In order to really get just how important good digestive health is to your horse’s overall wellness and performance, you need to understand the basics of how the horse’s digestive system works. Here’s a quick guide through the equine digestion process and some notes on where it often fails as a result of modern horse management.
Equine Digestion Step 1: Biting and Chewing
The first step in equine digestion is also one of the most important: chewing. (Mastication is the fancy word for chewing that you may come across in more scientific articles.) Horses are more able to digest their food when it has been ground into small pieces. Besides aiding in nutrient absorption, well-chewed food is less likely to get stuck in places it shouldn’t, causing choke in the esophagus and impaction colic in the intestines and colon.
Also, chewing produces lots of saliva, which further breaks down food and buffers the acids in the stomach to promote a healthy digestive system.
Equine Digestion Step 2: Stomach Acids Further Liquefy Food
The stomach’s main job is the pre-digestion of proteins and fats and the further physical breakdown of feed before it hits the small intestine, where nutrient absorption begins. It has several important jobs:
- Stomach acids break down food in preparation for digestion
- Acids activate the enzymes that break down the proteins in feed
- Stomach acids kill many microorganisms in feed which reduces the potential for infection
The stomach is actually quite small (only about 10% of the horses digestive tract), and food remains there for 30-45 minutes on average. The stomach is never more than two-thirds full and so food may pass into the small intestine before it has been treated by the stomach’s digestive juices.
Equine Digestion Step 3: Absorption In the Small Intestine
The small intestine of a horse is about 60-70 feet long, and is where most of the breakdown and absorption of feed occurs. The partially digested food from the stomach passes into the small intestine, where enzymes act on it to produce materials that can be absorbed into the bloodstream. The majority of what horses eat is absorbed into the bloodstream from the small intestines, including proteins, simple carbohydrates, fats, and essential vitamins.
Food spends little time in the small intestine, passing through within 1-3 hours after the horse ingests it. Factors such as meal size, feed type, and exercise influence this transit time. Processed feeds tend to move more quickly through the small intestine (in part because horse’s tend to eat them very quickly), which reduces the amount of starch that is available to be digested in the small intestine.
The biggest problem is that when we feed a large amount of processed feed, there is simply too much starch going through the intestines too quickly to be digested properly. The result is that undigested starch reaches the hindgut, which causes problems throughout the horse’s digestive system.
Equine Digestion Step 4: Fiber Fermentation & Energy Production in the Hindgut
The hindgut is composed of the cecum, colon and rectum. The cecum and colon together can hold up to 32 gallons of fibrous material that slowly ferments over 2-3 days. Microbial fermentation in the hindgut by billions of microorganisms (bacteria and protozoa) breaks down fiber, the structural components of the plants horses eat.
A key product of fermentation in the hindgut is volatile fatty acids that are absorbed into the bloodstream and provide an important source of energy for the horse. Horses on a complete forage diet get up to 70% of their energy from these VFAs.
Problems in the Hindgut
An overload of starch reaching the cecum and colon is a primary cause of digestive imbalance in the hindgut. Starch-digesting bacteria produce lactic acid and that increases the acidity of the hindgut, commonly known as hindgut acidosis. This increase in acidity kills the beneficial, fiber-digesting bacteria. Toxins released during this process can result in colic and founder. Furthermore, hindgut acidosis is also thought to be a precursor to colonic ulcers.
On the not-so-clinical side, upsetting the balance of microorganisms in the hindgut can lead to performance issues such as lethargy, irritability, girthiness, difficulty in bending or collecting, and general discomfort in the horse.