Rabbit Nutrition--Fiber
by Pamela Alley, RVT

Rabbit nutrition is one of the highest priority fields of study in rabbits; research performed in this area has led to much of our current knowledge of enteritis complexes and diet requirements. As more studies are done, fiber type and amount in the diet has assumed a new prominence and importance.

First of all, what is fiber? Fiber comes in two basic forms, digestible and indigestible. They are both carbohydrate chains made up of sugars bound tightly together. The body simply does not make the enzymes needed to break down cellulose and lignin (indigestible), whereas it is able to somewhat break down the hemicellulose, starches, and sugars (digestible).

Starches and sugars? Where did they come from? Starch and sugar are simpler versions of the complex carbohydrate called fiber, made up of some of the same sugars with different chemical linkages, which renders them highly digestible to the body, and thus, they are not 'counted' as fiber. Hemicellulose, cellulose and lignin all have tightly bound chains which the body cannot break down to any significant degree and thus they pass through the digestive process relatively unchanged.

Now, where do lignin and cellulose come from? Cellulose and lignin are often found together in a compound called lignocellulose, providing structural rigidity to plants. This rigidity, called lignification, increases with the maturity of the plant, and results in a lower 'food value' due to less readily available carbohydrate. Wood is extremely lignified, straw only slightly less so, and hay is less so yet. Hemicellulose is considered a digestible fiber, and does not exhibit the integrity of the lignocellulose in its passage through the gut.

Size of the fiber particle is also important; long fiber has been shown to be more beneficial and result in less enteritis than fine or short fiber particle size, The European term for it is 'scratch factor' and refers to the action of the coarse fiber as it moves through the gut, which is thought to stimulate gut motility (gut motility: the contractions of the intestine which move the feed through the intestinal tract). Rabbits fed a diet low in long fiber display a much higher rate of diarrheal disorders, which in rabbits is associated with lack of motility.

Growth rate is reduced on a low-fiber diet; fiber is also essential in the prevention of trichobezoars, or hairballs.  Animals on low-fiber rations will often chew their own and each others' fur, resulting in unsightly pelts.

Fiber is measured on most feed tags as Crude Fiber, and is listed as a maximum amount by percentage. Unfortunately, this is a poor reflection of the amount of digestible and indigestible fiber actually in the feed. More advanced measuring methods have begun to show that this number is reflecting not all of the fiber in the feed as well as some non-fiber fractions. Two processes, Acid Detergent Fiber (ADF) and Neutral Detergent Fiber (NDF), have made determining fiber content much simpler and more accurate.

ADF is the best measure of indigestible fiber, made up of lignin and cellulose; NDF also measures hemicellulose and the amount of digestible fiber may be determined by the difference between the two measurements,

When the tag reads 'Crude Fiber max 17%', what is it telling you? It's saying that the maximum percentage of fiber is 17, and it may be much lower, The requirement for this measurement on the tag is to ensure that the feed manufacturer didn't go overboard on adding nice, cheap fiber to the diet rather than wonderful, expensive protein and fats, which are listed as a minimum.

So what is best for your rabbits? High fiber, low fiber, and how are you supposed to get it into them? There is an easy solution: hay. Grass, oat, rye, timothy, barley, even stemmy alfalfa hay will provide plenty of long fiber to the rabbit as well as entertainment. You don't need to stuff the cage full of it; a relatively small amount added to a normal pelleted diet is sufficient.

Straws may also be used for the provision of indigestible fiber; remember that they are more mature when harvested, and so are more lignified, with higher fiber and lower food value. This can also result in lower acceptance by the rabbits, because the straw is less palatable than hays,

A novel idea from the tropics is to give each rabbit a coconut with most of the brown hairy hull material intact. They will happily consume this source of fiber and have a toy at the same time! Other options for hay feeding are specially made hay feeders that attach to the cage, cardboard boxes stuffed with hay and a hole cut to allow the rabbit access, placing hay on top of the cages so that the rabbits must pull it through, giving each animal a smaller amount directly on the floor of the cage daily to weekly, or even chopping the hay into smaller fragments of 1/4 to 1 inch in length and feeding in a separate hopper.

A common result of provision of high amounts of long fiber is an increase in cecotrope production. These small grape-like mucous-covered clusters are normally largely consumed by the animal, but as fiber supplementation increases the amount of feed moved through the cecum, the rabbit may not ingest all of them--which can seriously confuse the uninitiated. It's not a matter for worry, it's a sign of good gut movement. The rabbit will ingest what it needs to satisfy its vitamin and microbial needs, and let the rest drop,

However, no animal can survive on a diet with so much fiber that it cannot eat enough to meet its energy needs. Given a choice, they will eat only to meet their needs. Provided a high-density, high-energy diet, they may become obese. Adding fiber availability for these animals may result in the achievement of a normal weight.

Provided a low-density, high-fiber feed, some animals, particularly dwarf breeds, may not have the stomach room to process enough of the feed to meet their needs. These animals eat voraciously yet are bony and usually potbellied. These diets should be corrected to a better-balanced pellet. Happy Feeding!