Basic Understanding of the Horse Digestive System

Digestion is the process where feedstuffs that the horse consumes is broken down and converted to its simplest form so nutrients can be absorbed into the bloodstream. These nutrients provide fats, carbohydrates, vitamins, minerals, and amino acids (protein) required for growth and maintenance, or they can be stored until needed later. The digestive process in the horse primarily involved enzymatic action and fermentation. For you to feed your horse to its full potential, a basic knowledge of the digestive system is important. This information is based on a typical 1000-pound horse, but would be accurate for horses of any size and breed from miniature horses up to the draft breeds. Only the size and capacities of the various parts would differ.

A horse is classified and a non-ruminate herbivore and differs substantially from both humans and ruminates, such as cattle. The digestive system uses both high levels of enzymatic action in the small intestine and high rates of microbial fermentation in the large intestine. A horse functions best by grazing, eating small amounts of roughage products over an extended period of time. In fact, research has shown that pastured horses will spend about 70% of their time eating. The other 30% will be spent sleeping and socializing.

The digestive tract starts at the mouth and continues through the pharynx, esophagus, stomach, small intestine, cecum, large colon, small colon and ends at the rectum. Associated organs that aid in the total digestive process are the salivary glands, liver, and pancreas. The total digestive tract length is about 100 feet long that abruptly changes in diameter and is lined by mucous membranes along the way mostly excreting digestive fluids. The digestive tract also requires movement by the horse to help the various muscles mix and move digestive tract material. Research has shown that stall bound horses without much freedom of movement are more prone to impaction colic.

The digestive process begins in the mouth where teeth reduce the particle size. The lips are extremely nimble and can select specific plants, small particles of feed, and sort stones and pebbles from what they graze and eat. The incisors sheer the plant stalk and the molars grind to the appropriate size. The chewing process stimulates the flow of saliva, which lubricates the feed prior to swallowing. Geriatric horses who have depressed saliva glands or horses who are aggressive eaters and often do not chew their food long enough are prone to choke. Steps such as wetting the feed or slowing down consumption need to be considered with these horses. If a horse does choke, the esophagus and trachea are two separate tubes down the throat, so suffocation is not an immediate threat.

Once the horse swallows, the feed enters a simple stomach that is relatively small (8 to 19 quarts) and has an acidy pH. Stomach retention time is relatively small before passing into the small intestine. The small intestine is about 70 feet long, relatively small in diameter, and holds about 65 to 70 quarts. The pH in the small intestine and the rest of the digestive system is pretty neutral. The stomach and small intestine are primarily responsible for digesting and absorbing most of the starch, protein, fat, fat-soluble vitamins (A, D, and E), and most of the minerals. The starch (soluble carbohydrate) absorbed here is converted into glucose and used as energy or stored as glycogen to be used later.

Material not digested in the stomach or small intestine pass into the cecum, large colon, and small colon know collectively as the large intestine or hindgut. The large intestine is very large and holds from 21 to 24 gallons of liquid and feed material. This is the primary site for fiber digestion. Since the horse does not have any natural enzymes for digesting plant fiber, the large intestine contains billions of bacteria responsible for fermenting the fiber into energy (volatile fatty acids) and micronutrients B vitamins and Vitamin K) the horse can use. One of the primary volatile fatty acids is proprionic acid, which is also commonly used as a preservative in feed to prevent mold. The large intestine is also the primary site for phosphorus absorption, important for skeletal growth, muscle contraction, and energy utilization.

The large intestine is actually the main engine of the horse and is essential to the overall health of the animal. Without maintaining a healthy hindgut, significant problems such as colic and laminitis can easily occur. Also, since the large intestine is responsible for synthesizing and absorbing B vitamins, including biotin, the health of the hindgut has a big influence on things you would not normally attribute to fiber digestion such as appetite stimulation and hoof and hair quality. Things you can do to help maintain hindgut integrity are:

• Do not overfeed grain. Excess grain that does not get digested in the small intestine passes into the large intestine and ferments extremely well. The excess fermentation causes changes in the pH and excess gas. Both can dramatically increase to potential for colic or laminitis (founder). A good rule of thumb is not to feed more than 0.5% of body weight of grain in a single feeding.
• Make sure clean, fresh water is available at all times at the right temperature. Dirty water or water that is too cold in the winter or too hot in the summer will inhibit consumption. The large intestine also serves as a large reservoir providing a reserve of electrolytes and excess water essential for cooling the body to sustain exercise. A lack of water will reduce the water required in the reservoir and will inhibit fermentation and material flow.
• Fiber quality and quantity are primary in any horse’s diet. Poor hay quality will cause loss of weight and hay bellies. Lack of quantity will cause an imbalance between grain and fiber and increases the potential of colic or laminitis. Another good rule of thumb is to make sure that the fiber portion of the diet is always a minimum of 50% of the total or diet or a minimum of 1% of body weight.
• Reduce rapid fiber changes. Everyone knows not to change the type of grain overnight, but the same rule should also be followed with fiber. Ease into a new cutting or delivery of hay and gradually introduce the horse to new spring or different pastures. Early spring pastures can have as much soluble carbohydrates and grain.
• Allow adequate turnout time. The same research that has proven that rapid fiber changes are a leading cause of colic also indicates that lack of proper turnout is also a leading cause. The horse functions best by grazing, eating small amounts of roughage over an extended period of time and moving casually which aids the digestive tract muscles to move the roughage through the digestive system and expel unused fiber.

Some indications that the hindgut is not functioning very well or that other feeding options need to be considered are:

• Hay belly – indicates that poor quality forage is being provided. The large intestine will retain poor quality forages longer trying to get as much nutrition as possible. That will stretch the large intestine causing the hay belly appearance. Good quality forage will shrink the hindgut back to normal size.
• Cow flops rather than road apples. Manure that more resembles cow manure often indicates that the fermentation in the large intestine is not functioning optimally. This often occurs with rapid fiber changes, excess grain, or just poor intestinal health often associated with age. Do not overlook the potential for diseases or illnesses required the attention of a vet.
• Poor hair coat or hoof condition. Typically it would accompany any one of the problems listed above as well as gastric ulcers.

Research and practical experience has taught us that the healthiest horses are those that are allowed to be horses by feeding on quality fiber on a continual basis. However, outside influences such as pasture availability, energy and work requirements, owner life styles, and etc. have an influence on the digestive system of the horse that are not always positive. We know a lot more today about feeding horses in the 21^st century environment. Feeding for life styles, age and living conditions are now considered when designing feeds. Even forage alternatives and improved high fiber feeds are on the forefront of equine nutrition.

Horses, like people, are individuals. Each has their own metabolic rate and some are easy keepers while others are a bit more of a challenge. By understanding how the internal process works, we can better understand how to feed them to their maximum potential and limit feeding related problems.