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Fiber can supply a horse with 30-70% of its digestible energy requirements. Some feedstuffs are considered “super fibers,” as they provide greater levels of digestible energy than forages (due to their superior digestibility), with slightly less energy than cereal grains such as oats and barley. Soybean hulls are one of the most commonly fed super fibers in the United States.

Soybean hulls are high in pectin and other soluble fibers. Because they are digested mostly in the cecum and contain relatively small amounts of starch, their use in equine diets does not pose a high risk for colic and laminitis. The purpose of one study in 2004 was to evaluate soybean hulls as an alternate fiber source, and to determine the effect, if any, of different levels of inclusion in the horse’s diet.

How was the study conducted?

Four mature Quarter Horse geldings were used in the study. All horses had four hours of free exercise each day and were housed in stalls the remainder of the time. Horses had constant access to water and trace mineral salt blocks. Body weight was measured once a week.

The horses were given alfalfa/bromegrass hay as a fiber source with 0, 25, 50, or 75% replacement of the hay with unpelleted soybean hulls. Each horse was given one dietary treatment for 21 days, and all horses experienced all diets during the course of the study. Analysis showed that chemical composition of the soybean hulls was comparable to the hay being fed, although the soybean hulls had 70% less lignin than the hay.

The horses were cannulated so that cecal samples could be taken on the first and seventeenth days of each treatment period. The samples were taken about 90 minutes after the morning feeding and were analyzed for pH, ammonia, volatile fatty acid concentration, NDF (neutral detergent fiber; lignin + cellulose + hemicellulose), ADF (acid detergent fiber; lignin + cellulose), cellulose, and hemicellulose. Total fecal collection was done the last three days of each treatment period, and material was analyzed for dry matter (DM), protein, NDF, ADF, cellulose, and hemicellulose. Because of the study design, the authors reported the effect as cecal only; however, it is likely that the results would apply to the entire hindgut.

What results were found?

Palatability was good throughout the study, although a slight decrease was seen at higher inclusions of soybean hulls. In spite of the fact that unpelleted soybean hulls tend to be somewhat dusty, no respiratory problems were noted. There were no gastrointestinal health issues throughout the study. Digestibilities of DM, NDF, ADF, cellulose, and hemicellulose were not different with regard to level of soybean hull inclusion.

There was an increase in total cecal volatile fatty acid production as inclusion increased, indicating that soybean hulls are readily fermentable in the cecum. The horse uses volatile fatty acids to meet its energy requirement.

Digestion of nitrogen decreased somewhat at higher inclusion levels, probably because the cecum is beyond the site of protein digestion. Microbial nitrogen, while abundant in the cecum, is carried out of the body with the feces. This was verified by the fact that fecal nitrogen content increased with greater soybean hull inclusion.

Higher levels of soybean hull inclusion were associated with a change in proportion of propionate to butyrate, suggesting changes in microbial populations, pH, and available substrate in the cecum. A higher level of propionate favors gluconeogenesis, a process by which an animal can form glucose from noncarbohydrate molecules. An additional benefit is a drop in the amount of energy lost as gas during the fermentation process.

There was a small drop in cecal pH with increased inclusion of soybean hulls, but the decrease did not reach levels of either clinical or subclinical acidosis.

What does this study tell us about feeding horses?

The results of this study indicate that soybean hulls may be an acceptable replacement for up to 75% of total forage in diets for horses. This feedstuff, which is often readily available and economical, may find a use in areas where hay is unavailable due to drought or other climatic conditions. With a chemical composition close to that of medium-quality alfalfa/bromegrass hay, soybean hulls are suitable to replace part of the forage component in equine diets, at least in short-term situations.

NOTE: To avoid digestive upset, any change to a horse’s diet should be made slowly over a period of several days. The recommended method of introducing a new dietary component is to include a small amount in the first feeding and continue to increase the proportion of new feed to old feed for four or five days until the desired amount is being fed.

The full text of this article can be found in the Journal of Animal Science, June 2004, pages 1663 to 1668. The study “Soybean hulls as an alternative feed for horses,” was conducted by J.A. Coverdale, J.A. Moore, H.D. Tyler, and P.A. Miller-Auwerda.

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