A horse’s stomach has a volume of about 5 to 15 litres, which is about 7 to 8 percent of the entire digestive system. Compared to other animal species, this is a relatively small stomach. In comparison, a human stomach has an average volume of 1 litre and can stretch to about 3 litres. The horse has such a small stomach for a reason; because it is a flight animal. A large, full stomach only makes an emergency escape more difficult.
The construction of the horse’s stomach is different from our stomach. It is shaped like a bag with a strong bow in it. At the entrance of the stomach is a valve, which ensures that no food can return to the oesophagus. This valve normally prevents a horse from vomiting. Although the stomach is one whole, inside it consists of two different parts, separated by a fold. The first part contains no glandular tissue, it is actually a kind of extension of the oesophagus. There is a bacterial population in this part. These bacteria mainly live on starch and sugars in the feed, and produce lactic acid. If (too) much lactic acid is produced (by foods with a lot of sugars and starch), the stomach can become more acidic, which can lead to stomach ulcers.
The second part of the stomach does contain glandular tissue, where more digestion of the food takes place. The gastric glands are responsible for the production of gastric juice. Gastric juice consists of various components including stomach acid, pepsin and gastrin. Stomach acid includes hydrochloric acid. It breaks down food into smaller parts, denatures (deforms) proteins, destroys bacteria and converts pepsinogen into pepsin. Pepsin is an enzyme that breaks protein chains into smaller pieces so that they can be digested more easily. The gastric glands themselves do not produce pepsin, but pepsinogen. Gastrin is a hormone that produces stomach acid. This part of the stomach also moves a lot, in order to mix the food with the gastric juice.
When you look at a horse’s stomach with a camera, you will clearly see the differences between the two parts. Because of the difference in tissues, they have a different colour. But the acidity is also different. The first part has a pH of 6 or 7, while the second part is much more acidic with a pH of 2 or 3.
The shape and function of the stomach tells us a lot about how we can best feed our horses. The stomach is relatively small, so feed several smaller portions throughout the day. Therefore, do not feed a 600 kg horse more than 2 to 3 kg of concentrate per feeding,as the stomach cannot handle this. This is not a problem with roughage, because a horse takes longer to absorb the roughage and roughage leaves the stomach faster. When chewing roughage, more saliva is produced, which neutralizes the stomach acid. Roughage also contains less starch and sugars than concentrates, so the bacteria in the stomach will produce less lactic acid and the pH in the stomach will be better maintained. Therefore make sure that your horse’s diet contains sufficient good quality roughage.