Have you ever wondered what happens after your horse takes a bite of hay? Gaining a better understanding of her gastrointestinal tract will help you optimize her health and minimize complications.
You pay the utmost attention to what goes into your horse. You pore over which grain is most appropriate, poll your barn mates to see what their horses are eating, read through too many supplement labels to count, and ask your vet her opinion at every appointment. But have you ever thought about what goes on inside your horse, within her gastrointestinal tract?
Digestion begins in the oral cavity, aka the mouth. Adult horses have 36 to 44 teeth. At the front of their mouths are the incisors, designed for grasping hay and tearing grass blades. A strong tongue moves food from the incisors into the mouth, where the cheek teeth – premolars and molars – work to grind down the food. The average length of fibers in equine feces is just 3.7 mm, and no significant decrease in food particle size occurs between the stomach and anus. In other words, chewing is the most important determinant in food particle size! In addition to highly-efficient teeth, horses produce up to 12 liters of saliva per day. Unlike other animals, horses produce saliva as a result of chewing only. Saliva acts as lubrication to help the food bolus (mass of chewed food) pass easily through the oral cavity and down the esophagus, and also contains enzymes to begin the chemical breakdown of food.
After food is ground down into small particles in the mouth, it moves into the esophagus. The esophagus is a 50” to 60” muscular tube that extends from the pharynx, down the left side of the horse’s neck, through the thoracic cavity (home of the lungs and heart), through the diaphragm, and into the stomach. Food moves down the esophagus via peristalsis – regular rhythmic muscular contractions that push food towards the stomach. A strong gastroesophageal sphincter is found at the junction of the esophagus and stomach. This is a strong muscular ring that prevents the retrograde (backward) movement of food – thus, the horse cannot vomit or belch.
Once the gastroesophageal sphincter relaxes, food empties from the esophagus into the stomach. With an 8 liter to 15 liter (2 gallon to 4 gallon) capacity, the size of the stomach is relatively small compared to the rest of the horse’s digestive system – just 10% of the total capacity of the gastrointestinal tract! This small capacity means the horse’s stomach is best suited for small frequent meals.
The horse’s stomach is best suited for small frequent meals.
As the food sits in the stomach, hydrochloric acid and pepsin secreted by stomach cells contribute to the chemical breakdown of food. Retention time of food within the stomach is less than two hours. Typically, food moves through the stomach in about 12 minutes, which is why horses can graze for hours and hours during the day, and always seem to be asking for their next meal!
Within the abdomen, the horse also has a pancreas and liver – all of which play supporting roles in the digestion of feed.
The small intestine
The relatively simple design of the stomach leads into the twists and turns of the small intestine. At 60’ to 65’ long, the small intestine is the primary site for digestion and absorption of sugar, starch, protein and fat. Fat-soluble vitamins (A, D, E and K), calcium and phosphorus are also absorbed in the small intestine. With a 64 liter (17 gallon) capacity, this maze of tissue makes up 30% of the total capacity of the gastrointestinal tract. Food contents pass through all 65’ of the small intestine in about 30 to 90 minutes. In fact, the typical transit time of feedstuff through the small intestine is one foot per minute.
Three segments comprise this section of your horse’s gut. First, the duodenum, home to the enzymatic digestion of food. Then the jejunum, the longest and most mobile section of small intestine; here, enzymatic digestion continues and nutrients begin to be absorbed. Finally food reaches the ileum for further absorption of nutrients. The small intestine is attached to the body wall via a ligamentous attachment known as the mesentery. Within the mesentery are numerous vessels and nerves connecting to the intestines. The mesentery is attached to the small intestine along its entire length; but the mesentery itself has only one attachment to the abdomen, towards the spine – thus making it highly mobile and allowing the small intestine to move freely within the abdomen.
After food exits the ileum, it has reached an area of the horse’s gastrointestinal tract known as the hindgut. The hindgut is where fermentation of feed material takes place, and contains the gut microbiome. Beneficial fungi, bacteria and protozoa comprise the microbiome, helping break down feed material so it can be absorbed and used for energy by your horse.
The first part of the hindgut is the cecum. The cecum is a 30 liter (8 gallon) muscular sac that is essentially a huge microbial fermentation vat. It is here that all the “good bugs” of the microbiome live and work, ensuring the horse can utilize the food he eats. Feed material spends about seven hours in the cecum undergoing the fermentation process.
The large intestine
Food then passes into the large colon, or large intestine. The major function of the large intestine is the absorption of water. Every day, about 100 liters (26 gallons) of water (for an 1,100 lb horse) is secreted into the small intestine during the digestive process. As feed material moves through the large colon, much of this water is reabsorbed, forming semi-solid fecal material. While only 12’ long, it has a 75 liter (20 gallon) capacity – about 65% of the total capacity of the gastrointestinal tract.
As you might imagine, at this point, there is not much room left in the horse’s abdomen for the rest of the gastrointestinal tract. Therefore, the large colon is folded onto itself to fit, resembling two horseshoes stacked on top of each other.
From the cecum, the right ventral colon (on the floor of the right side of the abdomen) courses up towards your horse’s head. It then veers off to the left at the sternal flexure, becoming the left ventral colon (on the floor of the left side of the abdomen). The large colon then doubles back on itself in the pelvic region at a location called the pelvic flexure. Here, not only does the large colon change direction, but its diameter also narrows; this is a common site of impactions, or an accumulation of feed material obstructing the colon.
From the pelvic flexure, the left dorsal colon (which sits near the spine, on top of the left ventral colon) courses towards her head again, and turns right at the diaphragmatic flexure (which is above the sternal flexure). The right dorsal colon (on top of the right ventral colon) leaves the diaphragmatic flexure and heads back again towards her tail. The right dorsal colon turns left into the middle of the abdomen and becomes the shorter, narrower transverse colon.
The small colon
The transverse colon joins the small colon, which is the last segment of intestine before the rectum. It is in the small colon where fecal balls are made, and any remaining or excess water that wasn’t absorbed in the large colon gets absorbed here. Fecal balls pass into the rectum, then exit the body through the anus.
Your horse’s digestive tract is both a unique and complicated factory, working around the clock to process and convert feed into usable nutrients and energy. By understanding her digestive tract, you’ll be better equipped to manage her nutrition, optimizing her health and wellness while minimizing complications.
Dr. Justine Griesenauer is an equine veterinarian at Cedarbrook Veterinary Care, a mobile equine practice in Washington that takes a holistic approach to caring for horses. As a lifelong rider and horsewoman, she knows how important your horse is and this is reflected in her treatment. She is a graduate of the University of Tennessee College of Veterinary Medicine.