Fiber Facts

Grazing is a full-time job for horses. Given their druthers, they'd graze for 12 hours or more every day, their broad, flat teeth and sideways chewing motions making short work of the tough, stemmy grasses and weeds they favor. Like all true herbivores, horses get most of their daily energy requirements from eating plant fibers. Yet, ironically, horses can't digest fiber.

horses grazing

It's important to remember that horses are grazing animals, programmed to chew on stemmy, fibrous plants for at least 12 hours a day.

In fact, no animal can digest fiber on its own. Animals don't produce the enzymes needed to break the beta bonds of polysaccharide fibers and make the nutrients within available for use. Fortunately, horses, like most other animals, have an almost invisible ally--a population of intestinal bacteria, resident in the cecum and colon, that are specially adapted to digest the fiber that horses cannot. Through a fermentation process, these gut flora produce the necessary enzymes to convert fiber to volatile fatty acids (VFAs), which the horse can absorb. Not only do the bacteria benefit (making this a truly symbiotic relationship), but the VFAs they create provide 30-70% of the horse's total digestible energy needs.

While we often provide grain and supplemental fats to our domestic horses to give them the energy to do hard work, it's important to remember that it's fiber that horses were meant to use as fuel--and fiber remains the first and most important ingredient in every equine diet. It provides all the energy horses need for everyday maintenance metabolism--ordinary stuff like breathing, walking, grazing, and sleeping. Without adequate fiber, the horse's digestive system doesn't function properly--it loses the ability to move food particles efficiently through the gut, and its ability to conserve water and electrolytes also is compromised. Without fiber in the system, high-carbohydrate feeds tend to "pack" in the gut as well. The result is a horse at risk for dehydration, colic, and laminitis (not to mention stable vices like cribbing and wood-chewing, which often develop when a horse's fundamental urge to chew is not satisfied).

Except in the most strenuous circumstances (such as 2-year-olds in heavy race training), fiber should make up at least 50% (by weight) of your horse's daily diet. For the vast majority of adult horses, that percentage can be pushed up considerably higher--even to 100%, if the horse is an easy keeper and/or not being asked to do work. The basic principle is this: grain is an optional part of a horse's diet; roughage (fiber) is not.

Assessing Fiber Quality

Of course, not all fiber is created equal. Depending on its origins, it can vary widely in terms of quality and digestibility.

Fiber consists of three main substances: cellulose, hemicellulose, and lignin. Lignin is considered 100% indigestible by either horses or the bacteria they harbor in their digestive tracts; it's the very tough stuff that gives plant material its rigidity. (Oak trees are high in lignin; tender young grass shoots are low.) But cellulose, and to a certain extent hemicellulose, are digestible, and it's from these two that horses derive most of their digestible energy requirements.

Cellulose and hemicellulose are polysaccharide molecules, fairly complex chemical arrangements that need to be broken into smaller units in order to be absorbed through the gut wall. Breaking the "beta bonds" that hold the individual monosaccharide molecules together allows their conversion to VFAs. Cellulose and hemicellulose, which stem from the non-seed and non-fruit portions of a plant (such as the leaves, stems, and hulls), also are known as insoluble fiber. Soluble fiber (which makes up a relatively minor portion of the fiber in a horse's diet) is fiber stemming from the "liquid" portions of a plant: the resin, sap, pectins, and mucilages.

All plant-eaters use nearly all of the soluble fiber they ingest. But the degree of insoluble fiber that horses use varies. The sooner bacteria go to work breaking the beta-bonds, the higher the percentage of the fiber that is used by the horse; but even insoluble fiber that is not digested has its place in the equine diet. It helps maintain gut motility and function, as well as preventing the too-quick consumption of carbohydrates, which are readily digested and sometimes can cause digestive upset if not "cushioned" by the presence of fiber in the colon.

In practice, it's not terribly important to know how much of the fiber provided by a plant is soluble and how much is insoluble. An enzymatic test does exist, but it's rarely used because the differences are not significant from a feeding point of view. There are, however, a few ways of defining the fiber content of a feed--each with its own pros and cons.

Crude fiber is the value most of us are used to seeing on our feed labels or tags. It's an estimate of the total fiber in a feed, but it's not terribly accurate. The calculation that results in a crude fiber value tends to overestimate the non-fiber carbohydrate content of a feed, and underestimate the cellulose portion; this also leads to an overall overestimation of the feed's caloric content and thus its feeding value. The CF is a useful approximation, but not much more.

Another value commonly used to express the fiber content of a feed is the NDF, or neutral detergent fiber. Unlike the CF, an NDF value includes almost all of the cellulose in a feed sample, and more than 50% of the hemicellulose, but it also erroneously includes a high percentage of digestible starches in its calculations. Acid detergent fiber, or ADF, is generally considered the most accurate way of expressing fiber percentages. ADF removes starches from consideration, but also removes most of the hemicellulose--and because most of the hemicellulose in a feed is used by the horse, ADF analysis, which ends up being a measure of the cellulose plus the lignin, results in an underestimation of the feed's insoluble fiber content and an overestimation of its energy content and feeding value. Nonetheless, it's the best available indicator of fiber digestibility.

CF, ADF, and NDF values all can be generated by doing a lab analysis of your feed, a service provided by most feed companies and many universities. More about how to take a representative sample follows.

Finding Fiber

By far the most common fiber sources for equines, of course, are pasture grasses and hay (dried grasses and legumes). For the vast majority of horses around the world, this is where they get their all-important fiber fix; and rightly so, because these plants are precisely what horses have evolved to eat.

We discussed hay thoroughly in the August nutrition column; for our purposes now, we only need to understand that the fiber content of pasture and hay can fluctuate according to the environment, time of year, soil, and stage of growth of the plants. Hay cut early in its growth cycle, before it has developed seed heads, tends to be lower in overall fiber than hay cut late; but early-cut hay also is lower in lignin and higher in digestible fiber, proportionately, than late-cut. Once the plants have gone to seed, their stems tend to become tough and fibrous, and the palatability and digestibility plummet.

By contrast, the fiber content of most grains doesn't vary a lot. Regardless of the stage of growth of the plant, you can pretty much depend on the fiber values of grains to be within the ranges listed in the chart on page 80. As a result, most horse owners can rely on the information on the feed tag and forego getting a fiber analysis of their grain ration. But doing an analysis of your hay (and/or pasture) can be valuable, especially as even similar-looking batches of hay can be remarkably dissimilar in terms of fiber content.

The best way to take a sample of your hay is to use a tool that can take a core sample of several bales (ask your local feed store to loan you one). Insert the corer diagonally along the long axis of each of the bales rather than straight through the center, and take samples from at least 20 bales, ideally, mixing them together in a clean paper or plastic bag. If you don't have access to a corer, you can also get good results by doing a "grab sample," taking a handful of hay from 20 bales from different parts of the hay field, taking care to get some from the center flakes and some from the ends in order to get a real mixture. The proportion of leaves to stems can make a big difference to the resultant ADF and NDF values, so make sure that you have not grabbed too much of one and not enough of another. Most labs can return results within a week or two, at a cost of approximately $20-$40.

If you want to get a fiber analysis of your pasture, a bit of observation is called for first. There is no point in analyzing the fiber content of plants your horses don't eat, so begin by watching them to see which plants are favored. Then take handfuls of only those plants from several locations throughout your field.

How to interpret the results? As a rough guideline, forage with an acid detergent fiber value of more than 35% is considered of poor quality, and is probably past bloom. Its digestibility will be low--which is not to say that it cannot be fed, but that you will have to feed considerably more of it in order for your horses to extract the same quantity of nutrients that they could glean from a "younger" forage. (It's interesting to note that donkeys, who are adapted to living in harsh conditions, are considerably more efficient at extracting nutrients from poor-quality, highly indigestible forages than are horses and ponies.) ADF is considered a good overall parameter for assessing the maturity of forages.

Hay and pasture grasses are not the only fiber sources available to horses. One of the most popular alternatives is sugar beet pulp, a feed additive made from the fibrous portion of the sugar beet after the sugar has been extracted. Available in North America almost exclusively in a dehydrated format (either shredded or in pellet form), beet pulp can be re-hydrated by soaking it in water for a few hours before feeding. (A recent study has determined that soaking beet pulp, traditionally thought to avert the major digestive upset that would occur when the dehydrated product hit the liquid contents of the stomach and suddenly expanded, is not actually necessary. Horses fed varying quantities of dehydrated, unsoaked beet pulp demonstrated no ill effects--though many horsemen prefer to continue soaking beet pulp in the name of "better safe than sorry.")

Beet pulp has an ADF value of less than 28%, making it a very digestible fiber source and a useful supplement to hay or pasture for any of several circumstances. When soaked, its soft texture is easy to chew, making it a good choice for older horses, or any animal with a dental problem; it is also favored for putting weight on "hard keepers," and it makes a convenient place to hide oral medications. Many horse owners also serve it warm on cold winter nights, although one suspects that the comforting effect of such a meal does more for the owner than for the horse! Because the crude protein content of beet pulp is fairly low (averaging around 8%), it is appropriate for almost every type of horse. It is also fairly high in calcium.

Bran, another traditional way of supplementing fiber, is a less-suitable choice. Bran is the outer layer of the grain kernel that is removed in the process of milling; wheat bran is the type most commonly fed to horses (although rice bran sometimes is used as a fat supplement in small quantities). A fluffy, low-density feed, bran is only half as dense as oats (and thus delivers only half the digestible energy). Rice also is only a quarter as dense--and thus not as energy-rich--as corn or barley. So despite its ADF of approximately 15%, it takes a lot of bran to provide sufficient fiber for the average adult horse. Furthermore, its purported laxative effect has been shown to be a myth; whether fed dry or wet, bran has no demonstrated "loosening" or "regulating" effect on the bowel. (The loose manure many owners observe after the feeding of a weekly bran mash is, in fact, the result of a mild digestive upset from a sudden change in the diet!) The regular feeding of bran is likely to be detrimental, because it has a high concentration of phosphorus and a low concentration of calcium. This imbalance contradicts the horse's need for a 1:1 (up to 1.8:1) ratio of calcium to phosphorus; if large quantities of any type of bran are fed on a long-term basis, horses run the risk of nutritional secondary hyperparathyroidism (NSH) or "bran disease," in which calcium is leached from the limb and facial bones in order to try and balance the excess phosphorus. An occasional small bran mash probably does no harm, but as a fiber supplement, there are better choices. If you must feed it, make bran no more than 10% of your horse's total ration.

Two other fiber supplements are lignin-rich and largely indigestible. They are added to the diet mostly as "busy food"--useful in keeping obese or idle horses chewing away--and to some extent, to aid in digestive health by keeping gut motility up to speed. Chaff (chopped straw or low-quality hay) is a feed additive often used in the United Kingdom, mixed in with the grain to slow down a horse who bolts his feed, "fake out" an overweight or greedy horse who would like to be getting more grain than he needs, or "cushion" the system of a horse with a tendency to colic. Oat straw and barley straw commonly are used to make chaff, and while their ADF values are usually more than 35%, they certainly are harmless, even if they provide more bulk than nutrition to the diet.

Grain hulls are another inexpensive way to provide that same effect. High in crude fiber (up to 50% higher than grass hay) and low in energy, hulls can be used to replace some or all of the forage in a horse's diet. The hulls of most cereal grains can be fed safely to horses; oat hulls are particularly popular, and coarsely ground corn cobs are another similar product. One caveat: because hulls often are ground, they tend to be dusty. Blending them with a little water or molasses can help keep the dust down, or you can buy a pelleted version.

One downside to most of these alternate fiber sources is that they are consumed by the horse far faster than he can consume hay--so any time you substitute another fiber source for forage or pasture, you might be giving your horse less opportunity to satisfy that compelling urge to chew. Boredom can translate that urge into stable vices, coprophagy (eating manure), even munching on the stall walls or the neighbor's tail. Feeding small meals, often (at least four times a day), is a partial solution. But under most circumstances, forage is still the best and most natural choice for most horses. Other fiber sources can be used as supplements to hay or pasture, or as a complete substitute only in cases like advanced respiratory disease (such as heaves), or dental problems that make it impossible for the horse to chew and process forage.

About the Author

Karen Briggs

Karen Briggs is the author of six books, including the recently updated Understanding Equine Nutrition as well as Understanding The Pony, both published by Eclipse Press. She's written a few thousand articles on subjects ranging from guttural pouch infections to how to compost your manure. She is also a Canadian certified riding coach, an equine nutritionist, and works in media relations for the harness racing industry. She lives with her band of off-the-track Thoroughbreds on a farm near Guelph, Ontario, and dabbles in eventing.

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