Nutrition Fundamental Series: Structural Carbohydrates
In the horse, as well as companion and zoo industries, it’s not uncommon to observe feeding decisions based on anecdotes, folklore, fables or even human data that are not compatible or skewed from the known fundamentals. There are six major classifications of nutrients and depending on the animal there are approximately 45+ specific nutrients that are required in the diet. In addition there are components of the diet that are not “required nutrients” but are critical considerations in the nutritional management of the horse. The Nutrition Fundamental Series will address the fundamentals of nutrition.
Carbohydrates provide energy but the source of the carbohydrate and the type of digestion dictates the amount of energy that is ultimately available to the horse. The base unit of a carbohydrate is the monosaccharide (mono = one; saccharide = sugar) and how these basic units are connected to form different carbohydrates will determine the site of digestion and the nutritional value that is delivered.
Carbohydrates can be divided into two general categories, nonstructural and structural carbohydrates. Starches and sugars, known as nonstructural carbohydrates are digested by enzymes and are absorbed in the foregut. Fiber components, cellulose and hemicellulose, known as structural carbohydrates are digested by microorganisms throughout the gut but primarily in the hindgut. Structural carbohydrates are also important to gut health.
Structural Carbohydrates, what are they?
Structural carbohydrates make-up the plant cell wall and are resistant to digestion by enzymes produced in the horse’s foregut. For comparison, the nonstructural carbohydrates are found inside the plant cell (cell content) and are digested by horse’s enzymes. See the plant cell diagram below.
Facts about Feeding Structural Carbohydrates or Fiber
Fiber consists of structural carbohydrates. As fiber in a diet increase and starch, fat & protein feedstuffs decrease, the less energy consumed. There is not an established dietary requirement for structural carbohydrates. A requirement is inferred with minimum forage intake recommendations. Structural carbohydrates (fiber) are responsible for promoting gut contraction or movement. An increase in structural carbohydrates consumption will increase gut contraction.
Long stem fiber (hay) promotes the gut to contract with more vigor than hay pellet or cubes. Long stem fiber increases water intake. An increase in long stem fiber and water intake promotes gut health and integrity. If a horse is not on pasture, then long-stem hay (bale hay) should be at least 50% of the total forage consumed per day. Processed hay (pellets or cubes) should not exceed more than 50% of the total forage consumed per day. Alfalfa hay (bale, cube or pellet) should not exceed more than 50% of the total forage consumed per day. An all alfalfa forage diet usually provides significantly less fiber than traditional grass forage diets. Beet pulp is modest in structural carbohydrate content but the composition of the fiber content provides the benefits of promoting gut integrity. Integrity feeds are beet pulp/soy hull base balanced formulas that also contain oligosaccharides, which are three dietary components that contribute to a healthy gut.
Structural Polysaccharides in plants consumed by horses include cellulose, hemicellulose, pectin and gum. Collectively these cell wall components are called fiber. Fiber content can differ greatly in plants, which provides some of the challenges in feeding horses. Fiber is not digested by the horse’s enzymes. Rather it is microbially digested. The contributions from microbial digestion are influenced by gut design and the design of the horse’s gut is based on digesting plant materials that contain high levels of structural carbohydrates. Some sources of feed that can enhance microbial digestion also include beet pulp, and soy hulls, Timothy, orchardgrass and fescue.
Structural Carbohydrates – Polysaccharides
|hemicellulose||glucose, mannose, galactose, xylose, arabinose|
|pectins||different types of monosaccharides that form a gel|
|gums||mixed; can be found in seed coats/hulls|
Prebiotics can be short or long chain saccharides. The shorter chains are oligosaccharides and the longer chains are polysaccharides. The oligosaccharide, fructooligosaccharide (aka FOS), consists of several fructoses (a monosaccharide) but because it contains multiple units of fructose, FOS has also been listed as a fructan. Perhaps the major goal is to understand that fructooligosaccharide is marketed as prebiotic that nourishes and promotes favorable bacteria that resides in the gut. Also recognize that the potential benefits of prebiotics have not been established in horses.
Structural Carbohydrates – Oligosaccharides
|Oligosaccharides||fructooligosaccharide||several units of fructose|
|mananoligosaccharide||several units of glucose|
|raffinose||3 monosaccharides; glucose, fructose & galactose|
Structural Carbohydrate Requirements
There is not an established dietary requirement for structural carbohydrates but as already noted structural carbohydrates are essential to gut health and integrity and the minimum forage recommendations infer a requirement.
Sources of Structural Carbohydrates
Structural carbohydrates are a source of fiber. Fiber content in horse feeds is traditionally expressed as % crude fiber. The following table provides approximate values for feedstuffs fed to horses that have high levels of structural carbohydrates.
|Feedstuff||% Crude Fiber||% NDF||% ADF|
|Timothy hay - late||29||62||37|
|Orchard grass hay||32||57||34|
|Alfalafa hay - mid||26||42||34|
NDF – Neutral Detergent Fiber • ADF – Acid Detergent fiber
Fiber is the largest component of the horse’s diet yet today much emphasis is written and placed on nutrients that are required in either trace amounts or fed in small quantities. Digestion of structural carbohydrates is dependent on the microbes that reside in horse’s gut and that relationship of the gut’s microorganisms is not always stable. In addition, the digestion of food by the microorganisms that reside in the gut produce a by-product called volatile fatty acids (VFA). This by-product from microbial digestion is an excellent energy source for the horse.
I have always emphasized the importance of nutritional management and fiber or structural carbohydrate intake is the number one consideration in feeding horses. More information on nutritional management of forage intake can be found in Dr. Bray’s Corner fact sheet section in “Feeding Guidelines for Horses”.