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Carbohydrates are the main energy source in an equine diet. These macronutrients are naturally abundant in forages, grains and grain by-products.
Carbohydrates can be categorized based on their function in the plant and how they are digested by the horse.
Simple sugars and carbohydrates found within plant cells are digested in the foregut. Structural carbohydrates found in the cell wall of the plant are fermented by microbes in the hindgut.
The horse’s digestive tract is better suited for diets high in structural carbohydrates. These are indicated on a guaranteed analysis or forage report as neutral detergent fibre (NDF), acid detergent fibre (ADF) or crude fibre.
Diets high in starch and sugars are not typically recommended for horses because they can contribute to health issues such as insulin resistance, obesity, laminitis and hindgut acidosis.
Sugars are listed in a feed analysis as water-soluble carbohydrates (WSC) or ethanol-soluble carbohydrates (ESC). The calculated values of non-fibre carbohydrates (NFC) and non-structural carbohydrates (NSC) are also used to indicate carbohydrate levels.
This article will discuss carbohydrate sources in the equine diet, how they are digested and metabolized to impact equine health and how they are defined on a guaranteed analysis or forage analysis.
Carbohydrates in the Equine Diet
According to the Nutrient Requirements of Horses (2007), carbohydrates can be divided into three categories that reflect how they are broken down in the horse’s digestive tract. [1]
1) Hydrolyzable Carbohydrates
These are carbohydrates that are digested in the small intestine – the first section of the intestine after the stomach. Enzymes produced by cells of the intestine or from the pancreas break down these molecules to allow their absorption.
Simple sugars and starch are the most abundant forms of hydrolyzable carbohydrates in the equine diet. These are higher in grains than forages.
Starch is a long chain of hundreds or thousands of glucose molecules. Starch can exist as amylose (a linear chain of glucose molecules) or amylopectin (a branched structure).
Simple sugars (monosaccharides) and compounds of 2 sugars bound together (disaccharides) include:
- Glucose: A monosaccharide that triggers insulin secretion and is a primary energy source for most cells of the body
- Fructose: A monosaccharide that can be used for energy in the body or converted into fat
- Sucrose: A disaccharide of glucose and fructose
- Maltose: A disaccharide of two glucose molecules; this is a product of starch digestion in the small intestine
- Lactose: A disaccharide of glucose and galactose. this is the main sugar found in milk and is an important energy source for foals
Monosaccharides and disaccharides are low in forage-based diets. However, they can be linked together to form longer chains, such as starch and oligosaccharides.
Oligosaccharides consist of short chains of 3 – 10 sugars. Some examples of oligosaccharides in feed include fructooligosaccharides, raffinose and stachyose.
Depending on the type of bond between subsequent sugar molecules, the enzymes of the small intestine may or may not be able to break these into shorter chains or monosaccharides.
2) Rapidly Fermentable Carbohydrates
Besides starch, most complex carbohydrates cannot be broken down by the enzymes in the small intestine of the horse.
This is because the enzymes the horse produces can only digest alpha linkages, such as those found in starch. They cannot break the beta linkages found in other complex carbohydrates.
Instead, complex carbohydrates are broken down (fermented) by microbes in the hindgut.
Depending on their structure, some complex carbohydrates are rapidly fermented by microbes, including:
- Resistant starch: includes starch that escapes digestion in the small intestine due to starch overload or because it is not accessible to animal enzymes
- Oligosaccharides: short chains that are not digested in the small intestine
- Pectins, gums and mucilages: soluble fibre found in the cell wall of fruits, sugar beets, grasses and legumes
- Fructans: short chains of fructose that act as an energy storage form in plants, especially cool-season grasses
These complex carbohydrates are easily fermented by the microbes of the hindgut. This process produces volatile fatty acids (VFAs, also known as short-chain fatty acids) and lactic acid. The VFAs are absorbed and used for energy by the horse.
Excess lactic acid production in the hindgut can be detrimental. For example, excess starch reaching the hindgut rapidly increases lactic acid, which reduces pH (making the environment more acidic).
This disrupts the microbial populations and can lead to health issues, including hindgut acidosis, colic, and laminitis. [2][3]
3) Slowly Fermentable Carbohydrates
Structural carbohydrates that give plants their rigid structure are commonly referred to as fibre. These have a more complex structure and require more time for microbial fermentation.
The most abundant slowly fermentable carbohydrates in forages are: [1][4]
- Cellulose: a non-branched structure of glucose molecules organized into a microfibril structure
- Hemicellulose: a branched structure of various sugars
Fermentation of cellulose and hemicellulose produces volatile fatty acids, which are beneficial to other microbes in the hindgut and the horse. VFAs provide 30% to 70% of the horse’s energy needs. [5][6]
The three VFAs produced by hindgut fermentation are: [7]
- Acetate: a two-carbon structure that is metabolized in the liver to make fat to store energy in the body
- Propionate: a three-carbon structure that is metabolized in the liver to make glucose as an energy source for the body
- Butyrate: a four-carbon structure that is mostly used as an energy source by cells of the gastrointestinal tract
Nutrition: Carbohydrate Fractions
Feed or forage analysis is the only way to estimate how much of each carbohydrate your horse gets in their diet.
Laboratory methods to conduct these analyses are traditional wet chemistry or near-infrared spectroscopy. Wet chemistry is more expensive and time-consuming but is generally considered more accurate. [8]
The names given to carbohydrate fractions are based on wet chemistry procedures established by Van Soest in the 1960s and are still widely used today. [9] These fractions can be used to estimate the amount of hydrolyzable, rapidly fermented, and slowly fermented carbohydrates in the diet.
Below are descriptions of the main carbohydrate fractions reported on forage and feed analyses.
Neutral-Detergent Fibre (NDF)
What remains after the sample is dissolved in neutral detergent and filtered is called the neutral detergent fibre (NDF). The major components of NDF are:
- Hemicellulose
- Cellulose
- Lignin
Lignin is a structural carbohydrate that is neither digestible by animal enzymes in the small intestine nor by microbes in the hindgut. Some laboratories and feed tags also report the lignin content as a separate value.
NDF can be used to estimate forage intake in horses. As NDF content increases, voluntary intake tends to decrease, meaning horses given unrestricted access will consume less of the forage. [10]