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Digestive System

The digestive system is split up into the foregut and the hindgut. The foregut consists of the mouth, cannot forget that, that belongs to the foregut, the esophagus, the stomach and the small intestine. The hindgut then consists of the large intestine, the cecum, the large colon, the small colon and the rectum. We will talk about each one of them, what each one of them does to some degree.

Alright, if you look at the foregut of the horse, the esophagus, and then the stomach. How big do you think a horse's stomach is? It is about four to five gallons. In other wordsm if you ride a horse and get him a little bit dehydrated, often times you will see they can easily put away about one bucket of water, which is about four to five gallons. Anything beyond that they can’t hold in their stomach at any given time. But about four to five gallons is no problem.

How long is the small intestine of the horse? It says that up there, I don’t know if you can read it or not. It is about 70 feet long. For anything the horse ingests, how long does it take to go from the mouth to the large intestine? In other words it has to cover about 75 feet or so of intestine, how long does that take? It takes about 45 minutes to 6 hours. No longer than that. Then, how long does it take for all of that stuff to get out of the large intestine? That takes up to about two days. In other words, most of the time the feedstuff spends in the hindgut being fermented.

The mouth is useful for mastication or chewing and salivation. If you just feed a dog and a dog just thinks about food, does he start to salivate? Yes, dogs do people do. Do horses? Nope, horses don’t. They do not begin to salivate until they come into contact with the feed, that is one of the differences between herbivores and carnivores.

Then we have the esophagus. The esophagus is basically a very muscular pipe that has wave-like contractions called peristaltic contractions that push the feedstuff from the mouth, once the horse swallows, into the stomach.

In the stomach, like we said, there is a four to five gallon sack, basically, that is also very muscular. There is some hydrochloric acid present, which aids in the protein breakdown. And we have other gastric juices that begin the fat breakdown and a little bit of starch breakdown also occurs in the stomach. But primarily in the stomach we’ll have protein breakdown.

The small intestine is the major nutrient absorption sight, that is probably a pretty important thing that you need to remember. That is where most of the nutrients are being digested and absorbed, even though they do not spend a whole lot of time there. In other words, it needs to be fairly efficient. Soluble carbohydrates are turned into simple sugars at that location. Proteins are turned into amino acids. Fats are broken down into fatty acids and what? I left something off there, fatty acids and what? Glycerol. Okay, so it’s fatty acids and glycerol, but glycerol is not a very important part of energy production here. Fat soluble vitamins. A, D, E and K. They are absorbed with the fats, so that is why they are listed here. We also have calcium, zinc and magnesium being absorbed in the small intestine. There is a little bit of phosphorus absorption, but most of the phosphorus absorption will be in the large intestine. And then we have a little B vitamin absorption. The reason that we do not have a lot of B vitamin absorption is because most of the B vitamins that the horse uses are actually produced in the hindgut. And they are also absorbed there, so there is very limited amount of B vitamins in the horse's diet.

In the large intestine, which primarily consists of the cecum and the colon, cellulose is being fermented. And cellulose turns into volatile fatty acids which can be used for energy as well. The reason cellulose is not being digested at all in the small intestine is because it consists of a lot of cell wall that can’t be broken down by the enzymes. But the microbes in the large intestine actually can break it down and they will turn it into volatile fatty acids. And in turn those microbes will produce some B vitamins. They will produce some volatile fatty acids, but they will also produce some B vitamins which are being absorbed out of the large intestine or out of the cecum and colon. Also, this is the site of water absorption. So when a horse drinks water, 98% of it will be absorbed in the large intestine. Hardly any is absorbed out of the small intestine. There maybe a little bit, but not much at all. Most of it is absorbed right here. In other words, when the horse drinks, it will take about 45 minutes to an hour before that water really does him any good as far as rehydrating the horse because it takes that long for it to get back to the large intestine and absorb. And then what ends up in the rectum is totally indigestible and unfermentable material to the horse.

Let me ask you another question, what animal would you think is more efficient, a horse or a cow? Would you think that a cow is more efficient than a horse? You say no, why not? They’re the ones that have a rumen which does a lot of fermentation before the absorption site. Most people would say that cattle are more efficient. But you guys are actually right. There’s two ways to look at it. Cattle are more efficient based on the amount they consume. In other words, when you feed a cow a certain amount of feed, the cow will get more nutrients out of that amount of feed. If you look at it a per time basis. In other words, the total amount of feed that is being eaten over a 24 hour period, a horse will be more efficient. The reason for that is because it does not have to stop consumption while it’s fermenting. A cow will stop consumption while they are ruminating. They won’t eat anymore during that time, while a horse can continue to eat. That is why horses will do better on very poor pasture. If you look at some of the ranches in Eastern Oregon, the horses do pretty well up in the high country where it is so poor that cattle wouldn’t do very well up there because cattle simply could not eat enough, because they would eat, eat, and eat and then they would have to ruminate for a while. While they were ruminating, the horses are still eating. On a per time basis, horses are actually more efficient than cattle are.

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