Fiber is one of the most important components in the equine diet, providing up to 70% of a horse’s energy needs.

Fiber in the equine diet predominantly originates from forages such as hay and pasture grass. Fiber can also be obtained from highly fermentable sources such as beet pulp and soy hulls.

While horses do not have a daily requirement for fiber, nutritionists recommend maximizing horse’s fiber intake by feeding a forage-based diet.

In horses, eating a diet rich in fiber supports gut health, enables foraging behaviors, and reduces the risk of stereotypic behaviors. Continue reading to learn more about the role of fiber in your horse’s diet.

Fiber in the Equine Diet

As grazing animals, horses evolved to thrive on a diet primarily composed of fibrous plant material.

Also referred to as structural carbohydrates, fiber comprises the structural components of cell walls that give plants their rigidity and form.

Structural carbohydrates differ from non-structural carbohydrates (NSC), like sugar and starch, which plants use primarily for energy and other functions.

Several different types of fiber can be found in the equine diet, but all structural carbohydrates share one common characteristic: they cannot be digested by enzymes that horse produces in their gastrointestinal tract. Instead, horses depend on microbes in the hindgut to break down these dietary components. [1]

Fiber undergoes microbial fermentation in the horse’s hindgut, particularly in the cecum and colon, where beneficial microorganisms break it down into volatile fatty acids (VFAs).

These VFAs serve as an energy source for horses providing up to 70% of the horse’s energy requirements.

Fiber consists of long chains of sugars, such as glucose, linked together with beta linkages. The horse produces digestive enzymes in the small intestine and pancreas that can break alpha linkages (such as those in starch) but cannot break beta linkages in structural carbohydrates.

This is why starch can be digested in the horse’s small intestine, but fiber cannot. Instead, fiber passes undigested into the hindgut where microbes produce enzymes that can break beta linkages and release sugar.

Types of Fiber

In order to understand how fiber contributes to a horse’s diet, it is important to differentiate between types of fiber in forages and feedstuff.

Different fibers vary in terms of their chemical composition, physical structure, digestibility and energy content.

Some fibers are highly fermentable and easily broken down by microbes in the hindgut. Others are indigestible and remain largely unchanged as they pass through the digestive tract.

The following are descriptions of the different classifications of fiber typically reported on forage and feed analyses for horses.

Horse Feed Tag Fibre Content

Crude Fiber

Crude fiber is one of the oldest methods of estimating the fiber content in animal feeds. This value represents the portion of plant carbohydrates in a feed that are not digestible by the horse but can undergo fermentation in the hindgut.

Crude fiber is measured by chemically mimicking gastric digestion of feeds with solutions consisting of acids and bases. [1] This chemical digestion breaks down feed components, leaving behind fibrous material, which is then measured.

However, this analysis of crude fiber results in some loss of fibrous components such as cellulose, hemicellulose, and lignin, underestimating the fiber content of the feed. [1] For this reason, more advanced analytical methods are preferred to estimate fiber content.

Total Dietary Fiber

Total dietary fiber is defined as the sum of all the indigestible components of a horse’s diet, including both soluble and insoluble fibers.

While total dietary fiber provides a reasonable approximation of fiber in equine diets, it is used more commonly in human nutrition. [1]

Total dietary fiber represents the combined content of various fibers, including: [1]

  • Cellulose
  • Hemicellulose
  • Lignin
  • Pectin
  • Other soluble fibers such as gums and mucilages

Cellulose

Cellulose is a polysaccharide made up of long chains of glucose that are held together with beta-1,4 linkages. [2] Cellulose makes up a large percent of plant mass, with cool season grass pasture containing around 50 – 60% cellulose on a dry matter basis. [3]

Cellulose is an insoluble fiber, which means it doesn’t dissolve in water and isn’t directly digestible by the horse. Instead, cellulose adds bulk to the digestive contents, aiding in the movement of food through the digestive tract.

Hemicellulose

Hemicellulose is a polysaccharide with mixed alpha and beta linkages. Hemicellulose may consist of various simple sugar units, including glucose, xylose, mannose and arabinose. [2]

Like cellulose, hemicellulose makes up a large proportion of plants, with cool season grass pasture containing approximately 30 – 50% hemicellulose on a dry matter basis. [3] Hemicellulose is also an insoluble fiber.

Lignin

Lignin is a portion of the plant cell wall that has a phenolic (ring-like) structure that inhibits the digestion of polysaccharides. This makes it indigestible for both horses and microbes. [2]

Lignin can bind to cellulose and hemicellulose, forming lignocellulose, which is less digestible by microbes compared to cellulose and hemicellulose. [2][4]

Lignin is less abundant in plants than cellulose and hemicellulose, with cool-season grass pasture containing roughly five percent lignin on a dry matter basis. [3]

Pectins

Pectins are structural carbohydrates that help to bind plant cells together. They are made of simple sugar units including galacturonic acid, arabinose, and galactose. [2] Pectins are soluble fibers and are therefore quickly fermented in the hindgut. [2]

Pectins are less abundant in plants compared to cellulose and hemicellulose, with pasture typically containing about 2 – 4% pectin. [3] However, some feeds such as beet pulp may contain up to 30% pectin. [2]

Gums and Mucilages

Gums and mucilages are soluble fibers that are found in the woody parts of plants, as well as seed coatings. [2] Like other soluble fibers, they are not digested by the horse, but are rapidly fermented by microbes.

Neutral Detergent Fiber

Neutral detergent fiber (NDF) is a common measure of cell wall components and serves as a good approximation of fiber in a diet. [1] It is found in most forage analyses, making it easy to compare across samples.

The analysis of neutral detergent fiber includes: [1]

  • Cellulose
  • Hemicellulose
  • Lignin

However, NDF does not include soluble fibers such as pectins, gums, mucilages, and beta-glucans.

Acid Detergent Fiber

Acid Detergent Fiber (ADF) is a measure of the highly indigestible components of a feed, not including hemicellulose.

The analysis of acid detergent fiber includes: [1]

  • Cellulose
  • Lignin
  • Acid Detergent Insoluble Nitrogen
  • Acid Insoluble Ash
  • Silica

Because ADF does not include hemicellulose content, it is not a good representation of the total amount of fiber in a diet. However, it can be used to approximate the amount of hemicellulose in a diet by subtracting from NDF.

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Fiber Digestion

Fiber digestion in horses primarily occurs in the hindgut where microbial fermentation takes place. The horse’s digestive system lacks the enzymes necessary to break down complex carbohydrates like cellulose and hemicellulose.

Instead, specialized microorganisms in the hindgut produce enzymes capable of breaking down and fermenting these fibers into simpler compounds.

Fermentation allows horses to extract nutrients from fibrous plant material that would otherwise be indigestible.

Mouth and Small Intestine

Fiber digestion begins in the mouth, where feed is chewed and ground into smaller particles that microbes can more easily access to digest.

Chewing also stimulates saliva production, lubricating food to support transit through the digestive tract and buffering stomach acid. [5]

Since horses lack the enzymes necessary to digest fiber, the majority of it passes through the small intestine, where simple carbohydrates are digested and absorbed.

However, it’s worth noting that some bacteria are present in the small intestine, so some fermentative digestion may occur here. [6]

Hindgut

After passing through the small intestine, structural carbohydrates enter the cecum, where the bulk of microbial digestion occurs. Microbes attach themselves to fiber particles and excrete enzymes to break down beta linkages. [6]

The microbial population in the hindgut is diverse and specialized, with specific microbes preferentially digesting different types of fibers.

Cellulose is mainly broken down by anaerobic fungi but can also be broken down by cellulolytic bacteria (i.e. Ruminococcus albus, Ruminococcus flavefaciens, Fibrobacter succinogenes). [2][4] Compared to other types of fiber, cellulose is more difficult for microbes to degrade. [7]

Hemicellulose is mainly digested by bacteria, including F. succinogenes. [4] Hemicellulose is relatively easy for microbes to ferment, although the fermentation process is slow. [2]

Pectins can be broken down by both bacteria and protozoa, and are readily fermented by microbes in the hindgut. [7][8]

Digestion Products

The microbial digestion of fiber in the horse’s hindgut produces several byproducts, including: [6]

  • Fermentation gases like methane and carbon dioxide
  • Sugars that the bacteria use for their own cell growth and maintenance
  • Volatile fatty acids that horses can use for their own energy demands

Volatile Fatty Acids

Volatile fatty acids (VFAs) are organic acids with short-chain fatty acid molecules that can be used as energy sources for horses.

There are three VFAs produced by microbes in the hindgut that horses use to meet energy needs:

  • Acetate: Produced in the largest amounts, accounting for approximately 75% of VFAs in the hindgut. [6] Acetate can be used directly for energy in muscle and other tissues and can also be converted to fatty acids for energy storage. [9]
  • Propionate: Produced in lower amounts than acetate, accounting for approximately 18% of VFAs in the hindgut. [6] Propionate can be used for glucose synthesis in the liver, which is important for maintaining blood glucose levels in horses fed a forage-based diet. [9]
  • Butyrate: Produced in the lowest amounts, accounting for approximately 6% of VFAs in the hindgut. [6] Most butyrate is used by intestinal cells, but it can also be converted to B-hydroxybutyrate and used as an energy source by the horse. [10]

Because microbes in the cecum and large intestine are so efficient at producing volatile fatty acids from fiber, most horses can meet their energy needs from forage alone.

VFAs produced by the cecum alone likely meet more than 30% of a maintenance horse’s energy needs. [9] Additionally, propionate metabolism has been shown to supply 50 – 60% of horses’ glucose needs. [11]

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Fiber Sources for Horses

There are many different sources of fiber for horses, with varying nutritional profiles and degrees of digestibility.

Depending on which source you choose, levels of soluble and insoluble fiber, protein, and energy will vary, providing options suitable for meeting the nutritional needs of horses across different life stages and activity levels.

Hay and pasture grass are the main sources of fiber in equine diets, but horse owners can also incorporate processed forage products, beet pulp, and soybean hulls to increase their horse’s fiber intake.

Long-Stem Forage

Long-stem forages such as hay and pasture typically comprise the majority of structural carbohydrates in a horse’s diet.

The fiber content of forages is influenced by a number of factors, including grass type and environmental factors like temperature when the grass is harvested or consumed.

The fiber content of plants is also influenced by plant maturity. Younger plants tend to have lower fiber content, with relatively more hemicellulose. As plants mature, they deposit more fibers and grow longer stems, resulting in higher levels of cellulose and lignin in mature plants. [7][12]

Cool-Season Grasses

Cool-season grasses are grass species that thrive in cooler climates and typically experience peak growth during the spring and fall seasons. Examples used as forage for horses include timothy, orchardgrass, fescue, and ryegrass.

Cool season grasses generally have moderate to high fiber levels, containing: [3]

  • 50 – 60% cellulose
  • 30 – 50% hemicellulose
  • 2 – 4% pectin
  • 5% lignin

Legumes

Legumes are another common forage for horses, including alfalfa (lucerne) and clover. Legumes also provide significant amounts of fiber and are considered a high-quality feed for horses.

In general, legumes typically contain higher levels of soluble fiber, providing: [7]

  • 30 – 50% cellulose
  • 25 – 30% hemicellulose
  • Up to 30% pectin
  • 4 – 15% lignin

Warm-Season Grasses

Warm-season grasses thrive in warmer climates and grow vigorously during the summer months. Species used as forage for horses include Bermuda grass, Bahia grass, and switchgrass.

Compared to cool season grasses and legumes, warm season grasses typically mature faster. As a result, they typically have a higher proportion of cell wall constituents and higher fiber content on average. [7]

Hay Cubes and Pellets

Most horses can get all of the fiber in their diet supplied by long-stemmed forage. However, there are instances where feeding forage cubes and pellets may be recommended to increase fiber intake.

Hay cubes and pellets are made with forages that are ground into smaller particles and pressed into a cube or pellet shape. These processed feeds have a similar nutritional profile to long-stem forage, but a smaller particle size.

Forages with smaller particle sizes require less chewing compared to long-stem forage, so horses tend to consume them more quickly. [13] As a result, horses consuming mostly chopped or pelleted forage may go longer between meals without access to forage.

To avoid this, it is advisable to provide the majority of a horse’s forage intake as long-stem forage whenever possible.

Highly Digestible Fiber Sources

Highly digestible fiber sources are those that contain a higher proportion of soluble fiber and less indigestible material, such as cellulose and hemicellulose.

Digestible fiber sources contain higher levels of non-starch polysaccharides, pectins, and gums, that microbes can readily ferment. Common examples in equine diets include beet pulp and soybean hulls.

  • Beet Pulp: Beet pulp is a byproduct of sugar beet processing and is often used as a fiber-rich feedstuff for horses. It is highly digestible and can be soaked in water to form a palatable mash.
  • Soybean Hulls: Soybean hulls are another fiber-rich byproduct that is commonly fed to horses. They provide a good source of digestible fiber and can be included in concentrate feeds or fed as a standalone fiber source.

Deficiency

A daily fiber requirement hasn’t been established for horses. This means that unlike essential nutrients such as protein, vitamins, and minerals, there isn’t a strict threshold for fiber intake that must be met to prevent nutritional deficiency.

However, horses are grazing herbivores and it’s widely recognized that a significant portion of their diet should consist of fiber to support their overall health and well-being.

In general, horses should be fed a minimum of 1.5% of their body weight as forage dry matter per day to ensure sufficient fiber intake. [14]

Not meeting this requirement can lead to serious health issues, including hindgut acidosis, gastric ulcers, and stereotypic behaviors. [14]

Hindgut Acidosis

When fiber consumption is limited in horse diets, energy demands are often met with high-starch grains. Feeding high levels of starch in a single meal can overwhelm the digestive system, leading to starch overflow into the hindgut.

Excessive starch passage to the hindgut can cause shifts in microbial populations, increased lactic acid production, and ultimately causing hindgut acidosis. [15]

Ulcers

Horses chew more thoroughly and produce twice as much saliva when eating long-stem forage compared to concentrate feeds. [16][17] Saliva acts as a buffer in the stomach, preventing the development of gastric ulcers. [18]

Eating a diet with low fiber level and high proportions of concentrate grains may lead to reduced pH levels in the stomach and an increased risk of ulcers. [7]

Stereotypies

Low fiber diets can contribute to the development of stereotypic behaviors in horses, including bark stripping, wood chewing and crib-biting. [7] Stereotypies may arise due to boredom and less time spent eating or they may result from complications due to reduced saliva production.

Increasing fiber consumption by feeding more forage may prevent these issues by extending meal time and mimicking horse’s natural grazing behavior. [19]

Feeding Considerations

Fibrous forage is the cornerstone of the equine diet. but there are some considerations to keep in mind when adding fiber to your horse’s diet.

  • Forage Quality: Low-quality forages with high levels of indigestible fiber can contribute to gastrointestinal disorders in horses, such as hay belly. Choose the right quality and maturity of forage to match your horse’s nutritional requirements
  • Dental Issues: Dental issues, including sharp points or missing teeth, can hinder a horse’s ability to effectively grind and chew fiber-rich forages. Regular dental care by a licensed veterinarian is essential. Alternative forage options may help alleviate chewing problems.
  • High-Fat Diets: Feeding moderate amounts of fat will not negatively impact fiber digestion, but horses switched to high-fat diets without proper adaptation can experience reduced fiber digestibility. [20] Limit the total fat content of your horse’s diet and add fat sources slowly to allow the horse’s digestive system to adapt and avoid impacting fiber digestion.
  • High-Starch Diets: Feeding diets with excess starch content may reduce the population of cellulolytic bacteria and increase lactobacilli and streptococci in the hindgut. [9] This change in the hindgut microbiome may negatively impact fiber digestion.
  • Increased Gut Fill: High-fiber diets increase the amount of weight horses carry in their gut, which may not be ideal for race horses and performance athletes. [7] However, dietary fiber also helps reduce the risk of gastric ulcers, which are common in horses with very heavy exercise loads. [7] An equine nutritionists can help you strike a balance between optimizing performance and supporting gut health.

Summary

  • Fiber is a critical component in the equine diet, providing up to 70% of a horse’s energy needs and supporting gut health and natural grazing behaviors.
  • Mainly obtained from forages like hay and pasture grass, fiber can also come from highly fermentable sources such as beet pulp and soy hulls.
  • Different types of fiber in the equine diet include cellulose, hemicellulose, lignin, pectins, and gums, which undergo varying degrees of microbial fermentation in the hindgut.
  • Fiber is broken down by microbes in the hindgut, producing volatile fatty acids (VFAs) that are absorbed by the horse as energy sources.
  • Insufficient fiber intake can lead to hindgut acidosis, gastric ulcers, and stereotypic behaviors, highlighting the importance of feeding a forage-based diet.

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References

  1. National Research Council Chapter 10: Feed Analysis. Nutrient Requirements of Horses. 2007.
  2. Geor, R.J. et al. Chapter 8: Carbohydrates. Equine Applied and Clinical Nutrition: Health, Welfare and Performance. 2013.
  3. Longland, A.C. et al. Comparison of oven drying versus freeze drying on the analysis of non-starch polysaccharides in graminaceous and leguminous forages. Proceedings of the British Society of Animal Science. 1995.
  4. Wunderlich, G. et al. Understanding the microbial fibre degrading communities & processes in the equine gut. Animal Microbione. 2023. View Summary
  5. Staniar, W.B. What is dietary fiber, and why is it important in the equine diet?. The Pennsylvania State University. 2016.
  6. Geor, R.J. et al. Chapter 1: Gastrointestinal Physiology. Equine Applied and Clinical Nutrition: Health, Welfare and Performance. 2013.
  7. National Research Council Chapter 8: Feeds and Feed Processing. Nutrient Requirements of Horses. 2007.
  8. Bonhomme-Florentin, A. Degradation of hemicellulose and pectin by horse caecum contents. British Journal of Nutrition. 1988.
  9. National Research Council Chapter 2: Carbohydrates. Nutrient Requirements of Horses. 2007.
  10. Geor, R.J. et al. Chapter 2: Endocrine and Metabolic Physiology. Equine Applied and Clinical Nutrition: Health, Welfare and Performance. 2013.
  11. Simmons, H.A. and E.H.J. Ford Gluconeogenesis from propionate produced in the colon of the horse. British Veterinary Journal. 1991. View Summary
  12. Dulphy, J.P., et. al. Evaluation of voluntary intake of forage trough-fed to light horses. Comparison with sheep. Factors of variation and prediction. Livestock Production Science. 1997.
  13. Argo, C.M., et. al. Adaptive changes in the appetite, growth and feeding behaviour of pony mares offered ad libitum access to a complete diet in either a pelleted or chaff-based form. Animal Science. 2002.
  14. Geor, R.J. et al. Chapter 26: Controversial areas in equine nutrition and feeding management. Equine Applied and Clinical Nutrition: Health, Welfare and Performance. 2013.
  15. Medina, B., et. al. Effect of a preparation of Saccharomyces cerevisiae on microbial profiles and fermentation patterns in the large intestine of horses fed a high fiber or a high starch diet. Journal of Animal Science. 2002. View Summary
  16. Elia, J.B., et. al. Motivation for hay: Effects of a pelleted diet on behavior and physiology of horses. Physiology & Behavior. 2010. View Summary
  17. Meyer, H. Investigations on saliva production and chewing effects in horses fed various feeds. Proceedings of the 9th Equine Nutrition and Physiology Symposium. 1985.
  18. Geor, R.J. et al. Chapter 34: Gastric Ulceration. Equine Applied and Clinical Nutrition: Health, Welfare and Performance. 2013.
  19. Geor, R.J. et al. Chapter 25: Effects of diet on behavior- normal and abnormal. Equine Applied and Clinical Nutrition: Health, Welfare and Performance. 2013.
  20. National Research Council Chapter 3: Fats and Fatty Acids. Nutrient Requirements of Horses. 2007.