Thursday, December 9, 2010

The Equine Digestive System: Overview

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
Esophagus
Stomach
Small intestine
Cecum
Large colon
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.

15 comments:

  1. I thought to myself today, that perhaps luncheon would be nice while reading the blogs?
    So it was that half way through a rather nice corned beef and onion sandwich, with the trimmings, I clicked onto your wonderful blog. Ordinarily I would have read avidly the information therein contained. However, due to the content and the extended diagramatic, I feel I must defer the pleasure, until some time later. The Corned beef sandwich will have to wait! Sorry!

    Lol!!Couldnt resist!

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  2. Good post, Kate! Can't wait to read more. I am very interested in the inner workings of the horse. Always glad to learn more about these amazing creatures. Thank you!!!
    jane

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  3. Your descriptives look pretty spot on to me Kate ! Excellent post !

    I am a fan of 'Nutrient Requirements of the Horse' from National Academies Press for day to day nutrtion information. It's not technical but it's also not the sort of book one reads from cover to cover in one sitting !

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  4. Cheyenne - sorry to ruin lunch!

    Jason - thanks for the recommendation - I'll have to get a copy!

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  5. Thanks for the topic. My horses have the 24/7 turnout option, so movement is not a problem. What I lack is the 24/7 forage. My grass is limited, so I need to constantly supplement with hay. I keep thinking one of the big round bales might be an option.

    But I do feed three times a day, with a pelleted feed and, of course, hay.

    I am all too familiar with colic, unfortunately, and well aware of the somewhat "delicate" nature of the horse's digestive system. When all is well, it's a marvel. When things go wrong, a nightmare.

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  6. Fascinating post! I can't wait to hear more. :)

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  7. Looking forward to the rest of the series. Very informative! I think a lot of us horse owners know the basics, but posts like this really help fill in the blanks.

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  8. Excellent post! Have you ever heard of the "track system"? I think that's the right name. My friends introduced it to me. Apparently, even horses out on pasture 24/7 who are fed round bales, etc, don't get enough of the movement you described, so the track system is set up to keep them walking all day for food and water. Your pasture is fenced off in a track (center area for grazing in spring, summer and fall) and you have stations throughout and particular flooring to keep their hooves worn down. They delivered hay on the tractor or 4-wheeler by spreading it all along the track. I thought it was an awesome idea, but haven't done it myself. :(

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  9. Linda - I have heard of the track system - it sounds very good, both for digestion and hooves/bodies. We don't have anything like that at our barn - and it's true in the winter that the horses mostly stand around with their heads in the round bales.

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  10. Depending on the study you read some will show the track system increases movement and others will show it actually decreases it. It is amazing what a controversial topic that can be!!

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  11. Very awesome study Kate. Can't wait to read more from your findings!
    Thanks ~

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  12. Very, very interesting! Thanks for taking the time to share this. It's fascinating!

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  13. I know this is an old post, but today in class we were talking about Science Fair and Project Based Learning projects that we could do to get 100 pts in our science class. Immediately, I thought about doing something horsey. What jumped to my mind was the equine digestive system, and I knew you had a series about it so as soon as I got home I jumped on this blog and began reading! Thank you so much for writing these, they are going to be very useful. May I use them to reference in my project and research paper?

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