How to Feed the Small Herbivore Gastrointestinal Tract

Introduction

Small herbivores are able to utilize a high-fiber, low-grain diet. Larger herbivores, like the capybara (Hydrochoerus hydrochaeris) are grazers, able to subsist on poor-quality, low-energy bulk or roughage, however many small herbivores like the rabbit (Oryctolagus cuniculus) are “browsers” or concentrate selectors. When allowed to select their own diet in a natural setting, rabbits will select the most tender, nutrient-dense parts of the plant.

Gastrointestinal anatomy and physiology

Among small herbivorous non-ruminants, the gastrointestinal tract of the rabbit is the most specialized and this manuscript will focus on unique features of this species’ anatomy and physiology. The rabbit possesses a simple, non-compartmentalized stomach and a large hindgut. The rabbit cecum is relatively large with a capacity 10 times that of the stomach. The cecum forms a spiral that fills much of the abdominal cavity, making up approximately 40% of gut volume.

As digesta enters the hindgut at the ileocecal-colic junction, a sorting mechanism separates indigestible fiber from non-fiber components of the diet like protein and soluble carbohydrates. Muscular contractions facilitate the separation of large, coarse fiber particles from the small, non-fiber particles. Antiperistaltic waves move fluid and non-fiber to the cecum for fermentation. A series of peristaltic waves move indigestible fiber through the colon expelling it from the body as hard or day feces (Fig 1) approximately 4 hours after consumption of the diet.

Figure 1. Indigestible fiber is expelled from the colon as hard feces, also known as day feces. Image by Mr. Greenjeans. Click on image to enlarge.

Fiber

Insoluble dietary fiber plays a critical role in maintaining the health of the rabbit gastrointestinal tract. Dietary fiber stimulates gut motility, minimizes the risk of enteritis, and reduces the incidence of fur chewing in commercial operations. Rabbits require a minimum dietary fiber level of 25% to maintain gut health. Low fiber diets result in gut hypomotility, reduced cecotrope formation (see below), prolonged retention time in the hindgut, and possibly enteritis.

Fermentation

All mammalian herbivores lack the enzymes necessary to break down cellulose in their plant-based diets so they must rely on symbiotic microorganisms. The main site of fermentation in most rodents and lagomorphs is the cecum (Table 1). The hamster is an exception to this rule of thumb because it is a pre-gastric fermenter. The hamster stomach is compartmentalized with a distinct forestomach and a glandular stomach.

Table 1. Fermentative capacity expressed as percentage of total digestive tract (From Van Soest 1994)

The microbial population in the hindgut of the rabbit consists primarily of Bacteroides spp. Similarly to what occurs in the rumen of ruminants, these microbes breakdown plant materials converting them intovolatile fatty acids and some vitamins like vitamin B and K. Their bodies will serve as a source of amino acids to the host mammal Volatile fatty acids (VFA) such as butyrate serve as a major source of energy in the colon. More VFA are produced in rabbits on starch-based diets than on forage or hay, however, high-starch diets are incompletely digested within the small intestine resulting in more starch available for microbial fermentation. The excess starch allows extremely rapid growth of microbes. If toxin-producing bacteria like Clostridium spiroforme are present, high levels of starch can lead to enteritis and possibly death.

Cecotropes and coprophagy

Cecotropes, also known as “soft feces” or “night feces” (Fig 2), are usually voided within 8-19 hours after consumption of the diet. Cecotropes are formed after fermentation of non-fiber dietary components within the cecum. Cecotropes contain vitamins produced by the microbes and amino acids (protein) from the microbe bodies, and are normally consumed by rabbits directly from the anus. While dry or day feces appear as distinct, brown balls (Fig 1), cecotrophs tend to be passed in small clusters. They are soft and sweet smelling with an outer mucoid membrane. The practice of consuming cecotropes or “night feces” is called cecotrophy; while coprophagy is the consumption of stool. These terms are often used interchangeably and their practice is considered an integral part of the digestive process in small herbivores.

Figure 2. Cecotropes or “night feces” are formed after fermenation of non-fiber dietary components within the cecum. Click on image to enlarge.

Coprophagy is basically a form of nutritional recycling that allows small herbivores to capture microbial protein and vitamins that the stomach was unable to absorb during the first pass of a meal through the gut. Food moves through the gut relatively rapidly, and whatever nutrients are not absorbed the “first time around” have a chance to be captured through ingestion of the feces. A normal rabbit practicing cecotrophy can digest 75% to 85% of the protein present in alfalfa hay. If a rabbit is fitted with a collar that prevents coprophagy, digestibility of the diet is significantly reduced, even when a highly digestible diet is fed. Cecotrope ingestion increases when a rabbit is fed a low-energy, low protein diet. High levels of dietary protein reduce cecotrope consumption.

Practical feeding strategies

A balanced rabbit diet contains adequate fiber (minimum 25%), minimal starch, and low to moderate protein levels (12% to 16%). Alfalfa is relatively high in protein and calcium; therefore alfalfa hay and alfalfa-based pellets are recommended for young, growing rabbits. Feed mature small herbivores a maintenance diet of grass hay, which contains less protein and calcium.

Calcium metabolism is unique in rabbits compared to all other mammals allowing this species to absorb dietary calcium without the help of vitamin D or activation of calcium-binding proteins within the gastrointestinal tract. This results in elevated blood calcium levels that must be excreted in the urine, a pathway unlike other animals. Chronic elevations in dietary calcium can result in calciuria, calcium-based uroliths, and in severe cases, calcification of soft tissues such as the aorta and kidney. Hypercalcemia may be intensified when rabbits are over-supplemented with vitamin D, as would occur when commercial rabbit pellets make up the bulk of the diet.

Feed adult small herbivores only small amounts of a high-fiber pellet. If rabbits at maintenance are fed a high-quality pellet ad libitum, they will become obese. Feed up to 26 g of high-fiber pellet (25% crude fiber) per kilogram body weight (approximately ¼ to ½ cup per 6 lbs) to maintain body condition. The amount of pellets fed can be increased to meet energy demands for gestation and lactation.

Not all pellets are created equal. A coarse grinding process is recommended for optimal health. Grains such as soybean hulls and wheat middlings are often ground up and incorporated into pellets. Grains processed too finely can lead to rapid bacterial fermentation of starch and subsequent enterotoxemia.

Probably the most important additive seen in small herbivore diets is vitamin C to guinea pig pellets. Unlike the vast majority of mammals, guinea pigs are unable to synthesize endogenous vitamin C or ascorbic acid. The additive, copper sulfate, is also commonly used in feeds given to small herbivores. Copper sulfate acts by inhibiting the growth of pathogenic bacteria and its use may lower the incidence of enteritis.

Summary

Small, herbivorous, non-ruminants possess a simple stomach paired with a large cecum and colon. Microbial fermentation within the hindgut allows digestion of the cellulose component of the plant-based diet. Small herbivores have a relatively high feed intake and a relatively rapid gastrointestinal transit time. They are able to meet their nutritional needs by practicing coprophagy. Coprophagy in rabbits is particularly specialized due to the ingestion of cecotrophs or night feces. Cecotrophs are protein and vitamin-rich droppings normally consumed directly from the rabbit’s anus.

A balanced small herbivore diet contains adequate fiber (minimum 25%), minimal starch, and low to moderate protein levels (12% to 16%). Fiber is essential to maintain gastrointestinal health as insoluble fiber stimulates gut motility. Alfalfa is relatively high in protein and calcium, and is recommended for young, growing animals. Feed mature animals a maintenance diet of grass hay, which contains less protein and calcium, along with small amounts of high fiber pellets.

References