Equine Digestive Tract Structure and Function
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A horse has the same requirements for energy, protein, vitamins and minerals as other animals but differs in the type and function of its digestive system, falling between a ruminant and non ruminant.
Non ruminants (humans, pigs and dogs) digest carbohydrates, protein and fat by enzymatic action. Ruminants (cattle, sheep and deer) use bacteria in the fore stomachs to digest fiber by fermentation and use enzymatic digestion in the small intestines.
In the horse, all true digestion is by enzymatic digestion and takes place in the fore gut ahead of the cecum. This accounts for 52-58% of the crude protein digestion and virtually all soluble carbohydrate digestion (fiber excluded). (1) In addition, bacterial or microbial digestion of fibre occurs in the cecum and colon where large quantities of volatile fatty acids are produced through fermentation and are subsequently absorbed. This dual system allows the horse to digest simple carbohydrate sources such as starch from grain in the fore gut. Fibrous sources such as oat hulls, soy hulls, beet pulp, hay and pasture are digested in the hind gut.
For enzymatic and microbial action to digest feed efficiently, the horse needs healthy teeth to grind feed and allow enzymes and bacteria to attack the plant cell walls. Teeth should be examined during the annual health check to ensure that they are wearing normally and are effectively grinding the feed.
The capacity of the stomach of the horse is only about 8-15 litres (eight quarts or two gallons), which makes it difficult to understand how a horse can consume large amounts of food or water. The emptying time of the stomach after filling can be about 12 minutes, and the rate of passage down the small intestine is about 1 ft/min. The net result is that food can go from the mouth to the cecum in about 1½ hours. The small volume of the stomach and rapid passage of food from the stomach is the reason horses eat almost continuously, thus the name "hay burners."
The rate of passage of pelleted or wafered hay is faster than for loose hay. Various studies on total passage time, indicate two to three days is required from when food is ingested until it is passed in the manure. The mature horse's large intestine makes up more than half of the total volume of the digestive tract, is important for microbial digestion of food and is a major reservoir for water.
The foal and growing horse have undeveloped cecal and colonic digestion as compared to the adult horse. There is very little microbial digestion before three months of age. Therefore, the foal requires a diet low in fibre and easily digested in the fore gut. Foals who are seen eating their mothers manure are thought to be obtaining a bacterial culture necessary for future microbial digestion.
In the mature horse, the exit and entrance to the cecum (blind gut with a capacity of about 28-36 litres or approximately 7-9 gallons) are separated by only about 2 inches. This creates a certain amount of difficulty due to the two-way movement of feed in this region. As a result, the cecum can be a site of colic, which may develop when a horse is shifted from a poor quality ration to one which is rapidly digested.
Bacteria which are best at digesting high fibre type diets will be replaced with a population of bacteria which are more suited to convert high quality and easily digested fibre to soluble products. As easily digested hay enters the cecum, microbial populations flourish and there is an increase in fermentation rate. The coarse roughage will cause a relative occlusion at the exit of the cecum and will result in gas accumulation and the pain associated with colic. This is the principle behind the recommendation that "a horse should be shifted from one type of feed to another over a period of one to two weeks."
Impaction of the cecum and colon is common and results from the ingestion of poorly digestible material. This, coupled with inadequate water intake or exercise, will predispose horses to impaction. To prevent impactions, a similar one to two weeks should be allowed for conversion from a high quality, easily digested, ration to one of low digestibility. This allows time for the gastrointestinal micro flora to adjust to new substrates. Very low quality roughages should not be fed since they predispose to impactions.
Young lush fast-growing plants are low in fibre, can contain 22-25% protein and equally large amounts of highly digestible carbohydrates. Sudden switches between different feed qualities result in rapid changes to the bacterial flora and the sudden death of the less favoured bacteria. Death of large numbers of bacteria will, in turn, lead to the release of large amounts of endotoxin from the bacteria. Horses placed directly on pasture after being fed hay all winter may develop laminitis due to the sudden shift to a highly digestible feed. Grass founder or laminitis is the sequella. Climatic conditions, such as high rain fall and hot weather, can also result in the rapid growth of plants such as white clover which are high in digestible protein and energy and low in fibre.
Horses normally consume 2 to 2.5% of their body weight in dry matter daily. However, it has been estimated that horses on pasture 24 hours a day, can consume up to 3.3% of their body weight in dry matter. As the dry matter and nutrient proportion derived from grain increases, horses will reduce their total dry matter intake. In addition, abrupt incorporation of grain into a horse's diet will increase the lactate concentration and lower the cecal and colonic pH from 6.7 to 6.3. This was evident when the grain content of the ration was greater than 30% of the dry matter intake. (2) Abrupt feeding changes, such as giving horses an extra proportion of grain prior to an event, can result in significant accumulation of feed related lactic acid.
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