Synopsis of The Digestive Anatomy and Physiology of the Horse

Horses are non-ruminant herbivores, meaning they eat mainly plant material. The fibrous portion of the plant material ingested through fermentation in the horse’s hindgut (cecum and colon). The horse’s foregut is comprised of the mouth, esophagus, stomach and small intestine, which has responsibility for digestion and absorption of most non-fiber nutrients.

The digestive tract of the horse is designed to process small meals, obtained by eating most of the date, as in a natural grazing situation. Many domesticated horse’s intake is restricted to two meals a day. This practice is the basis of many digestive issues we see in our equine population.

Described below is a synopsis of the functions of the main segments of the horse’s digestive tract and potential problems related to the tract’s anatomy and functions as they relate to modern feeding practices.  See the image below for the anatomy of the digestive tract.

Anatomy of the Horse. Photo credit: stock.adobe.com

Anatomical Parts of the Equine Digestive Tract

Mouth

1. Anatomical Features: teeth, tongue, and salivary glands.
2. Main Functions:
   1. The process of digestion begins as soon as food enters the mouth.
   2. Chewing includes prehension (incisors grasp feed) and mastication (molars grind feed).
   3. Chewing grinds food into smaller particles and mixes it with saliva to begin the digestive process.
   4. Horses secrete large amounts of saliva, only when the horse is chewing. Saliva lubricates the bolus of feed for easier passage through the esophagus, helps buffer the acid in the stomach, and produces amylase, which begins the breakdown of carbohydrates.
   5. Forage promotes increased production of saliva. Forage should be fed at >1% of the horse’s body weight.
3. Related Issues:
   1. Teeth will develop sharp points over time. Routine dental care will enhance digestion by improving mastication.

Esophagus

1. Anatomical Features: 4-5 feet long
2. Main Functions: Transports the bolus of feed by one wave peristaltic waves from the mouth to the stomach.
3. Related Issues:
   1. Choke occurs when feed becomes lodged in the esophagus. Choke is most common in greedy eaters (bolting) and those with dental issues. Choke may require veterinary intervention.
   2. Owners can slow feed intake for greedy eaters by placing large stones in the feeder, mixing some forage with concentrate and wetting the hay to soften it.

Stomach

1. Anatomical Features:
   1. The stomach comprises approximately 10% of the total digestive capacity.
   2. 2-4 gallons in capacity. The horse has the smallest stomach in relation to body size of all domestic animals.
   3. The rate of passage is variable depending on the meal size. Retention of feed is normally less than two hours. Larger meals reduce residence time
2.  Main Functions:
   1. Stores feed
   2. Regulates passage of feed into the small intestine
   3. Secretion of hydrochloric acid and pepsin contributes to the chemical breakdown of food. Secretion of pepsin begins protein digestion,
   4. Very little absorption of nutrients occurs in the stomach.
3. Related Issues:
   1. The small capacity is not conducive to large, infrequent meals. Feeding large meals can lead to gastric distension and colic.
   2. Secretion of gastric acid continues for many hours even when the stomach is empty, which can contribute to development of ulcers.
   3. Microbial population will rapidly ferment dietary sugar and starch to lactic acid, which can also contribute to ulcer development.  

Small Intestine

1. Anatomical Features:
   1. The small intestine comprises approximately 28% of the total digestive tract.
   2. Approximately 70 feet in length and consists of  the duodenum, jejunum and ileum. Accessory organs include the pancreas and liver.
   3. Food can pass through the small intestine in as little as 30 to 90 minutes, but most require 3 to 4 hours.
2. Main Functions:
   1. Primary site of the digestion of NSC (starch and sugars), proteins and fats (oils) via enzymes from the pancreas and intestinal lining. Fat-soluble vitamins (A, D, E and K), calcium and phosphorus are also absorbed in the small intestine.
   2. Enzyme action for fat metabolism facilitated by bile from the liver (horse does not have a gall bladder)
   3. Some microbial activity occurs on disaccharides and polysaccharides
   4. Absorption of end products of digestion are:
      1. NSC – absorbed primarily as simple sugars – high levels result in significant increases of blood glucose and insulin.
      2. Protein – absorbed as amino acids.
      3. Fats (oils) – absorbed as fatty acids and glycerol.
      4. Vitamins/Minerals – absorbed as individual vitamin and mineral complexes, including chelates (organic minerals).
3. Related Issues:
   1. Enzyme secretion adapts to the diet
   2. Volume of feed consumed and rate of passage affect digestion and absorption of nutrients – larger volume will increase the  rate of passage and will decrease digestion and absorption
   3. Numerous turns and folds increase risk of impaction

Hindgut (Cecum/Large Colon/Small Colon)

1. Anatomical Features:
   1. Comprised of the cecum, large colon, and small colon
   2. The cecum is 12-15% of total tract capacity. Feed remains in the cecum for about 7 hours.
   3. The large and small colon is 40-50% of total tract capacity. Feed may reach the large colon in as little as seven hours post-feeding and will stay here for 48 to 65 hours.
   4. The small colon reclaims excess water and returns it to the body. This is where fecal balls are formed. Fecal balls are the undigested and mostly indigestible portion of what the horse was fed.
2. Main Functions:
   1. Digestion is the hindgut is primarily microbial, performed by billions of symbiotic bacteria, protozoa and fungi. The diet composition will affect the makeup of microbial population
   2. Most feedstuffs are composed of dietary fiber (structural carbohydrates) that cannot be digested efficiently and must instead be fermented via the microbial populations.
   3. Plant fibers and undigested starches are broken down by fermentation to volatile fatty acids (acetic, propionic and butyric) – important energy sources
   4. The microbes will also produce vitamin K, B-complex vitamins, and some amino acids (minimal, if any absorption of these amino acids occurs).
   5. Water reabsorption
3. Related Issues:
   1. Passage of NSC from the small intestine to the hindgut is undesirable; causes overproduction of gas and lactic acid
   2. Change in pH favors pathogenic bacteria which can lead to discomfort, colic, laminitis and enterotoxaemia
   3. Numerous turns and folds increase risk of impaction and entrapment of gas contributing to “gas colic” and twisted-intestine

Management Suggestions

1. Make sure acceptable water is available at all times (except when hot from work). Water must be clean and at an acceptable temperature (> 45 degrees F is suggested).
2. The feeding program should be based on high quality forage – only feed sufficient concentrate to meet nutrient requirements not met by the forage.
3. Minimize NSC content (15-25% NSC) in the concentrate while assuring adequate supply of other non-calorie nutrients (amino acids, vitamins, minerals). Horses in more intense work, lactating mares and growing horses, in general, can tolerate higher levels of NSC than idle horses.
4. Supply increased calorie needs in performance horses, lactating mares and growing horses with highly digestible fiber and fat wherever possible.
5. Try to mimic the intake behavior of the grazing horse by feeding 3 to 5 times per day, if possible.
6. Feed small meals, especially concentrates, in small amounts (< 4-5 pounds of concentrate/meal)
7. Mimic trickle feeding by ad-libitum hay feeding or using double hay nets/slow feeders.
8. Monitor pasture consumption to prevent overconsumption
9. Make changes in feed type and amount gradually (preferably 7 to 14 days), especially when increasing concentrate amount or NSC content or going from grass/mixed hay to higher legume hay. Allow time for the digestive system to adjust to pasture in the spring or moving a horse from a stall to pasture.
10. Avoid moldy, dusty feed.
11.  Follow a regular schedule of deworming, vaccinations, dental care and exercise