Written By Dr. Kris Hiney
Equine disorders related to carbohydrate consumption have received much attention by owners and researchers alike, as of late. This has resulted in almost a mistrust or fear of feeding horses carbohydrates. But in reality, almost all of the horse’s calories come from carbohydrates – there is no way to avoid them in the horse’s diet. What one must do is understand all of the forms in which CHO (carbohydrates) are found, identify horses at risk for CHO disorders and select the appropriate feeds to keep them healthy.
To begin, carbodydrates are simply molecules composed of carbon, hydrogen and water. Monosaccharides are single units of sugars which vary slightly in their structure. Common monosccahrides in the horse’s diet consist of glucose, galactose, fructose, mannose, arbinose and xylose. While these monosaccharides are not normally found in their single form in plants, they are joined together to make longer polysaccharides. However, monosaccharides are produced through enzymatic digestion by the horse. Disaccahrides, then, are just two sugar units linked together. Common disaccharides include lactose (found in mare’s milk and is formed by glucose and galactose linked together) and maltose (two glucose units linked together).
Figure 1. Glucose and galactose. The two structures only differ by the location of the hydroxyl group on the left side of the structure.
Oligosaccharides are longer chains of a variety of monosaccharides linked together, typically between three and ten sugar units. The primary oligosaccharides in the horse's diet are stachyose, raffinose and fructo-oligosaccharides (FOS). FOS have received attention in animal nutrition as a way to supply pre-biotics to the animal. Pre-biotics are often oligosaccharides which are resistant to digestion in the foregut of the horse but are digested by bacteria in the hindgut. These supply a source of nutrition which supports the growth of beneficial bacteria and perhaps reduces the population of disease causing – or "pathogenic" – bacteria. In fact they are looked at as an alternative to feeding antibiotics in livestock. FOS are believed to alter the pH of the colon to a more favorable environment for the most productive bacteria. Mannose specific oligosaccharides are also thought to reduce the adherence of pathogenic bacteria to the epithelium of the gut wall. In yearling horses, feeding FOS reduced fecal pH and increased the production of volatile fatty acids from the hind gut. FOS supplementation also decreased the incidence of diarrhea when fed to foals. It has also been shown to have a protective effect on the development of foal diarrhea when fed to their dams. However, it is not known if that was an indirect effect passed through the milk, or if the foals simply ingested some of their dams' feed containing the supplement. While feeding oligosaccharides does not appear to have an immune boosting effect that has been suggested in other species, it does appear to have beneficial effects on gut health in the equine. Horses receiving FOS and challenged with a large barley meal had less lactobacilli in their colon compared to controls. Thus FOS may help prevent GI disturbances due to diet changes or CHO overload.
Fructooligosaccharides also belong to the category of carbohydrates labeled as fructans. Fructans are polysaccharides which have multiple fructose units. Inulin is also classified as a fructan. Many horse owners have heard of fructans as a risk factor for pasture associated laminitis. A sudden increase in fructans in the diet can alter the microbial population in the hindgut which may then subsequently lead to the development of laminitis. Fructan concentrations in grasses vary with both season and time of day. Fructans and other starch concentrations are highest in the spring, lowest in the summer and intermediate in the fall. During the day, the process of photosynthesis results in the highest concentrations of fructans in the afternoon with sometimes half or less in the morning or evening hours.
Other CHO include longer chains of sugar units and are known as polysaccharides. Most commonly we think of starches and fibers as the common polysaccharides in the equine diet. Starch occurs in either linear form known as amylose or branched form, amylopectin. It is composed of only glucose linked by bonds that can be enzymatically digested by the horse. In contrast, cellulose is also a straight chain of glucose but is linked by a different type of bond , a beta bond, which must be broken by microbes. Fermentation of this fiber fraction results in formation of volatile fatty acids which are metabolized by the horse to produce energy. Pectin and hemicelluloses are also common polysaccharides found in the equine diet.
Figure 2. Amylose is a chain of glucose units linked by alpha bond.
Figure 3. Cellulose is a similar chain of glucose units, but linked by beta bonds instead, making it indigestible by mammals.
Those CHO linked with alpha bonds can be digested in the foregut, allowing the monosaccharides to be absorbed intact. In contrast, cellulose, hemicelluloses, pectin, raffinose and stachyose, which contain beta bonds, will all need to undergo microbial fermentation to provide energy to the horse. Hemicellulose, compared to cellulose, is a mixture of arabinose, xylose, glucose , mannose and galactose. Pectin is made up of beta linked galacturonic acid, arabinose and galactose. Pectin and hemi-cellolose are more rapidly fermented than cellulose and increase the digestibility of the feed if present in a greater proportion.
Now that we know what different types of carbohydrates exist in the horse’s diet, let’s look more closely at some differences that occur in forages. Typically, forages should always make up the bulk of the horse’s diet. They are made up of structural CHO which make up the cell wall as well as some indigestible lignin. The plant cell wall is made of cellulose, hemicelluloses and pectin. Forages also have non-structural CHO or NSC in the cell content, though certainly not as much as concentrates. The NSC is a mixture of monosaccharides (glucose, fructose, etc.) and disaccharides as well as starch and fructans.
If we compare common forages, cool season grasses are made up of primarily cellulose, then hemi-celluose and the fairly small amounts of pectin. Cool season grasses include Kentucky Bluegrass, orchard grass, fescues and ryegrass. Legumes, which are typically high in digestible energy are relatively higher in pectin. Legumes would include alfalfa, clover, lespedeza and peanuts. Warm season grasses grow and mature more rapidly and have much more cell wall/kg DM and thus much more fiber. Warm season grasses include Bermuda grass, switchgrasses, and bluestem. Therefore warm season grasses at a later stage of maturity may be ideal for horses with carbohydrate sensitivities. In general, there is a higher proportion of cell content in a younger, or more immature plant. This makes grasses or hays harvested at an earlier stage more digestible.
Interestingly, the storage form of CHO in legumes and warm season grasses is primarily starch, while cool season grasses prefer to store energy in the form of fructans with much less starch. There is also a limit to how much starch the chloroplasts of warm season grasses and legumes can contain, yet there is no limit to fructan accumulation. Fructan also accumulates more to the base of the plant and more so in the stem than in the leaf. Cool temperatures and droughts (which typically don’t go together) may also increase the fructan production by the plant. Anything that promotes photosynthesis but retards growth ends up increasing NSC (lots of light with cool temperatures). Therefore, be especially careful to observe growing conditions, especially if the horses are consuming cool season grasses and have carbohydrate sensitivities.