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A basic understanding of digestion can help you
plan an appropriate diet for your horse.
The Ins and Outs
of Digestion
Story & Illustration
by Lori K. Warren, Ph.D.
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As
a horse owner, you likely make a conscientious effort to select quality
feeds for your horse to eat. You are also probably well acquainted
with the by-products of digestion each time you clean your horse's
stall. But how often do you think about what takes place in your horse's
body in between?
Selecting the Best
Digestion begins with the prehension (grasping) of food by the lips,
tongue and teeth. Horses have very mobile lips that help them select
desirable feeds and avoid those that are undesirable. Many of us have
witnessed this lip dexterity-ever wonder how some horses skillfully
sort out powdered medications you have tried to hide in their sweet
feed, or what about the soggy pellets left behind in the feed tub
that were deemed unworthy?
Chew Your Food!
The next step is chewing, which is an extremely important part of
the digestive process. Chewing helps to break down large food particles
into smaller ones so that digestive enzymes and microbes have a greater
surface area to extract the nutrients. Horses with poor dentition
(a common ailment of older horses) or horses that "bolt"
their feed and fail to chew properly are at greater risk for choke
and impaction colic. Feed that is inadequately chewed is also harder
to digest, so weight loss may become an issue because the horse gets
less out of the feed than horses that chew thoroughly.
Horses spend more time chewing forages (hays or pasture grasses) than
grains or pelleted feeds. The average horse will spend about 18 minutes
eating one pound of long-stem hay, whereas it will take him less than
5 minutes to eat a pound of oats. Therefore, whenever grains are added
to the diet, the horse spends less time eating. An unfortunate side
effect of reducing the horse's "chew time" is boredom and
behavioral vices, such as wood or tail chewing.
The nature of the feed also affects saliva production. Twice as much
saliva is produced when horses eat forage compared to grains and other
concentrates. Saliva moistens the feed, making it easier to swallow,
and it's also rich in bicarbonate, which helps buffer the acid produced
in the stomach.
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Acid
Attack
After being chewed and swallowed, the ingested feed passes into
the stomach. The horse's stomach is relatively small for its large
body size, approximately the size of a small watermelon. A horse's
stomach functions similar to a human's stomach, producing acid and
the enzyme pepsin, which initiates the breakdown of proteins in
the feed.
In humans, dogs and pigs, acid is only secreted by the stomach in
response to a meal, but in the horse acid is produced continuously.
Horses evolved as grazing animals, often foraging for 16 or more
hours per day. Therefore, a steady production of acid was necessary
to process small meals consumed at frequent intervals. However,
with today's practice of feeding a distinct breakfast and supper,
many horses spend a significant amount of time between meals without
anything in their stomachs to absorb the acid. Furthermore, when
the horse isn't chewing, there is no saliva produced to help buffer
the constant flow of acid. Adding insult to injury, the top one-third
of the horse's stomach is made up of tissue similar to that found
in the esophagus and can easily be destroyed by the potent stomach
acids (basically the acid digests the stomach lining). Consequently,
limiting intake to one or two meals per day allows acid to build
up in the stomach between meals and increases the risk of gastric
ulcers.
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The
Horse's Digestive Tract
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Extraction
Extraction and absorption of nutrients begins in earnest once the
ingested feed reaches the small intestine. Although feed spends a
very short amount of time traversing this 70-foot long tube (1-3 hours),
the small intestine is the primary site for the digestion and absorption
of sugar, starch, protein, and fat. Vitamins A, D, E and K, as well
as calcium and some phosphorus, are also absorbed from the small intestine.
The horse's small intestine is very good at breaking down and absorbing
simple sugars, such as those found in molasses or very leafy pasture
grasses. However, the horse is not as efficient at digesting starch,
which is a major component of cereal grains (oats have ~50 percent
and corn ~70 percent starch). Rolling, flaking and other forms of
grain processing help to increase the amount of starch digested in
the small intestine. Nonetheless, with large grain meals, there is
a risk that undigested starch will reach the large intestine (more
on this later). As a result, a meal of grain should typically not
exceed 0.5 percent of the horse's body weight (about 6 pounds for
a 1200-lb horse).
In comparison to starch, the horse's small intestine is very good
at breaking down proteins into their constituent amino acids. Similarly,
even though the horse lacks a gall bladder, he is quite capable of
digesting fats. Forages and cereal grains are naturally low in fat
(~3 percent), but studies have shown that horses can effectively digest
10 percent fat in the total diet. If you choose to use fat to provide
additional calories, make sure to increase the amount gradually over
two or three weeks to allow the digestive system to adjust.
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The
Fermentation Vat
Once digesta leaves the small intestine it flows into the large intestine
(or hindgut). The first segment of the large intestine is the cecum,
which is equivalent to the human appendix. However, in the horse,
the cecum is very well-developed, with a capacity of about 12 gallons.
In fact, the cecum, combined with the large and small colon, make
up over 65 percent of the total capacity of the equine digestive tract.
The hindgut houses billions of bacteria, protozoa and fungi, which
assist the horse in the digestion of fiber found in hay or pasture.
No animal possesses the enzymes needed to digest these fibrous feeds;
therefore, herbivores like the horse rely on these microbes to process
the fiber into something useful they can absorb.
Microbes break down cellulose and hemicellulose, both of which are
forms of fiber, into volatile fatty acids. These volatile fatty acids
serve as an important source of energy for the horse. In addition,
the microbes in the hindgut produce vitamin K and the B vitamin complex.
As a result, a healthy horse consuming a high-quality, high-forage
diet will generally not require these vitamins in their diet.
Lignin, another form of fiber, cannot be processed by either the horse
or the microbes living in the hindgut, and will be passed in the manure.
The amount of lignin, as well as other types of fiber, greatly influence
the nutritional value of the forage consumed by the horse. For example,
overly mature grass hay will be relatively high in lignin, which depresses
the overall digestibility of the fiber. In contrast, young leafy grass,
beet pulp and soy hulls contain lower amounts of lignin and higher
amounts of hemicellulose and soluble fibers, making them highly digestible.
Another important function of the large intestine is the absorption
of water. The horse's digestive tract secretes a large volume of water
(~35 gallons) to aide in the digestion process. This fluid is reabsorbed
as digesta passes through the hindgut, allowing the formation of semi-solid
fecal balls in the rectum.
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Proper
function of the horse's large intestine is highly dependent upon the
health of the microbial population. These microbes do their job best
when the horse is in its natural environment grazing pasture or when
the horse is fed an all-forage diet. Heavy grain feeding can upset
this delicate balance. When the capacity for starch digestion in the
small intestine is overwhelmed, undigested starch from grain enters
the large intestine. Starch is rapidly fermented in the hindgut, producing
lactic acid and decreasing the pH. As the hindgut becomes more acidic,
some of the microbes die off and produce harmful toxins that may lead
to diarrhea, colic, and/or laminitis.
What the Digestive Tract Tells Us About Feeding
The horse's digestive system functions best when it is fed a predominantly
forage diet on an almost continual basis. Problems are more likely
to arise when a horse is fed a high-grain, low-forage diet, particularly
when given in one or two large meals per day. If your horse needs
more calories than can be supplied by an all-forage diet, try to reduce
the amount of starch by using grain mixes that include highly digestible
sources of fiber (e.g., beet pulp, soy hulls) and by adding fat to
the diet. If possible, spread the daily grain allotment over three
or more meals. Finally, allow your horse to nibble on hay or pasture
as much as possible throughout the day.
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Lori
K. Warren, Ph.D, PAS
Assistant Professor, Equine Nutrition
University of Florida |
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