Journey Through the Equine GI Tract

Follow fodder’s fate through a horse’s digestive tract

When someone mentions the equine gastrointestinal (GI) tract, what do you think of first? Maybe you immediately picture the abdomen, where the bulk of this body system lies, moving ingesta along through its twists and turns of intestine. Or, your mind might roam to the mouth, responsible for consuming the forages, concentrates, and supplements you have worked hard to source. Conversely, you consider the hind end, producer of the mounds of manure you spend hours mucking, picking, and transporting from one place to another. 

Unlike some organ systems, the GI tract changes immensely from one section to the next, with each segment aimed at one specific goal: providing energy (calories) for the horse’s body. In this article we’ll explain how each part of the equine GI tract is designed to break down plant products to produce energy. We’ll also describe each region’s special characteristics and the medical conditions that can develop there. Note that all measurements included here refer to an average 500-kg (1,100-lb) horse.

Head First

Horses use their lips to painstakingly procure food, sifting through this plant and that to pick the perfect stem or leaf with their lips and incisors to deliver to the oral cavity. Once the plant or feed is within the mouth, enzymes in the saliva begin to break down its tough cell walls while simultaneously wetting and lubricating it for the upcoming voyage. The molars and premolars also assist by physically breaking down the plants to create a moist, semidegraded food bolus the horse then swallows.

Health Risks

• Dentition Because teeth play such an integral role in preparing forages for the remainder of the digestion process, be sure to have them examined at least annually. Don’t wait for quidding (when horses drop chunks of partially chewed feedstuffs) or weight loss to develop before having the veterinarian out.
• Salivary gland issues Although rare, several conditions can impair the salivary glands. Foreign bodies, such as awns from plants, can puncture the glands or ducts or block saliva flow. Salivary stones (sialoliths) can develop in the glands or ducts, also blocking flow. Head trauma can damage ducts, and tumors such as melanomas and sarcomas can develop there. Infections of the glands and ducts can also occur, most notably due to the rabies virus and the bacterium Streptococcus equi that causes strangles.
• Temporomandibular joint (TMJ) problems Some horses develop pain or arthritis in this joint at the junction of the upper and lower jaws, which can negatively impact mastication (chewing).

Down the Hatch

Once the horse swallows the neatly packaged food bolus, it travels down the length of the muscular, tubelike esophagus, which begins at the epiglottis at the back of the oral cavity near the larynx. The esophagus simply serves as a conduit to funnel food from mouth to stomach. Peristalsis—regular, rhythmic muscular contractions—propel the food in a timely manner. No further feedstuff breakdown occurs here because the esophagus is largely devoid of salivary glands, but it does house mucous glands that help lubricate the potentially rough food bolus as it travels. When the bolus reaches the end of the esophagus, it pops through the muscular ring between the esophagus and stomach known as the esophageal sphincter. 

Health Risks

• Choke An esophageal obstruction can cause discomfort, excessive drooling, lethargy, and malaise, as well as more serious long-term complications. For instance, a bolus of food stuck in the esophagus can damage the esophageal lining. As a result, scars can develop, causing the esophagus to contract, or a rent can form (a tear that causes diverticulitis).

The Bulky, Brawny Abdomen

Foregut

Once dispelled from the esophagus, the food bolus hits the stomach. The equine stomach only holds 8-15 liters (picture eight milk cartons), accounting for a mere 9% of the GI tract. Alternatively, the human stomach makes up about 17% and typically holds about 1 liter, though it can expand to hold up to 4.

Veterinarians estimate it takes only 12 minutes for food to move through the stomach, which explains how horses can graze for so many hours each day (approximately 17 hours in a 24-hour period with free access to pasture). The hydrochloric acid produced in the stomach’s glandular region (the lower half) breaks the food bolus down into small pieces. In addition, some enzymes begin the actual digestion process.

Digestion involves breaking down protein and simple water-soluble (also known as nonstructural) carbohydrates into amino acids and individual sugar molecules, respectively.

Once the small, partially digested pieces of food pass from the stomach into the small intestine (not to be confused with the small colon we’ll discuss later), things start to get interesting. Much like a marble run game, the partially digested feed materials course through the abdomen, twisting and turning through coiled loops of intestine.

The first section of small intestine is called the duodenum. Enzymes released into it continue the digestion process initiated in the oral cavity and stomach. Thanks again to peristalsis, partially digested food travels along the duodenum before passing seamlessly into the longest section of the small intestine, the jejunum. Here, enzymatic digestion of feedstuffs continues to produce additional amino acids and sugars and other nutrients that are absorbed in the latter regions of the jejunum. The ileum, the third and final section of the small intestine, also helps absorb digested nutrients.

The entire trip down the 22-meter-long (60-65-foot) small intestine is remarkably fast—up to 0.3 meters (1 foot) per minute. Compare this to the human small intestine, which measures 6 meters (10 feet) and moves at a rate of about 0.04 meters per minute (0.13 feet).

Hindgut

Most feedstuffs are composed of structural carbohydrates that cannot be digested efficiently and must instead be fermented. To review, digestion requires enzymes to break down feed and produce amino acids and simple sugars, along with small fat particles. Fermentation, on the other hand, uses microorganisms such as bacteria, fungi (including yeast), and protozoa to break down feed materials. The process produces volatile fatty acids (VFAs, such as butyric and acetic acid) that are absorbed into the bloodstream and used to fulfill most of the horse’s energy requirements. Fermentation provides far more energy to horses than does digestion, which is the opposite of what happens in humans. This explains why horses fall into the “hindgut fermenter” category, along with rhinos and rabbits.

The collection of microorganisms responsible for fermentation is referred to as the hindgut’s microbiome. Research into this microbiome continues at a furious rate, and an improved understanding of it has the potential to maximize horse health from the inside out, says Shannon Pratt Phillips, MSc, PhD, an associate professor in North Carolina State University’s Department of Animal Science.

The cecum and large intestine make up the bulk of the GI tract and contain millions of these microbes that ferment food materials to produce VFAs. The voluminous cecum serves as the gateway to the large intestine. It has a capacity of approximately 30 liters—almost triple what the stomach can hold. The food bolus of undigestible but fermentable material passes from the ileum through a tiny hole, or ileocecal orifice, into the cecum where fermentation begins. Essentially, the cecum is a muscular sac, equivalent to a human’s tiny 3-4-cm appendix that serves as a venue for initiating fermentation. After about seven hours of fermentation, feed passes into the large intestine for additional fermentation.

The 12-foot-long large intestine (often referred to as the large colon), although relatively short compared to the small intestine, boasts an impressive volume—a whopping 75 liters (20 gallons). This is equivalent to the amount of fuel required to fill a minivan’s or large SUV’s gas tank. From the cecum, the large intestine becomes somewhat of a whirling dervish, yet still not as circuitous as the looping small intestine. The large intestine courses up the right side of the abdomen toward the diaphragm, which separates the thoracic (which contains the lungs) and abdominal cavities, before turning left and traveling down the left side of the abdominal wall. These two sections of the large intestine are referred to as the right and left ventral colons, respectively. Next, the large intestine doubles back on itself in the pelvic region, at a location referred to as the pelvic flexure, and becomes markedly narrower. The left dorsal colon continues back toward the diaphragm, becoming the right dorsal colon before crossing into the middle of the abdomen as the transverse colon and finally racing toward the rectum.

“In my opinion, the cecum/large colon is the most delicate part of the GI tract,” says Pratt Phillips. “It is a wonder of evolutionary development to suit what horses were consuming throughout history compared to ruminant species, for example. But it also causes issues in modern horses with its twists and turns and changing diameters.”

Health Risks

Entire textbooks are devoted to the list of things that can go wrong in this part of the horse’s body. The intestinal tract’s massive length, volume, and continual change in diameter and direction beg for disaster at every section.   

• Gastric ulcers Almost all performance horses and even some pleasure horses suffer from painful ulcers caused by splashing hydrochloric acid in the stomach’s squamous and glandular regions.
• Colic A fancy way of saying abdominal pain, colic can strike at any time. As you can guess from the seemingly haphazard way that the intestines appear stuffed in the equine abdomen and their sudden changes in size, especially in the large intestine, blockages and twists aren’t uncommon.
• Internal parasites Roundworm (asacarid) and small strongyle (cyathostomin) infections can occur in adult horses, with the latter being the most concerning. A less common but more serious condition is larval cyathostominosis, caused by mass eruption of encysted larvae.
• Diarrhea A potentially life-threatening condition, diarrhea has a variety of causes, including bacterial infections, such as Clostridium difficile or Salmonella spp, and diet changes.
• Hindgut acidosisA sudden decrease in pH—often tied to large amounts of rich concentrate feeds—can make the large intestine’s contents more acidic than normal. This will alter the population of microorganisms in the large intestine, interfering with fermentation and potentially causing ulceration of the lining of the cecum and large intestine, as well as diarrhea.

The Final Few Feet

The small colon descends from the transverse colon, terminating at the rectum and anus. Here, fecal balls form, and excess water is absorbed before manure is expelled. Thankfully, once we reach this portion of the GI tract, we are home free, with few worries.

Health Risks

• Melanomas These tumors can develop in the horse’s anal region. In some cases, melanomas grow so large as to block manure passage. Gray horses are particularly susceptible, with approximately 80% affected.

Take-Home Message

Having a clear understanding of equine GI tract structure and function maximizes an owner’s ability to recognize the wide array of digestive conditions that can threaten horse health. For anyone who has suffered through the roller coaster of colic or battled with gastric ulcers, chronic choke, hindgut acidosis, and more, the balance between GI clockwork and ticking time bomb is tenuous. Assess horses daily, maintain a consistent forage-based diet, integrate feed changes slowly to allow the microorganisms in the fermentation vat to adapt to those alterations, and consult a veterinarian immediately if you discover any abnormality.

“Horses should have plenty of forage in front of them at all times, unless they are overweight,” says Pratt Phillips. “The fiber within helps keep food moving through the gut and helps maintain the microbial ecosystem.”