Nutrition
for Maximum Performance
By Eleanor Richards
Copyright © 2003
LESSON
UNDERSTANDING THE EQUINE DIGESTIVE
SYSTEM
The equine
digestive system is like a disassembly plant.
It takes feed (forage, concentrates, supplements, water) and breaks them
down into a form the body can utilize.
There are two methods of processing: mechanical and chemical. The equipment used are
the organs that make up the digestive tract.
Mechanical processing
starts in the horse’s mouth as he uses his lips to sort and choose the desired
feed. While grazing he will use his
incisor teeth to bite off the grass. The
tongue then moves the material back to the molar teeth where it is chewed and
swallowed. The stomach continues to
break down food with a churning action.
Chemical processing is present through out the digestive tract. It starts with the salvia in the horse’s
mouth and ends in the large intestine.
Bacteria, enzymes, and other substances are used to release the nutrients needed by the horse.
Horses are non-ruminant herbivores – they eat plants, but do not have a
rumen. Animals with a rumen (cows,
sheep, and goats) swallow the feed directly into a fermentation vat, the rumen,
where it is broken down by bacteria. It
is then regurgitated and chewed again before being re-swallowed. But, horses are hindgut fermenters. The feed a horse eats goes through the
chemical and enzyme process before it is fermented in the hindgut. Horses do not chew a cud. This method of digestion is the reason horses
need plenty of good quality fiber.
The best way to learn about the equine digestive system is to follow a
meal, as it is broke down and processed.
SALIVA
Once the horse has gathered the food into his mouth he begins to
chew. The chewing action stimulates
three pairs of salivary glands – the parotid, the submaxillary, and the
sublingual. A horse will produce
approximately 5 to 10 gallons (19 to 38 liters) of saliva per day.
If a horse does not get enough chew-time the feed is not moistened
sufficiently. Senior horses or horses
that bolt their feed can have problems with the feed causing an obstruction in
the esophagus (choke).
TEETH
The teeth are very important to the health of the horse. Many nutrition problems start here, so an
understanding of the healthy equine mouth is important.
By the time a foal is a week old he will have 4 incisor teeth – 2 on the
top and 2 on the bottom, these will be in the center. In another week the second set of incisors
will come in and by 6 months of age the corner incisors will be in. Also, the pre-molars will be coming in at
about the age of 2 weeks. There will be
6 pre-molars top and bottom. So, by the time the foal is 6 months of age he
will have all his baby teeth, a total of 24 teeth.
The baby teeth, or caps, will start to shed by the age of 2 ˝
years. The erupting permanent teeth will
shove the caps out of the way. You may
find them in the feed box. There are
times when the caps will not be pushed out and this can lead to problems. An equine dentist or veterinarian will need
to remove the cap, so the permanent tooth will come in properly.
Most adult mares will have 36 teeth and males can have 38 to 40. The extra teeth males have are called canine
teeth. Mares do not usually get them and
if they do they will be very small.
Canine teeth will not erupt until 4 to 5 years of age.
A type of tooth that may cause problems is a wolf tooth. They are found in front of the molars. Wolf teeth do not serve any purpose. They may interfere with the bit. Many equine dentists and veterinarians
re-commend they be removed. They have
short roots and are easily extracted.
Wolf teeth usually erupt between the age of 12 and 18 months.
The upper jaw is wider than the lower jaw and at rest the back teeth do
not meet. As a horse chews his lower jaw
moves side to side and this action causes the teeth to grind together. This grinding action causes the teeth to wear
away. Below the surface of the gum-line
is almost 4 inches of tooth, which erupts continuously as the exposed surface
wears away. So, by the time a horse is
in his twenties the amount of tooth left to erupt is a lot less than a horse
that is 4 years old.
Routine dental examination is important to how well a horse will utilize
his feed. If all works well the teeth
will wear away evenly, but that is rarely the case. Malocclusion, a term used to describe when
teeth do not meet properly can occur.
Overtime, points (sharp edges) can develop on the inside of the lower
teeth and the outside of the upper teeth.
These sharp edges inhibit the side-to-side movement of the lower jaw and
soon the horse has lost the full range
of the chewing action. These points are
also very sharp and can lacerate the inside of the mouth and side of the
tongue. Horses will pack material
between the teeth and cheek to try to pad the sore area. An equine dentist or veterinarian will
“float” or file off these points. This
procedure does not hurt, because the adult equine tooth does not have nerves or
blood supply.
A missing tooth can also cause problems.
The corresponding tooth continues to erupt, but is not being wore off.
Soon it is sticking above the rest of the arcade of teeth and the horse cannot
chew properly. This tooth will need to be filed on a regular basis to prevent
malocclusion.
The type of feed the horse eats will also affect the wear of his
teeth. A horse that has a diet of only
pellets or complete feed without any long stem fiber will have uneven wear of
teeth. A horse that eats this type of
diet or only hay will also develop long incisors. Because he is not nipping grass off with his
incisors they can become long, which prevents the molars from grinding the feed
properly.
Malocclusion can also be a genetic problem. Horses with inherited tooth problems will
need to be monitored regularly and offered feed they can utilize.
Once the food has been chewed and softened with salvia the horse
swallows.
ESOPHAGUS
The esophagus is a muscular tube that leads to the
stomach. The length is about 50 inches
for an average sized horse.
Choke, an obstruction of the esophagus, is a feed related concern. A horse that bolts his grain or does not have
enough salvia to soften the feed can get the dry material stuck in his
esophagus. Also, a horse that has tried
to swallow a corncob or other large object can develop this problem. As the horse continues to eat the feed cannot
pass by the obstruction and the material builds up in the esophageal tube.
The symptoms of a horse choking are frequent attempts to swallow,
stretching out of the neck, excessive salivation, and feed material mixed with
saliva coming out of the nose. A
veterinarian should be called immediately as severe damage to the esophagus can
result. Unlike humans, horses can still
breathe while choking, but immediate care is still needed.
Choke can be avoided. A horse that bolts his feed should have several
large rocks placed in his feeder. He will need to move them around to eat his
ration. A horse that has an inflamed esophagus or does not have enough moisture
to break down the feed should be provided with a feed that can be
pre-soaked. This will aid in the passage
of the feed to the stomach. Soaking the
feed should also be done for seniors or horses that have tooth problems which
result in them not being able to chew properly.
Avoid feeding horses large pieces of apples,
carrots, cubes, and other items.
A unique feature of the equine esophagus is the very strong cardiac
sphincter valve. This valve is located
where the esophagus joins the stomach and prevents food from being
regurgitated. It also prevents horses from burping. The horse’s stomach will usually rupture
before this valve allows the material to be regurgitated. This results in death and is why feed must be
of the quality and quantity the horse can handle.
STOMACH
The stomach of the horse is very small in relationship to the size of
the horse. It only makes up 8-10% of the
capacity of the digestive system. The
average horse’s stomach will only hold about 4 ˝ gallons (17 liters) and is
about the size of a football. The
stomach breaks food down mechanically and chemically.
There are four sections to the equine stomach: the esophageal section,
the cardiac section, the fundic section, and the pyloric section. The oesophageal is basically a storage area
and no digestive action takes place here.
The cardiac section produces mucous used to coat the stomach and protect
it from acidic secretions. The fundic
section produces pepsin enzymes, gastric lipase enzymes, and hydrochloric
acid. The pyloric section produces a
small amount of enzymes and mucus. The
stomach has an overall low pH level, but the pyloric section is highly acidic.
Most of the digestion, which occurs in the stomach, takes place in the
fundic section. Pepsin breaks down
protein into amino acids, which can then be utilized by the horse. The feed is also mixed with hydrochloric acid
and bacteria that produces lactic acid. These stomach secretions do not break down
fiber, so very little forage is digested in the stomach.
The muscles in the stomach uses rhythmic contractions called
peristalsis. This mechanical action
helps break down feed into smaller particles.
The design of the equine stomach is such that a horse needs small
frequent meals. It fills quickly and
empties quickly. Feed usually only
remains in the stomach for about 20 minutes.
Ulcers can result if the stomach is allowed to become empty.
Feeding a horse too much grain
at one feeding causes the excess to be pushed into the small intestine before
the digestive juices have a chance to break it down. Colic will be the result.
The pyloric sphincter is located where the stomach joins the small
intestine. Once the meal is broken down
the valve relaxes and the feed passes through to the small intestine.
SMALL
INTESTINE
The small intestine is a tube that measures about 70 feet
(21 meters) in an average sized horse.
Its capacity is approximately 15 gallons (56 ˝ liters). Most of the nutrients from the grain portion
of the diet are absorbed in the small intestine.
The breakdown of these nutrients relies on chemicals secreted by the
pancreas, liver, and intestinal mucosa.
Pancreatic enzymes help digest the feed.
Because the horse has no gallbladder, bile is constantly flowing in from
the liver. Bile breaks down fat. The available nutrients are then absorbed
through the walls of the intestines.
Feed passes very quickly through the small intestine. In as little as an hour some types of feed
will have moved through. However, it
usually takes up to 8 hours for most of the feed to reach the end of the small
intestine. It is very important to the
health of the horse that soluble carbohydrates are absorbed in the small
intestine. If they reach the large
intestine fermentation can occur which will result in gas and colic. To avoid overloading the small intestine
small frequent meals are strongly recommended.
Starch, complex sugars, protein from the grain portion of the diet, fat,
fat-soluble vitamins (A, D, and E), and most minerals are absorbed in the small
intestine. Protein from roughage, fiber,
and phosphorus is not digested well in the small intestine.
The small intestine consists of three sections: the duodenum, the
jejunum, and the ileum.
The duodenum prepares the food for absorption by mixing it with enzymes
and chemicals. Very little of the nutrients are utilized here.
The jejunum is the longest portion of the small intestine. Most of the nutrients from the grain part of
the diet are absorbed in the jejunum.
Lining the inside of the intestinal wall are tiny projections called
villi. The villi absorb the nutrients,
which are now molecules and allow them to be utilized by the horse.
The ileum is the last section of the small intestine. The absorption of water and water-soluble
vitamins takes place here.
The insoluble carbohydrates (fiber), any starch, or other material that
was not utilized in the small intestine now moves on to the large intestine.
LARGE
INTESTINE
The large intestine is 25 to 30 feet (7 ˝ to 9 meters) long in an
average sized horse. It has the largest
capacity of the equine digestive system at approximately 20 to 30 gallons (75 ˝
to 113 ˝ liters). Feed will remain in
the large intestine for about 48 to 65 hours.
Digestion of insoluble carbohydrates takes place in the large intestine. Protein from the fiber portion of the diet is
utilized. Phosphorus is absorbed. Any starch that was not digested in the small
intestine will be worked on by microbes – which can cause gas and colic. Some B vitamins and vitamin K will be synthesized
in the large intestine. (Definition of synthesis: the combining of parts to
form a whole.)
There are five sections that make up the large intestine: the cecum, the
large colon, the transverse colon, the descending colon (small colon), and the
rectum.
The cecum is a fermentation vat.
It has been described as a comma shaped organ. The cecum is about 4 feet
(1.2 meters) in length and has a normal capacity of approximately 8 gallons (30
liters). The size can vary according to
the quality of roughage being fed to the horse.
The entrance and exit openings are both located at the top of the
cecum. This can cause problems resulting
in colic if the organ becomes compacted with non-digested material. Most feed remains in the cecum for about 7
hours.
Within the cecum are microbes that aid in the digestion of cellulose,
fiber, and any remaining soluble carbohydrates.
These microbes are somewhat specific as to what they digest. If a drastic feed change occurs the balance
of the bacteria and protozoa is disrupted and colic can result. Any change of feed (concentrates or forage)
must be made over a period of about 3 weeks, so the microbe population can
adjust.
A horse that has a hay belly is the result of an expanded cecum. Poor quality fiber has accumulated in the
cecum and can remain there for up to 72 hours.
Switching to less mature hay will be more beneficial to the horse and
prevent a hay belly from developing.
The cecum then leads to the large colon.
The opening leading from the cecum to the large colon is about a 3-4 cm
hole. It is a prime spot for blockages
to occur.
The large colon is approximately 20 feet (6 meters) long and is shaped
like a horseshoe. Microbial digestion
continues in the large colon. Any
remaining vitamins, fatty acids, and water are absorbed.
The large colon is made up of 5 segments (listed in the order in which
the feed travels): the right ventral colon, the left ventral colon, the pelvic
flexure, the left dorsal colon, and the right dorsal colon. The large colon folds back on itself several times and is held in place by the bulk, which
it contains. At several points the colon
narrows and blockages can occur in these narrow passageways.
The design of the large intestine requires plenty of good quality forage
be available to the horse.
If the colon becomes empty the chances of it becoming twisted increases.
The remaining material then travels through the transverse colon and
into the descending colon (small colon).
The transverse colon is a very narrow tube and another primary spot for
blockages.
The descending colon uses contractions (peristalsis) which results in
fecal balls forming. Any remaining
moisture is also extracted in the small colon.
It measures about 12 feet (3 ˝ meters) long and is very narrow at about
3-4 inches (7 ˝ - 10 centimeters) in diameter.
The small colon is also free to move and can become twisted.
The descending colon then leads to the rectum. The rectum is a short straight tube and acts
as a storage area for the fecal balls until the horse expels them.
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