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Equine Science
Digestive Physiology
The Digestive Tract
Horses are non-ruminant herbivores
“simple stomach” animal
Utilize 1 stomach
Able to utilize large amounts of
roughage due to their relatively large
cecum
Cecum is a section of the colon where
digestive bacteria break down roughage
The Digestive Tract
The 5 functions of the digestive system
are:





Ingestion
Grinding
Digestion
Absorption of nutrients
Elimination of waste
Mouth
Mouth- first part of
digestive system.
Mouth has 2 main
functions- masticate
food and breakdown
food with saliva.
Functions of Saliva:
 Digest buffer for pH
 Moisten mouth
 Aids in taste
Teeth
Top and bottom
bridge.
Should be examined
by professional to
check for sharp
edges that must be
floated or filed down
Breakdown feedstuff
mechanically
Digestive tract cont.
The esophagus- 50- 60 inches long in
an adult horse.

Muscular tube that extends from the
pharynx to the cardia of the stomach
Horses have striated muscles in the
beginning, and smooth muscles at the
end of the esophagus.
Digestive tract cont.
Sphincter - one way door that allows
food to come in to the stomach, but not
out.
Stomach may actually rupture before
animal will vomit.
Once in the stomach, the matter in
considered “digesta”
Stomach Cont.
Stomach- relatively
small.
Therefore horses
should be fed several
small meals per day.
Should feed by
volume not
weight!
Stomach - made up
of 4 layers

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Serosa (outer)
Muscularis
Submucosa
Mucosa (inner)
Stomach Cont..
Mucosa is the most inner layer, it is a muscle
Muscle contractions causes the digesta to
mix with Chemicals and enzyme secretions
Mucus - functions to protect muscles from
hydrochloric Acid (HCL), and aids in
digestion.
Hydrochloric Acid (HCL) - aids in digestion,
1st form of chemical digestion, kills bacteria
Stomach Cont..
5 Regions





Cardia - sphincter region
Esophageal - non glandular
Cardiac- produces and secretes mucus
Fundic- produce HCL and mucus, also
produces pepsin and rennin
Pyloric - Secretes Prodeolytic enzymes,
sphincter muscle as well
Small Intestine (SI)
Also known as the “upper gut”
The Small Intestine- contains 30% of
the capacity of the entire tract.
Digesta in the small intestine is 93-95%
liquid.
Most digestion and absorption takes
place in the small intestine
Small Intestine
The same layers in the stomach are present
in the intestine.
Mesentery - membrane that holds the
lymphatic and blood vessels to the intestine
Small Intestine
Mesentery
Small Intestine
The rate of passage is rapid through the
SI.
Grinding or chopping feed will increase
the rate of passage, therefore
decreasing????
Absorbs soluble carbohydrates (CHO)

Soluble
Small Intestine
Lumen- folds inside
the small intestine.
Villi - fingerlike
projections on the
lumens

Increase surface area
Microvilli - fingerlike
projections on the
villi that absorb
nutrients
Small Intestine
Villi and Microvilli
Small Intestine
There are 3 parts to the small
intestine.
 Duodenum
 Jejunum
 Ileum
Small Intestine
Duodenum (longest portion)



The pancreatic duct (pancreas) and bile
duct (liver) empty digestive juices into the
duodenum
A majority of absorption of nutrients occurs
in this section.
Extends to the pelvic region, along the
lower right side of the animal. It then
crosses over to the left and turns back to
the head
Small Intestine
Jejunum and Ileum

A lower percentage of absorption takes
place in these 2 regions of the small
intestine
The only way to differentiate the
different regions of the small intestine is
through a microscope.
Absorption
Proteins, CHO, fats, minerals and
vitamins are all digested in the small
intestine
Water is absorbed in the large intestine
Absorption Methods
Passive Diffusion = requires no energy
Active Transport = requires energy
Passive Transport = requires a carrier to
get across membrane
Large Intestine
Also known as the “hind gut”
Composed of Cecum, colon (large and small),
and rectum
Horses have the largest and most complex LI
in domesticated animals, due to their large
cecum
Functions to move undigested material from
the SI to the anus for elimination
Makes up about 50 percent of the total
capacity of the digestive tract.
Large Intestine
Approximately 25 feet long
Can digest cellulose, starch, and sugars into
volatile fatty acids
The horse's large intestine is designed to
utilize plant fiber.
Volatile fatty acids - Acetic, buteric, and
propionic acids that are produced by
microbial digestion of CHO
This VFA can supply up to 1/4 of the energy
used by the horse
Large Intestine
Vitamins E and K are synthesized in the
large intestine. These are fat soluble
vitamins.
Bacteria in the LI produce some amino
acids, but mostly fatty acids
Fatty acids = energy
Amino acids = protein synthesis
Large Intestine
Cecum - primary water absorption site.

Fermentation occurs in the cecum, similar
to the action that takes place in the forestomach of the ruminant
Rate of passage through the LI is
relatively slower than other parts of the
digestive tract.
Large Intestine
Rectum



Connects the small colon with the anus.
Receives feces from the small colon, which
formed the characteristic balls of horse
manure.
Reabsorbs water that was not absorbed in
the cecum.
Large Intestine
Feces
The size, shape, and consistency of feces
give an indication of the general health of
the horse.
 If the feces is hard and dry, then the horse may
be deficient in water or protein.
 If the feces is moist or soft, the horse may be
sick or consuming a diet that is too laxative.
Rate of passage
95% of all food eaten will be excreted
in the form of feces within 65-75 hours
of ingestion
Factors Affecting Digestion
Processing of feedspellets require less
digestion than whole
grains.
Feed processing
Grinding feed
speeds rate of
passage therefore
decreasing
absorption.
Feed Processing
Crimping oats will
increase rate of
digestion.
Factors affecting Digestion
cont.
Level of intake- the more grain eaten,
the less digested
Frequency of feeding- frequent feeding
can increase level of digestion.
Terminology to know
Non- Ruminant
Roughage
Ingestion
Digestion
Mouth
Teeth
Esophagus
Pharynx
Stiated muscle
Smooth muscle
Peristalsis
uppergut
hindgut
mesentery
CHO
HCL
pepsin
rennin
Terminology to know
Sphincter
Lumen
Villi
Microvilli
Absorption
methods (3)
Volatile Fatty
acids
Amino Acids
Fat soluble
vitamins
Digesta
Saliva