The horse's digestive system is important for digesting feed and absorbing nutrients, each of which has an important function in the body. A healthy and well-functioning digestive system is therefore important for the horse. Roughage forms the basis of the horse’s diet. The digestive system is thus adapted to digesting this. The horse's digestive system consists of the mouth, oesophagus, stomach, small intestine, large intestine, appendix and rectum.
The Mouth
Digestion of feed starts in the horse's mouth. A mature horse has 44 permanent teeth to grind feed for better digestion and absorption of nutrients, allowing the feed to pass easily through the oesophagus into the stomach 1. Healthy and well-maintained teeth are therefore essential for digestion. Chewing motions stimulate the production of saliva in the mouth of the horse 2. The saliva that mixes with the feed in the mouth contains a buffer that neutralises stomach acid in the stomach 3. When a horse chews 1 kilogram of roughage, the horse makes more chewing movements compared to when the horse chews 1 kilogram of concentrate. Roughage has a more fibrous structure, compared to concentrates, therefore it takes the horse longer to chew it finely. To chew a kilogram of roughage, the horse makes between 3000 and 3500 chewing motions 4. If a horse chews on a kilogram of concentrates, the horse will make between 800 and 1200 chewing motions 4. Therefore a greater amount of saliva and the buffer are produced when the horse chews on roughage. As a result, roughage has a better buffering effect on stomach acid.
The Oesophagus
Once the forage has been chewed, the forage passes through the oesophagus to the stomach. The oesophagus makes peristaltic movements that move the feed towards the stomach. Between the oesophagus and the stomach is a muscle that closes the oesophagus from the stomach 5. Because of this muscle, feed can only move in one direction towards the stomach. In nature, wild horses should be able to flee abruptly with a full stomach. The muscle makes sure feed does not flow back into the mouth when a horse needs to flee. So, unlike cows, for example, horses cannot ruminate the feed either.
The Stomach
Compared to the rest of the digestive system, the horse's stomach is small. Horses naturally graze between 14 and 16 hours a day. As a result, they frequently eat small portions, preventing the stomach from becoming overfull. The horse's stomach has a capacity of about 7.5 to 15 litres 2. Because the stomach is small, the feed that enters it does not remain for long, and the stomach is emptied within 2 to 6 hours each time 6. The advise is therefore to frequently feed your horse smaller portions throughout the day instead of providing them with one or two large meals.
The stomach is divided into two parts: the non-glandular region and the glandular region 7. Both parts are covered with mucosa. The glandular region consists of glands that produce stomach acid 8. Stomach acid has multiple functions and is for example important for protein digestion. Because stomach acid is produced in the glandular part, which creates an acidic environment, this area contains a thicker layer of mucosa to protect the stomach wall from the acid. The non-glandular region does not have this thicker mucosal layer and therefore does not have the same protection as the glandular region against stomach acid 7. The environment in the glandular region is much more acidic due to the production of stomach acid in this region. As mentioned, saliva contains a buffer that neutralises the acidic stomach acid, preventing the environment in the stomach from becoming too acidic 9. Stomach acid is continuously produced, even when the stomach is empty 10. If stomach acid is not sufficiently buffered, it can result in the development of gastric ulcers in the non-glandular region of the stomach. For more information on gastric ulcers: see the blog ‘Equine Gastric Ulcers in Horses: Causes, Treatment, Prevention and the Role of Good Nutrition’.
The Hindgut
After leaving the stomach, the feed first enters the small intestine. The horse's small intestine is divided into three parts: the duodenum, jejunum and ileum 3. In this section of the digestive system, enzymes mix with the feed for digestion 6,11. After the food is broken down, the nutrients are absorbed in the small intestine. These nutrients travel through the bloodstream to various parts of the body, where they carry out essential functions.
Fibre, derived from roughage, passes into the hindgut where it is digested by micro-organisms 12. These micro-organisms are also known as the hindgut microbiome. The microbiome consists of different types of bacteria, fungi and yeasts. The composition of the microbiome affects fibre digestion and gut health 13. Through fermentation, fibre is converted into the volatile fatty acids acetate, butyrate and propionate 12. Volatile fatty acids provide energy and are essential for hindgut health and function 14. For more information on the hindgut and the microbiome, see: ‘The Importance of Optimal Gut Health of the Horse’.
Feed affects the overall health and function of the digestive system of the horse. A diet that does not meet the horse's nutritional requirements can lead to digestive system health issues. Feed management that meets the horse's requirements is crucial for the horse's well-being and health. If you're unsure whether your horse's diet meets its requirements, consult a nutritionist for advice.
References
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Strauch, S., Wichert, B., Greef, J.M., Hillegeist, D., Zeyner, A., Liesegang, A. (2017) Evaluation of an in vitro system to simulate equine foregut digestion and the influence of acidity on protein and fructan degradation in the horse′s stomach. Journal of Animal Physiology and Animal Nutrition, 1: 51-58.
Collinet, A., Grimm, P., Julliand, S., Julliand, V. (2021) Multidimensional Approach for Investigating the Effects of an Antibiotic–Probiotic Combination on the Equine Hindgut Ecosystem and Microbial Fibrolysis. Frontiers in Microbiology, 12: 1-14.
Julliand, V., Grimm, P. (2016) Horse species symposium: The microbiome of the horse hindgut: History and current knowledge. Journal of Animal Science, 94(6): 2262-2274.
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