Download The Digestive System

The Digestive
System
The main parts of the cow’s digestive system are illustrated in
the diagram below. Ingested feed is chewed, mixed with saliva
and formed into a bolus before it passes down the esophagus
into the reticulum and rumen, the first two
rectum
dorsal sac of rumen
sections of the four compartment
anus
esophagus ruminant stomach. Later, boluses (cuds)
duodenum
of feed are regurgitated and further
broken down by chewing. Whenever
omasum
cecum
feed is chewed, saliva is secreted from
reticulum
salivary glands in the mouth. Over the
abomasum
course of a day, a lactating cow will
spiral colon
ventral sac of rumen
secrete more than 200 litres of saliva
small intestine
which has three main functions:
•
it
lubricates
feed as it moves down the esophagus;
The main parts of the cow’s
• it provides the liquid flow which carries feed particles into
digestive system.
the omasum, and;
• it contains buffers which prevent rumen pH from falling too
low as microbes produce acids from feed fermentation.
STARCH
GRANULE
BACTERIA
PROTOZOA
Electron photomicrograph of
bacteria and protozoa digesting Barley starch granules.
On entering the rumen, an
extremely varied population of
bacteria, protozoa and fungi
attach themselves to feed and
begin the breakdown process.
Enzymes are secreted onto the
feed and into the fluid
contents of the rumen.
Distinct microbial populations
digest specific types of feed.
For example, starch digesters
are distict from fibre digesters.
BACTERIA
The inner lining of the rumen is
not unlike pile carpet, having innumerable small, flat
projections called papillae. These serve two primary functions.
They vastly increase the area for absorption of nutrients and
they also provide attachment sites for additional, specific
populations of bacteria. For example, the bacteria responsible
for digesting urea are mainly found attached to papillae.
continued...
The Digestive
System (continued)
Continual mixing of rumen contents is essential to efficient
fermentation. The muscular walls of the rumen and reticulum
produce waves of contraction travelling their combined
lengths at about half-minute intervals. This process, in
addition to mixing the rumen contents, facilitates both
regurgitation for further cud-chewing and eructation, which
releases gases produced by fermentation (mainly hydrogen
and methane). Under some conditions (e.g., grain overload)
these contractions may stop, resulting in rumen stasis and
placing the animal at serious risk of bloating.
After the feed has been sufficiently chewed and broken down
by microbial action, digesta enter the omasum. Flow into this
third segment of the stomach is regulated by a small opening
called the reticulo-omasal orifice which prevents large
particles from leaving the rumen. The omasum is a muscular
organ which is thought to have two main functions. The first is
the extraction of water from the digesta, yielding a product for
further digestion which has a significantly higher proportion
of dry matter. Secondly, the omasum serves as a pump,
propelling digesta from the rumen and reticulum into the
fourth segment of the stomach, the abomasum.
The ruminant abomasum is analogous to the true stomach of
monogastric animals such as humans, pigs, dogs and cats. The
digestion of proteins, fats and oils is initiated by acids and
enzymes secreted by the tissue lining the abomasum. As
these initial stages progress, the digesta pass to the duodenum,
the first segment of the small intestine. Here, bile from the
liver and gall bladder along with digestive enzymes from the
pancreas are added. Digestion and absorption of its products
progress as the digesta pass through the two lower segments
of the small intestine, the jejunum and the ileum.
Digestion is almost complete by the time the unabsorbed
digesta reach the large intestine. One of the main functions of
this part of the system is the absorption of water and minerals.
In addition, some further breakdown is carried out here by a
permanent population of microbes (bacteria and protozoa)
with some of the products being absorbed into the blood.
Food material which has escaped both enzymatic and
microbial digestion is excreted.
source: Dairy Production Primer - Feeds & Feeding Management
©2004 Western Dairy Science Inc.