An understanding of the equine digestive system is essential when establishing proper husbandry practices. Unfortunately, many modern day feeding and horsekeeping practices go against the horse’s natural way of being.
Horses are herbivores and hindgut fermenters. They are built to eat around 1.5-2% of their body weight in forage (hay, grass, etc.) per day. Sadly, numerous equine facilities feed grain-heavy diets and steer away from a forage-first mentality.
In this post, our goal is to give you a general understanding of the journey your horse’s feed takes through the equine digestive system. At the end, we will draw some conclusions about common husbandry practices and their effects on equine welfare.
Overview of the Equine Digestive System
- Lips/Teeth/Mouth: mechanical digestion of food (chewing) and lubrication with saliva
- Esophagus: tube that transports feed from the mouth to the stomach
- Stomach: enzymatic digestion of proteins and lipids (fats) begins
- Small Intestine – Duodenum: digestion and absorption of soluble carbohydrates and proteins begin in this first part of the intestine
- Small Intestine – Jejunum: absorption continues
- Small Intestine – Ileum: the final portion of the small intestine
- Ileocecal Junction: feed enters the cecum via this opening
- Cecum: substantial fermentation of fibrous contents (hay/grass) begins here
- Large Colon: fermentation and digestion of fibrous components occur here as well as the beginning of water resorption
- Transverse Colon: connects the large colon to the small colon
- Small Colon: continuation of water resorption and formation of fecal balls
- Rectum: stores feces before they leave through the anus
Now that we’ve gone through the basics, let’s dive into more details about each section of the equine digestive system.
Lips/Teeth/Mouth
Lips
First off, let’s talk lips! Horse lips have a tactile and prehensile muscle covered by skin and long hair. Prehensile simply means capable of grasping. Think of your horse’s lip like a finger that can poke around and grab things.
When your horse sifts through their grain to find all the good bits (and avoid those expensive supplements), they are using their amazing prehensile lip mechanism.
Another example is when horses sweep their lip back and forth across your hand in search of treats. Check out an adorable application of this mechanism here.
Teeth
Next, let’s talk teeth. Horses have hypsodont teeth. This means that they have high crowned teeth and enamel that extend past their gum line.
Their teeth will continue to erupt (grow) throughout their life (about 3-4 mm/year). This is why horses need their teeth floated regularly by a veterinarian!
Types of Teeth
There are four types of teeth present in a horse’s mouth: incisors, premolars/molars, wolf teeth, and canines.
Horses have 12 incisor teeth (6 upper and 6 lower) whose purpose is the biting and tearing that is ideal for pasture grazing. These teeth are located at the front of the mouth and are the ones you will see most often.
Next are the premolars and molars. Horses have 12 of each (6 upper and 6 lower) for a total of 24. These teeth are responsible for the lateral grinding of grass and other feedstuff.
Wolf teeth are not present in all horses (only about 70% of horses develop them). They are technically premolars that may erupt from 5 to 12 months of age (up to 4 teeth possible).
The purpose of wolf teeth is not well known. Many hypothesize that they may have been for eating twigs and brush in early equids. They serve no purpose to the modern horse.
Finally, we have canine teeth. These teeth are typically only in stallions and geldings and up to 4 teeth can be present. Mares can sometimes develop them but this isn’t very common. They were potentially developed for fighting use in wild herds. Oddly, they serve little purpose to domesticated horses.
Mouth
In the mouth, food mixes with saliva to make a moist bolus that is easy for the horse to swallow. The saliva’s main purpose is to lubricate the bolus (little to no digestion occurs in the mouth). Unlike dogs, horses only produce saliva through chewing. There is no psychological response that would cause a horse to drool.
A horse can produce 10 to 12 liters (3 gallons) of saliva per day. The more roughage/dry matter they are fed, the more saliva they produce. Horses have three salivary glands: parotid, mandibular, and sublingual.
Equine saliva is comprised of 99.5% water and contains enzymes, electrolytes, IgA, and lactoperoxidase. The enzymes present (amylase) begin the breakdown of starch but in very minute amounts. The electrolytes regulate pH and act as a buffer for the stomach. IgA and lactoperoxidase help disable bacteria and viruses to help prevent disease.
When the horse is done chewing their feed, they swallow and the bolus heads to the esophagus. This is the beginning of the horse’s foregut.
Foregut – Esophagus
The horse’s esophagus is a 4 to 5 foot muscular tube that connects the throat and the stomach. The bolus travels through this region in about 4 to 5 seconds (liquids pass even faster).
Peristaltic contractions move the bolus down the esophagus. Once it reaches the end, it passes through the cardiac sphincter to enter the stomach.
Side note: Many people may have wondered whether or not horses can vomit. Horses are actually incapable of vomiting and there are a few reasons why. First, the cardiac sphincter is extremely tight making it nearly impossible for anything to go back through it.
The location of the stomach is also to blame. The equine stomach lies deep within the ribcage where muscles simply cannot reach it. Even if horses had a vomiting/gag reflex (which they don’t), there is nothing their body could do about it.
Foregut – Stomach
After passing through the cardiac sphincter, the bolus enters the stomach. The stomach has a capacity of 8 to 15 liters and makes up 8% of the entire GI tract. The bolus spends 30 to 90 minutes here while its proteins and fats degrade.
Two regions make up the equine stomach: non-glandular and glandular. An imaginary line called the margo plicatus splits the two regions.
In the non-glandular region, there are no glands (hence its name). This region does not secrete any mucus, acids, or enzymes. It relies heavily on mucus and bicarbonate from the saliva to protect it. This region is where gastric ulcers can form.
In the glandular region, there are glands present that CONTINUOUSLY secrete enzymes and hydrochloric acid. Three histological sections make up this region: the cardiac mucosal, the fundic mucosal, and the pyloric mucosal.
Once processed in the stomach, the bolus will exit through the pyloric sphincter into the duodenum (small intestine).
Foregut – Small Intestine
The equine small intestine is the primary site for the digestion and absorption of protein, fat, and simple carbohydrates. This section of the GI tract is 15 to 22 meters long and can hold up to 55 to 70 liters!
The bolus will spend 1 to 7 hours here and will pass through the three sections of the small intestine: the duodenum, the jejunum, and the ileum.
The duodenum (1 – 1.5 m) is where bile salts released by the liver neutralize the bolus so that digestion can occur.
The jejunum (21 m) is the primary site of absorption of fatty acids, amino acids, glucose, vitamins and minerals, and electrolytes.
Further absorption and digestion occur in the ileum (0.5 to 1 m).
Once the small intestine finishes processing the bolus, it passes through the ileocecal junction into the cecum.
Hindgut – Cecum
The cecum is a 1 meter long, blind fermentation pouch, meaning the bolus enters and exits through the same spot. Feed does not flow through the cecum. Instead, it is churned, worked, and fermented.
The bolus spends 3 to 7 hours here while fermentation takes place. Additionally, microbes synthesize vitamins B and K here as well.
The cecum has a capacity of 25 to 30 liters with a pH of approximately 6.6. It is extremely sensitive to pH changes which is why it takes approximately 3 weeks for the microbial population to adjust to a new diet.
Once the cecum finishes fermenting the bolus, it sends it back out to the large colon.
Hindgut – Large Colon
The equine large colon is the 10 to 12-foot portion of the hindgut where fermentation continues. It efficiently digests fibrous materials with a capacity of 60 to 90 liters. Interestingly, the bolus will spend 28 to 65 hours here!
Unfortunately, these pouches/sacs of the large colon can fill with gas and/or twist, which leads to colic. Once the bolus has finished fermenting in the large colon, it travels through the transverse colon into the small (descending) colon.
Hindgut – Small Colon
Near the end of the GI tract, you will find the small colon. The small colon’s main purpose is water resorption. No nutrient absorption occurs and fecal balls form here as well.
Rectum
Everybody out! This is the final stop for our little bolus. Finally, it is ready to be evacuated as manure through the anus. When it hits the ground, it is finally ready for you to scoop up with a smile. 😉
Key Points and Takeaways
Finally, the feed has been digested. The total transit time through the equine digestive system can range from 36 to 72 hours! How cool is that?!
Now, we need to discuss a couple common husbandry practices and their benefits and drawbacks. We will look at two feeding situations: a forage first diet and a grain first diet.
Grain First Diet
Let’s look at the grain centric diet first. Many modern equine facilities rely heavily on commercial grains during feeding. The equine stomach is not designed to take on these infrequent, large meals.
Horses are grazing animals. They need a constant trickle of feed into their system. Let’s think back to the design of the equine stomach. It is CONSTANTLY producing hydrochloric acid. If there is nothing in the stomach, the HCl will begin to eat away at the stomach lining, causing ulcers.
When feeding a grain heavy diet, the stomach passes it to the small intestine quickly. This undigested feed ends up in the hindgut, disrupting the delicate microbial balance, possibly leading to colic.
Forage First Diet
When feeding a forage first diet, you are mimicking the horse’s natural feeding behaviors. They can gradually consume hay/grass and it can pass through their digestive system normally. However, the frequency of forage fed plays a key role in a healthy equine digestive system.
The best feeding practice is to feed forage little and often. You can also utilize numerous hay feeders and nets to slow down your horse’s consumption. This further mimics natural grazing behavior.
When people choose to ignore a horse’s natural needs, welfare can suffer and pathological issues (like colic) can occur. It is important to have a keen understanding of the equine digestive system and adapt your husbandry practices around it.
Remember, your horse’s welfare is more important than human convenience.
Thank you so much for reading about the equine digestive system. Equine digestion and nutrition are two of my favorite topics and I love to discuss them. Let me know if any of the information in this post surprised you! I’d love to hear from you.
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