Carbohydrates in Forages

By J. Kathleen Young, Ph.D

Due to the incidence of several diseases that have been associated with "carbohydrate intolerance" in horses, there has been increased attention paid to the amount of carbohydrates in the diet. However, there appear to be some misconceptions in the horse industry regarding carbohydrates in forages.

Carbohydrates include sugars, starches and fibers, all of which are found in plants, including grains, pasture and hay. Dietary carbohydrates include those carbohydrates found as components of the cell walls of plants, or structural carbohydrates, and those found within the cell contents, or nonstructural carbohydrates (NSC). Sugars and starches (hydrolyzable carbohydrates) fall into the NSC category, while dietary fibers are considered structural carbohydrates.

Simple sugars are usually only a minimal presence in horse's diet, even when the horse is eating a sweet feed with molasses. In plants, simple sugars are utilized as fuel for growth or respiration; when excess sugars accumulate they are stored as polysaccharides. In the horse, simple sugars are primarily digested and absorbed in the small intestine. Starches are polysaccharides formed of several simple sugars linked together, and are found primarily in grains and in the immature leafy portions of plants. During digestion, starches are broken down into the simple sugar building blocks (primarily glucose), which are then absorbed primarily in the small intestine. Glucose may then be used as fuel immediately, stored as glycogen by the horse, which is usually the major fuel source for aerobic and anaerobic activity, or stored as fat, which is also a fuel source for aerobic activity. NSC that passes through the small intestine undigested will be fermented in the hindgut. If starch intake exceeds 0.4% of the horse’s body weight per feeding, the amount of starch digestion in the horse’s hind gut increases considerably. Starches that are fermented in the hind gut will not be broken down into glucose molecules, but instead will result in volatile fatty acids (VFA’s), which will then be absorbed.

Fibers are also formed by the linkages of simple sugars (again primarily glucose), but the linkages are different from those in starches, so fibers are digested differently. Digestible fibers (including hemicellulose and cellulose) are fermented by the microbes in the horse's hindgut into VFA’s, which are then absorbed. These VFA's are also a source of energy for aerobic activity. Some fibers (lignin) are indigestible in the horse's digestive system, and pass on through, providing bulk in the diet.

Nonfiber Carbohydrates (NFC) in Forages

The category of nonfiber carbohydrates includes nonstructural carbohydrates and also rapidly fermentable carbohydrates such as fructans that are not included in the structural carbohydrate grouping.

During the process of photosynthesis, plants manufacture sugars that are either used for metabolic processes such as growth, or are stored as polysaccharides such as starches or fructans. The storage form of the sugars depends on the plant species. For most plants, the principal storage polysaccharide is starch, however for a significant number of species (primarily the cool season grasses) much of the excess sugars are stored as fructans. In certain situations, such as the warm sunny days and chilly nights that we see in the spring and the fall, the plants use fewer sugars for growth, and therefore store more as polysaccharides. In one study, hydrolyzable carbohydrate content of pastures peaked in April and November, and was lowest in June and September.

Other environmental conditions that can affect the amount of polysaccharide storage in plants include drought stress, duration and intensity of sunlight, salinity (salt content) of soil, and overall health of the plant. Overgrazed pastures also may have higher NFC content. Many grass species store NFC in the stem base, allowing the plants to maintain reserves for growth after grazing animals have removed the upper portions.

Some suggested strategies for managing grazing for horses that appear to be at risk for laminitis and/or colic include:

• Utilizing a grazing muzzle to reduce intake of pasture
• Choose grasses that tend to be lower in NFC
• Limit grazing time to when plants tend to be lowest in NFC (approximately 3:00 am to 10:00 am)
• Limit or eliminate grazing when grass is stressed due to cool temperatures
• Do not allow horses to overgraze pastures
• Maintain a system of rotational grazing to minimize grazing of grasses that are starting to head out (seed heads tend to be higher in NFC)

Suggestions for choosing hay lower in NFC include:

• Choose hay that was cut at a later stage of maturity
• Avoid hay that was cut under conditions stressful to the plant
• Utilize warm season grass hays (do not contain fructans)
• Utilize hay that has been allowed to dry slowly, as cut plants will continue to metabolize sugars until moisture content is low

Soaking hay before feeding has been suggested for high risk horses. Simple sugars and fructans are soluble in water, so soaking in cold water for at least 60 minutes or hot water for at least 30 minutes appears to reduce those carbohydrate fractions. Starch is not as water soluble, so soaking hay has little effect on the starch concentration of hay.

An important aspect to keep in mind when choosing and recommending hays for horses is that the factors that tend to reduce NFC in hays also tend to reduce the amount and availability of other nutrients. For instance, lower NFC hay tends to be higher in indigestible fiber, thus protein will less available for digestion in the upper gut. It is therefore essential to offer another source of nutrients to ensure that the horse receives adequate protein, vitamins and minerals to meet nutritional demands.

References

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Freeman, D.W., and L. Rommann. Use of forages for horses. In: Texas Horse Owner’s Reference Guide. Texas A&M University, Equine Sciences Program, Department of Animal Science, College Station, TX.

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