 1997).
Under no circumstances should the inophore monensin
be fed to rabbits; it is toxic even at low concentrations
(McNitt et al., 1996; Martin, 2000).
Utilization of Protein
In ruminants, microbial protein satisfies the major
amino acid requirement for the animal. However, this
is not true for rabbits. Even though amino acids produced
by bacteria may be available via coprophagy (especially
lysine, sulfur amino acids, and threonine; Carabano and
Piquer, 1998), research has shown that microbial protein
plays only a minor role in meeting a rabbit’s protein
and amino acid needs (McNitt, 1996). The majority of
microbial protein utilized by the animal is digested in the
colon (Stevens and Hume, 1995). As a result, synthetic
amino acids are often added to commercial rabbit diets
to fully meet amino acid needs, particularly lysine and
methionine, which may be limiting amino acids in tradi-
tional alfalfa-corn diets (McNitt et al., 1996). Cecotropes
do, however, contain approximately 28% crude protein
(Stevens and Hume, 1995).
Rabbits are able to digest protein in forages quite well;
rabbits can digest 75 to 85% of alfalfa protein, whereas
hogs digest less than 50% (McNitt et al., 1996). This
capability of utilizing protein from a forage source may
be an added asset in developing countries where less
grain and protein sources are available for animal con-
sumption. Urea is recycled by the rabbit large intestine
in a manner similar to that occuring in the rumen (Ste-
vens and Hume, 1995). However, when dietary urea is
fed to rabbits, it is not well utilized by microbes. Pro-
longed feeding of 0.5% urea in the diet of rabbits will
result in liver or kidney lesions (Cheeke, 1994). Urea is
converted to ammonia in rabbit gut, and when absorbed,
it results in toxicity.
Ingestion of cecotropes is influenced by dietary protein
and energy. When an animal is fed a low-energy diet,
cecotrope ingestion is maximized (Jenkins, 1999). When
an animal is fed a diet for ad libitum consumption, di-
etary protein and fiber concentration affect cecotrope
consumption. Low levels of dietary protein fed to rabbits
increase cecotrope consumption and high levels of pro-
tein decrease consumption, which seems to be a protein-
sparing mechanism (Cheeke, 1994). Coprophagy has
been found to increase protein digestibility (50 vs 75 to
80% for alfalfa) of forages in rabbits. As indicated earlier,
feces are excreted according to a circadian rhythm. Data
indicate that the internal cycle differs when shifting from
ad libitum to restricted feeding, which compromises
growth. Care should be taken when feeding high levels
of dietary protein because excess protein may increase
cecal ammonia levels, causing an increase in cecal pH
(Cheeke, 1994). This rise in pH may allow pathogens to
flourish and may increase the potential for enteritis.
Utilization of Starch
High-starch diets are often incompletely digested in
rabbit small intestine due to rapid transit times (McNitt
et al., 1996). Incomplete chemical digestion of the starch
results in the availability of starch for microbial fermen-
tation (Stevens and Hume, 1995). Excess starch in the
gut results in an extremely rapid growth of microbes. If
toxin-producing microbes (primarily Clostridium spiro-
forme) are in residence, high levels of starch may lead
to enteritis and possible death (McNitt et al., 1996; Jen-
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IrlbeckE346
kins, 1999). Because of potential incomplete starch di-
gestion, low-energy grains such as oats are preferred
over corn or wheat (Cheeke, 1994). Grains processed too
finely can lead to rapid bacterial fermentation of the
starch and cause enterotoxemia. Thus, a coarse grind is
recommended. The addition of copper sulfate (125 to 250
ppm) to rabbit diets sometimes lowers the incidence of
enteritis (Cheeke, 1994). Copper sulfate is commonly
used in swine and poultry diets and acts by inhibiting
the growth of pathogenic bacteria.
Pellet Quality
Rabbits seem to perform better when fed pellets than
when they are fed mixed grains or textured feeds, pri-
marily because the animals are not able to sort out pre-
ferred items (Cheeke, 1994). For example, pelleted dehy-
drated alfalfa is preferred to alfalfa in its natural form.
Rabbits, like most other animals, will select only the
alfalfa leaves and leave the stems uneaten. This feeding
practice results in a low-fiber diet and potential enteritis.
Pellets need to be hard and durable, because rabbits
prefer not to eat the fines. If an animal does eat too
many fines or if the particle size is too small, there will
be an increase in retention time in the gut, reduced gut
motility, and enteritis. Large indigestible fiber particles
are needed for normal cecal-colonic motility (Cheeke,
1994). Hypomotility of the gut predisposes an animal to
enteritis. Feeding pellets of small diameter (<0.25 cm)
will lower intake and ultimately weight gain due to in-
creased feeding time (Maertens and Villamide, 1998).
Feeding larger diameter pellets (>0.5 cm) results in
greater feed wastage. Maertens and Villamide (1998)
recommend a pellet length for rabbits of 0.8 to 1.0 cm,
because longer pellets will cause greater breakage and
production of smaller pellets. McNitt et al. (1996) suggest
that a solid and firm pellet of 0.63 cm in length and 0.47
cm in diameter is optimum for rabbits.
Calcium Metabolism
Rabbits have an unusual calcium metabolism, ab-
sorbing calcium without vitamin D facilitation and acti-
vation of calcium-binding proteins in the gut (McNitt et
al., 1996; Jenkins, 1999), resulting in excess calcium
being excreted in the urine. In most mammals, less than
2% of dietary Ca is excreted in the urine, but in rabbits
it is much higher. In one study cited by Jenkins (1999),
the fractional excretion of Ca was 44% when animals
were fed a “typical” commercial diet. Because rabbits
can absorb Ca without the facilitation of vitamin D, a
mechanism is needed to regulate serum Ca levels. Para-
thyroid hormone and calcitonin are thought to prevent
serum Ca levels from becoming dangerously high due to
dietary influence. Diets high in calcium (alfalfa-based)
may result in kidney damage for animals at maintenance
levels (Cheeke, 1994) because homeostatic mechanisms
are not as effective as in other species. Prolonged high
dietary calcium will result in calcification of soft tissues
such as aorta and kidney (Cheeke, 1994) and formation
of kidney stones. This calcification is intensified if rabbits
are supplemented with vitamin D, as is often found with
commercial rabbit pellets.
Implications
Feeding a traditional alfalfa and corn diet or an alfalfa-
based pelleted diet is not the key to feeding or managing
the rabbit gut for optimal production and maintenance.
Traditional alfalfa is high in protein and calcium, which
are both of concern for rabbit production. Corn is high
in starch, and high levels of starch result in enteritis.
Alfalfa is recommended for growing rabbits; however,
one should feed mature animals a maintenance diet of
grass hay with less protein and calcium along with mini-
mal amounts (26 g) of high-fiber (25% crude fiber) pellets
per kilogram of body weight. Amounts of high-fiber pel-
lets can be increased as energy demands for gestation
and lactation increase. Ultimately, correct feeding of rab-
bits includes high fiber from grass hay, low starch, and
moderate protein and calcium levels. This feeding strat-
egy helps maintain balanced gut microbial populations
and coincides with potential feed sources available in
developing countries.
Literature Cited
Brooks, D. 1997. Nutrition and Gatrointestinal Physiology. In: E. V.
Hillyer and K. E. Quesenberry (ed.) Ferrets, Rabbits and Ro-
dents—Clinical Medicine and Surgery. p 169. W.B. Saunders
Company, Philadelphia.
Carabano, R., and J. Piquer. 1998. The Digestive System of the Rabbit.
In: C. de Blas and J. Wiseman (ed.) The Nutrition of the Rabbit.
p 1. CABI Publishing, London.
Cheeke, P. R. 1987. Rabbit Feeding and Nutrition. Academic Press,
New York.
Cheeke, P. R. 1994. Nutrition and Nutritional Diseases. In: P. J. Man-
ning, D. H. Ringler and C. E. Newcomer (ed.) The Biology of the
Laboratory Rabbit. 2nd ed. p 321. Academic Press, New York.
Gidenne, T., R. Carabana, J. Garcia, and C. de Blas. 1998. 5. Fibre
Digestions. In: C. de Blas and J. Wiseman (ed.) The Nutrition of
the Rabbit. p 69. CABI Publishing, London.
Jenkins, J. R. 1999. Feeding Recommendations for the House Rabbit.
Veterinary Clinics of North America: Exotic Animal Practice. vol.
2. p 143. W.B. Saunders Company, Philadelphia.
Maertens, L., and M. J. Villamide. 1998. 13. Feeding Systems for
Intensive Production. In: C. de Blas and J. Wiseman (ed.) The
Nutrition of the Rabbit. p 241. CABI Publishing, London.
Martin, N. 2000. Personal Communication. Nutritionist, Ranchway
Feeds, Fort Collins, Colorado.
McNitt, J. I., P. R. Cheeke, N. M. Patton, and S. D. Lukefahr. 1996.
Rabbit Production. Interstate Publishers, Inc., Danville, IL.
NRC. 1982. United States-Canadian Tables of Feed Composition. 3rd
ed. National Academy of Press, Washington, DC.
Okerman, L. 1994. Diseases of Domestic Rabbits. 2nd ed. Blackwell
Scientific Publications, Oxford.
Stein, S., and S. Walshaw. 1996. Rabbits. In: K. Laber-Laird, M. M.
Swindle, and P. Flecknell (ed.) Handbook of Rodent and Rabbit
Medicine. p 183. Pergamon Press, England.
Stevens, C. E., and I. D. Hume. 1995. Comparative Physiology of
the Vertebrate Digestive System. 2nd ed. Cambridge University
Press, Cambridge, United Kingdom.
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