I've always been fascinated by all things equine, including their anatomy and how their various body systems work. As a project to learn more about these amazing animals, I've decided to do a series of posts on the equine digestive system. As these come along, they'll be put in a new sidebar. (Please keep in mind that I am neither a vet nor an expert on these topics - if you have corrections and additions or resources to direct us to, please put that in the comments.)
Before getting started on a discussion of the equine digestive system, it makes sense to talk about the diet that goes with that digestive system and that makes that system work best. Many of the problems our horses have with their digestive systems, including many dental issues, choke, ulcers, colic, laminitis and other hoof problems, relate to the mismatch between how the digestive system works and how we feed and house our horses. The horse's complex digestive system is designed to accommodate almost continuous intake - think conveyer belt - of a high-volume variety of quality forage selected by the horse that is high in structural carbohydrates (and abrasive silica) and relatively low in non-structural carbohydrates (think sugars, although that isn't he exact definition), and that contains an appropriate amount of vitamins and minerals from the soil in which the forage grows. The functioning of that digestive system is enhanced by the ability to access fresh, clean water at all times and to continuously move - movement improves the functioning of the digestive system - while foraging in the company of other horses - being safe in a herd reduces equine stress. (Note: for those of you who are technically-minded, non-structural carbohydrates (NSCs) may or may not be the correct term to use as thinking on these topic develops, but think easily-digestible carbohydrates. I will use the term NSC in these posts. For more information on these topics, please visit safergrass.org - this site is an excellent resource.)
Many horses, including my two that live with me, have conditions that only approximate what is required for the optimum functioning of their digestive systems. They are stalled at night, and do not have the opportunity to eat or move continuously. They are on a low NSC diet and do have free-choice access to high-quality forage all day and for most of the night. Our grasses are too profuse in the summer resulting in over-consumption of too easily digestible non-structural carbohydrates (NSCs) and weight gain, to which some horses are more sensitive than others. But my horses are better off than some who are stalled almost continuously and fed higher-NSC diets, often from large amounts of small-particle grain concentrates. Most of the ways we handle and manage our horses are a compromise between the needs of the horse and our convenience, and this is a fact of life for many of us and our horses.
Horses are also very choosy eaters, unlike many other grazing animals. They search out and eat only the forage and browse that they find palatable - not only grasses, and their choices may vary based on nutrient needs and season. Our horses will often selectively graze high vitamin C forage - like dandelions and emerging thistles - when it is available.
The equine digestive system is a marvel - its structure and functioning cope effectively with a high-volume, extremely abrasive and hard-to-digest forage diet. There's a lot going on inside your horse as he eats and digests, so I'll be dividing these posts into a series covering separate parts of the digestive system. Here's a list of the components of the digestive system:
Mouth - including lips, teeth, tongue and salivary glands
Small colon, rectum and anus
You'll notice there's something in this list that looks odd - cecum. Horses are what are know as post-gastric fermenters - most of the hard work of digestion takes place in the hind-gut, particularly the cecum and large colon. This is unlike ruminants like cattle, which have the rumen available for pre-digestion and even detoxification. (This is one reason horses may be more sensitive to toxins than cattle - they don't have the rumen for detoxification functions.) The digestion/fermentation in the hind gut not only produces calories and nutrients for the horse, it also serves an important heat-production function - there's some truth to calling horses "hayburners" - the digestive process serves as a furnace.
Here's a diagram of the equine digestive system:
Here's another diagram that is useful:
This article has excellent images showing where the various digestive organs lie in the horse's body, and the spatial relationships of these organs one to the other.
The next post in this series will be on the equine mouth.