The horse’s digestive system is a complex grouping of organs that is designed for the free-grazing animal. Because of man’s constraints on the movement and feed choices of domestic horses, problems can arise ~ Les Sellnow, 2006.
In the wild, horses are naturally very active animals and they rarely stay in one place. Horses will slowly graze as they roam, consuming small amounts of forage as they go to attain their required nutrients. The horse is a prey animal so even when they come across a patch of luscious forage, they are very aware not to stay there for a long period of time. We know very little about the wild horse diet but we do know that it appears to be founder free.
"Much of their diet appears to be range grasses and grass-like plants and probably a wide variety of high desert type legumes" ~ Jaime Jackson, 2015.
So how have humans altered what nature has intended for the horse?
Firstly, we've taken away his ability to forage for his own nutrients. Secondly, we've placed the horse in a small paddock where he is subject to the same grass type day in and day out. He can no longer eat small amounts in a grass pasture and so eats constantly with little movement. Many domesticated horses are also subject to one or two meals per day (a solitary haynet and/or bucket feed). This practice is the basis of many digestive issues we see in the domesticated equine population.
The digestive tract of the horse is designed to process small meals obtained by eating as a pick and go behaviour. The horses gastrointestinal tract consists of the mouth, esophagus, stomach, small intestine and the large intestine.
A few important notes to consider about the horses digestive system - firstly it starts at the mouth. According to Peggy Auwereda of Iowa State University, horses secrete large amounts of saliva when the horse is chewing. The saliva lubricates the feed for easier passage through the esophagus.
As explained by Peggy Auwerda, the horse has the smallest stomach in relation to body size of all domestic animals. Its capacity is approximately 2-4 gallons and comprises of around 10% of the total volume of the digestive tract. Therefore, feeding large infequent meals can lead to gastric distension and colic. Secretion of gastric acid continues for many hours even when the stomach is empty which can contribute to development of ulcers.
The small intestine is the tube that connects the stomach with the large intestine. According to Les Sellnow 2006, horses are susceptible to a variety of ailments, including colic, if they ingest toxic materials. With the horse the toxic material that lands in the small intestine is absorbed into the bloodstream before it can be detoxified.
As specified by Peggy Auwerda, the major functions of the hindgut are the microbial digestion of dietary fiber. Important end-products of the fermentation are volatile fatty acids which can serve as an energy source for horses. Fermentation also produces methane, carbon dioxide, water, as well as most of the B-vitamins and some amino acids.
The diet composition affects the makeup of the microbial population. When starch is delivered to the hindgut the starch fermenters begin to rapidly ferment the starch, producing large quantities of lactic acid and volatile fatty acids. Because of the acidic nature of these products of fermentation, the pH in the hindgut begins to fall. A low pH favors pathogenic bacteria which can then contribute to serious diseases such as founder, colic and endotoxemia.
A take home note: Horses are foragers, they need to be fed little and often. A choice of different meadow hays should be on offer at all times, as well as removing our horses off grass pasture and onto a track system of some sort to encourage movement.
References
Les Sellnow, 2006. https://thehorse.com/126346/the-equine-digestive-system-a-food-factory/
Jackson, J 2015, Paddock Paradise: A Guide to Natural Horse Boarding. Star Ridge company, Arkansas.
Peggy Auwerda, https://www.extension.iastate.edu/equine/blog/dr-peggy-m-auwerda/digestive-anatomy-and-physiology-horse
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