Last month I described the first parts of the equine digestive tract, including the itty bitty equine tummy. This month I’ll cover what I’ve learned from the Equine Soundness Hoof Care Course about the rest of the digestive system. In the previous article, I pointed out three clues to possible problems with the equine digestive system. Well, guess what? There are more potential dangers to be aware of, and I’ll provide some clues to those in this article.
The not-so-small small intestine is nearly 70 feet long (~20 meters) and has a capacity of 15-18 gallons (55-70 liters). Additionally, it has finger-like structures within it that enlarge the surface area. Obviously all the organs of the digestive system are necessary, but the small intestine is actually where most nutrients are digested and absorbed. The pancreas and liver each have a channel which converge and enter the duodenum—which is the first part of the small intestine—together as one tube. Pancreatic secretions help to break down proteins, fats, sugars, starch, fat soluble vitamins and some minerals. Additionally, since horses don’t have gall bladders, bile constantly flows into the small intestine from the liver, which helps to break down fats and suspend them in water for easier absorption.
More than 90% of the fat consumed is digested in the small intestine, and nearly 60-95% of the starch consumed is digested here as well. The digested nutrients are absorbed into the bloodstream through the walls of the small intestine. Since the upper gut (the stomach and small intestine) comprises nearly 40% of the digestive tract, and feed moves through rapidly, it is relatively easy to overwhelm the digestive capacity of these two organs. If large amounts of soluble carbohydrates reach the large intestine, they will quickly ferment. This—and too much fat in the total food intake—are clue number four that an inappropriate diet can cause problems. For example, too much concentrated feed can lead to undigested starch moving to the large colon and upsetting the bacterial flora, possibly resulting in colic, founder and other metabolic problems. It’s important to note, too, that horses are very susceptible to colic or death from toxins in their feed, because along with all the good nutrients, toxins are also quickly absorbed into the bloodstream in the small intestine.
Yes, this the same diagram I used before, but I made it larger. I’m including it again because it is the only one I’ve found that shows a very complex system in a fairly simple diagram. Other images show a jumble of multi-colored organs crammed into the horse’s body, looping back and forth over and under each other…it’s no wonder they end up with impaction and twisted guts. But the thin tube labeled “small intestine” in this image really does not do justice to what an amazing organ it is.
The small intestine moves undigested food into the large intestine, or hindgut, which is comprised of the cecum, the large (or ascending) colon and the small colon. Together they represent about 60% of the equine digestive tract, and the cecum, which is next in line after the small intestine, makes up about 25-30% of the entire large intestine.
At first glance the cecum seems like a pretty lousy design. It’s a big sack, about four feet long that can hold up to 40 quarts (38 liters) and—get this—the entrance and exit are both at the top! Since the horse has survived for many millennium, it obviously works. But this, and a couple of other things, make the cecum a potential trouble spot in the equine digestive system. We’ll lump these all into clue number five of the things that you need to be aware of.
First, while the walls are somewhat wavy, they are less wavy and muscular than the rest of the colon walls. This means food can get stuck—it is one form of impaction colic. Second, the cecum is a huge fermentation vat, containing millions of microbes to break down feed that is not digested in the small intestine, especially fibrous feed. Fermentation of course results in gas, which must pass all the way through the rest of the digestive system. Excess gas is a potential problem for the horse, no matter where it is generated. Additionally, the pH of the cecum—and actually the entire colon—must be just right in order for those millions of microbes to survive and do their job. Any imbalance in this can cause problems with digestion and/or absorption of nutrients. And since horses get 75% of their energy requirements from the fermentation of the colon symbiotic activity of the microbes, it is vital that they have a healthy environment in the colon.
After fermentation in the cecum, feed enters the colon. The (large and small) colon is only about 12 feet long, but it holds up to 80 quarts (76 liters). This is where food stays the longest, moving through it in 35-50 hours. Most of the nutrients remaining in the feed are absorbed here, as well as any excess water. Cellulose—the crude fiber from hay—is the food substance that benefits most from fermentation in the colon. The microbial breakdown of non-carbohydrates and crude fiber and non-volatile fatty acids that takes place in the colon results in a reserve energy source. Furthermore, the intestinal bacteria itself requires protein, minerals and vitamins for their own metabolism. Plant protein is partially transformed into bacterial protein in the colon and cecum, and the bacteria themselves are later digested, providing additional protein to the horse. Undigested (and mostly indigestible) feed is formed into fecal balls and passed from the rectum.
The large intestine is also an important part of the immune system and it can activate defensive cells which can travel throughout the body. The mucus inside the intestinal walls stop unwanted germs from entering the body. Certain waste products will also be denied entry into the bloodstream. When the environment in the intestine is not healthy, the immune system will not function optimally.
In summary, you might agree with me that it seems truly amazing that horses have survived all these years with such an incredibly complex and sensitive digestive system. But they obviously have, when left to their own devices in the wild. It is generally agreed that domesticated horses have far more health problems than their wild kin. So taking that into consideration, and noting the clues provided in this and the previous article, there are really only a few simple things to be aware of when you are responsible for feeding and caring for horses:
- Horses do best when allowed to graze 24/7. When that isn’t possible, small meals provided several times over the course of a 24 hour period is much better for them than one, two or even three larger meals.
- It is important that horses have access to water (2-4 lbs for each pound of ration consumed) at all times.
- Too much carbohydrate or fat in their feed can cause problems.
- Change of feed should be done over a period of weeks to allow the microbial population to adjust to the new diet, thereby preserving the proper pH in the colon.