Digestive Physiology - CRF Horse Nutrition Guide
Digestive Physiology
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The digestive tract of the horse consists of the mouth, esophagus, stomach, small intestine, cecum, small colon, large colon, rectum and anus. Each segment of the horse’s digestive tract performs specific functions that are important in the digestion and absorption of nutrients as well as to the general well-being of your horse.
Food is ground in the mouth by cross action of the jaws. Saliva, which aids in moistening the food for mastication and helps food move down the esophagus to the stomach, is secreted into the mouth. There are three salivary glands in the horse’s mouth, which can produce up to ten gallons of saliva daily. Saliva contains bicarbonate, which buffers or raises the pH of the stomach contents, providing protection from ulcer formation. Saliva also contains a small amount of the enzyme amylase; this enzyme breaks down starch and complex sugars into simple sugars for absorption in the small intestine.
The esophagus is a simple muscular tube that is between four and five feet long in the mature horse. Its function is to convey food from the mouth to the stomach. When a large piece of an improperly chewed feedstuff lodges in the horse’s esophagus, it is called a choke.
The equine stomach is a weakly muscled organ and is used primarily as a holding vat. The capacity of the stomach depends on the size of the horse, but generally the range is from eight to 16 quarts. Hydrochloric acid and pepsin are secreted into the stomach; both initiate the breakdown or digestion of protein.
Approximately 70 percent of the feed leaves the stomach within eight hours after it is eaten; the stomach empties completely within twelve hours after eating. However, large grain meals result in more rapid gastric emptying and food can begin leaving the stomach in as quickly as fifteen minutes after ingestion.
Because of the stomach’s relatively small capacity and the rate at which food passes through it and the small intestine, it is advantageous to feed the working horse more than twice a day.
The small intestine is approximately 70 feet in length. It is divided in to three major segments – the duodenum, jejunum and ileum. Most digestion of sugars and starches from cereal grains such as oats, barley and corn occur in the first portions of the small intestine. The protein from cereal grains as well as that from protein supplements, such as soybean meal, flaxseed meal and cottonseed meal is also digested here. Vitamins and minerals from cereal grains, protein sources and vitamin/mineral supplements are also broken down in this part of the tract. Specific enzymes break down starch, sugar, fat and protein to smaller constituents, which are then absorbed across the intestinal wall in to the blood or lymph (fat) in the latter portion of the small intestine, along with the vitamins and minerals.
High starch/sugar feeds cause dramatic increase in blood glucose (glycemic response), causing dramatic increase in blood insulin levels and numerous adverse effects. Often the capacity of the small intestine to process sugar and starch is surpassed, resulting in sugar and starch passing into the cecum and large intestine, where the fermentation can lead to diarrhea, shifts in the microbial population and pH changes which can cause colic and laminitis.
The rate of flow in the small intestine is related to intake level of feed (large meal faster than small meal) and the physical form of feed (pellets faster than coarse grains and forage).
The cecum, or uppermost segment of the horse’s large intestine, has a capacity of eight to ten gallons. It is here that microbial digestion, where bacteria and protozoa break down fibrous feedstuffs through a fermentation process, begins.
Microbial digestion continues in the large colon, which comprises right and left ventral colons and the dorsal colon. The ventral colons have a sacculated construction, which means they are made of a series of pouches. These structures are designed to efficiently digest large amounts of fibrous materials, but these pouches can fill with gas and become twisted during the fermentation process if something upsets its proper function. The result can be a serious case of “twisted gut” or strangulation colic.
The microbial population of bacteria and protozoa become specific for digestion of the type of feed ingredients that the horse regularly ingests. Therefore, it is very important to change a horse’s diet gradually in order to allow the microbes sufficient time to adjust to new dietary ingredients, or a digestive upset or colic can occur.
The cecum and large colon are important areas for transforming feed into energy for the horse. In both areas, large quantities of volatile fatty acids (acetic, propionic, butyric) are produced by bacterial action on carbohydrates. It is estimated that 30 percent or more of the energy requirement is provided by these volatile fatty acids for a horse on a typical hay/grain diet.
Bacteria and protozoa in these organs also create B-complex vitamins during the fermentation process, supplying some 30-40 percent of the horse’s requirements for these important nutrients. B-complex vitamins and some minerals are absorbed in the large colon. Protein that enters the lower digestive tract is broken down to ammonia and volatile fatty acids which are used to make microbial protein. There is no conclusive evidence to indicate that microbial protein is available to the horse.
The small colon is mainly the site of water absorption and formation of fecal balls or manure that are then passed out through the rectum and anus, completing the digestion process.
The amount of feed required for maintenance, growth and reproduction depends on how digestible it is. Processed cereal grains and protein concentrates such as soybean meal have the highest digestibility followed by good quality forages and then poor quality forages. However, the quality of the feed is more important than the physical form. In fact, CRF research shows there is no difference in the utilization of alfalfa hay when fed as long hay or pellets. It usually costs more to feed a poor-quality diet than one of higher quality because the horse receives fewer usable nutrients from the lesser- quality feed. In addition poor quality grain may cause digestive upsets and other problems due to the presence of mycotoxins (mold toxins), and poor-quality forages have a greater risk of causing impaction colic.
Horses are susceptible to a variety of ailments, including colic, if they ingest toxic materials. The reason is a basic difference between the horse and ruminant animals, like cattle and sheep. With ruminants, food is passed from the esophagus into the rumen, where microbial activity can detoxify many toxins before they reach the small intestine for absorption. With the horse, the toxic material reaches the small intestine and is absorbed into the bloodstream before it can be inactivated by microbial action in the large intestine.