Carbohydrates in horse feed

Low Carb and Low Starch have become buzzwords in equine feeds recently. The reasons for needing a feed with these properties usually centre around important equine health and behavioral issues. For this reason we feel it’s important to help you understand the different types of carbohydrates and their essential role in equine diets.

Classification of Carbohydrates

The equine digestive tract is anatomically classified as that of a non-ruminant herbivore. This digestive arrangement allows feedstuffs to be broken down with enzyme digestion in the small intestine and microbial fermentation in the cecum and colon, commonly referred to as the hindgut. Plant material (hay/pasture), cereal grain and commercial grain concentrate, the cornerstones of modern equine diets, consist of a wide array of carbohydrates and may contain up to 75% carbohydrate. However, not all of this carbohydrate is digested or absorbed in the same manner within the equine digestive tract.

Structural Carbohydrates

Structural carbohydrates (fibre) are typically found in the cell wall of the plant and are often referred to as fibre. Analytically the major carbohydrates associated with the cell wall are cellulose, hemicellulose and lignin. Technically, lignin is not a carbohydrate since it is based on alcohol rather than glucose. These carbohydrates are represented on a laboratory analysis reportas Neutral Detergent Fibre (NDF). Baled hay, mature pasture grass, beet pulp and soybean seed coats are good sources of structural (fibrous) carbohydrate. Structural carbohydrates are resistant to enzyme digestion in the small intestine and must be fermented by bacteria in the horse’s hindgut. Bacterial fermentation of fibre yields volatile fatty acids (VFA’s). VFA’s are absorbed from the hindgut and are transported to the liver were they are converted to energy substrates for the horse. The overall digestibility of fibrous carbohydrate is quite variable, depending on the distribution of cellulose, hemicellulose and lignin in the carbohydrate fraction. Since lignin is non-digestible by bacterial fermentation the higher the degree of lignin present the lower the overall digestibility. Thus, as plants mature and increase their lignin content, their digestibility is decreased. The overall digestibility of NDF in good quality forages by horses varies from 40-50 percent.

Non-Structural Carbohydrates (NSC)

Non-Structural Carbohydrates (NSC) are carbohydrates associated with the inner portion of the plant cell, or plant cell contents. Sugar may also be found in intercellular spaces. This sugar can be lost from hay when it is soaked in water prior to feeding. The plant cell includes NSC along with protein, lipids, organic acids and soluble ash. NSC is made up sugars, disaccharides, starches and fructans. In warm season grasses (C4 type plants), starch is the primary storage carbohydrate, whereas in cool season grasses (C3 type plants) fructan is the primary storage carbohydrate. As a practical point, commonly fed legumes such as clover and lucerne do not contain fructan, and store carbohydrate as starch. Enzymes in the horse’s small intestine break down sugars and starch to monosaccharide’s (simple sugars) that are absorbed and circulate in the blood as glucose. Fructans are resistant to mammalian enzyme digestion and must be fermented by bacteria in the horse’s hindgut. Sugar and starch are highly digestible, greater than 95%, within the length of equine digestive tract. Bacteria located in the hindgut ferment any starch or sugar that is not digested by enzymes in the small intestine. Unfortunately, fermentation of sugar, starch and fructan by hindgut microorganisms can produce lactic acidosis that destroys the environment within the hindgut leading to death of the microorganisms and health concerns such as colic and laminitis.

Carbohydrate definitions

WSC – This is an acronym for Water Soluble Carbohydrate. These carbohydrates are extracted from a sample by dissolving them in hot water and then hydrolysing with a strong acid. It consists of simple sugars + all fructans + some glucans and pectin. The simple sugars are digested in the small intestine and influence glycaemic response (blood sugar). The fructans are typically fermented in the large intestine and should not impact blood sugar in horses, although research is lacking.

ESC – This is an acronym for Ethanol Soluble Carbohydrate. These carbohydrates are extracted from a sample in an 80% ethanol solution. This carbohydrate fraction is a subset of the WSC number and represents the sugar fractions that directly influence blood sugar, and the most rapidly fermentable fructans. It consists of simple sugars and short-chain length fructans.

Starch – A polysaccharide composed on repeating glucose units. Starch is found mainly in grains and is typically digested in the small intestine. Starch is also found in small amounts in many types of hay.

Fructans – carbohydrates that are made of fructose chains, sometimes with glucose molecules included. These can vary in the number of fructose molecules (chain length). Fructose is considered to be a complex carbohydrate that requires fermentation to be digested. Thus, fructans are thought to be digested in the large intestine. This issue requires some research to clarify the digestive process in horses. Fructans are rarely analysed separately in a feed sample since they are completely included in the WSC fraction and partially included in the ESC fraction.