Ruminant Digestive System
By Susan Schoenian
sheep101.info
Why are sheep always chewing?
Sheep belong to the ruminant classification of animals. Ruminants are characterized by their "four"
stomachs and "cud-chewing" behavior. The cud is a food bolus that has been regurgitated.
There are about 150 different ruminant species
including cows, goats, deer, buffalo, bison,
giraffe, moose, and elk. Ruminant species can
further be classified as grazers, browers, or
intermediates. Grazers, such as sheep, cattle,
and buffalo consume mostly lower quality
grasses while browsers such as moose and
mule deer stay in the woods and eat highly
nutritious twigs and shrubs. Intermediates,
such as goats and white-tailed deer have
nutritional requirements midway between
grazers and browsers.
Awassi Ewe Chewing Her Cud
Kazakhstan
The primary difference between ruminants and simple-stomached animals, like people, dogs or pigs, is the
presence of a four-compartment stomach that includes the rumen, reticulum, omasum, and abomasum.
Often, it is said that ruminants have "four" stomachs. Llamas and alpacas are "pseudo-ruminants"
because they have a three-compartment stomach instead of four like ruminants. Horses are also not
ruminants; however they have a "cecum" that performs a similar function as the cow or sheep's rumen.
The Ruminant Digestive System
The rumen occupies a large percentage of the
abdominal cavity of the ruminant animal. It is a
large storage space for food that is quickly
consumed, then later regurgitated, re-chewed,
and re-swallowed in a process called cudchewing. Rumination or cud-chewing occurs
predominantly when the animal is resting and
not eating. Healthy mature sheep will chew
their cud for several hours each day.
Capacities of the Digestive Tract
Compartments in a Mature Sheep
Compartment
Capacity
Stomach
Reticulum
Rumen
Omasum
Abomasum
1.5-2.0 quarts
5.0-10.0 gallons
0.5-1.0 quarts
2.0-3.0 gallons
Small Intestine
2.0-2.5 gallons (80 ft)
Large Intestine
1.5-2.0 quarts
Source: Sheep Production Handbook (2002)
American Sheep Industry Association
The rumen is also a large fermentation vat. It
contains billions of micro-organisms, including
bacteria and protozoa, which allow ruminants
to digest fibrous feeds such as grass, hay, and
silage that other animals cannot efficienty
utilize. Fermentation in the rumen produces
enormous quantities of gas that ruminants
must get rid of by belching (burping). Anything
that interferes with belching is life threatening
to the ruminant and may result in a condition
called "bloat." Mild cases of bloat can be
treated with an antacid such as Mylanta.
The reticulum is closely associated with the
rumen. Contents mix continually between both
sections. It looks like a "honey comb."
Relatively little, if any, digestive activity occurs
in the omasum. It is also called "many plies"
because it contains many layers of tissue. The
abomasum is the "true stomach" of the
ruminant. It is similar in function to the
stomach of a non-ruminant: secretion of
enzymes and acids to breakdown nutrients.
Young Ruminants
At birth, the lamb's rumen and reticulum are
not yet functional. As lambs begin to nibble on
dry feeds, these two compartments become
"innoculated" with microogransims. As the
microbes multiply and begin to digest feed,
they stimulate the growth and development of
the rumen and reticulum. The lamb's rumen
and reticulum are usually functional by the
time it is 50 to 60 days old.
Young Lamb
Because baby lambs are not born with a
functioning rumen, supplemental feeds, such
as "creep" feed need to be very digestible.
Creep rations typically consist of cracked or
rolled grains and soybean meal. Creep feeding
enhances development of the rumen.
Grazers by Design
Katahdin Ewe and Lamb
Though ruminants can digest grain (starch),
their more natural diet is forages: grass,
weeds, browse, hay, and silage. If too much
grain is consumed at one time, a large amount
of lactic acid is produced in the rumen and the
pH of the rumen drops. This can be a fatal
condition to the ruminant animal. Grain must
be introduced gradually to ruminant diets to
give the rumen time to adjust. Sheep "love"
the taste of grain (It is like candy!) and will
overeat, if grain consumption is not regulated.
If grain is introduced slowly to the ruminant's
diet, grain can successfully be substituted for
forage, though some forage is always
necessary in the diet to keep the rumen
functioning properly and to keep ruminants
content.
Greenhouse Gases
Katahdin Ewe and Lamb
A global impact of ruminant livestock
production is that when ruminants belch, they
produce methane gas, one of the greenhouse
gases. A small amount of methane is produced
by manure. Scientists are currently studying
ways to reduce methane production from
domestic livestock. For example, it is known
that livestock fed certain plants produce less
methane. Australian scientists are testing a
vaccine to reduce livestock methane emissions.
Greenhouse gases are believed to contribute to global warming. The largest source of greenhouse gases
(by far) is fossil fuel burning.
. . New words . .
Ruminant - An animal with a multiple stomach (polygastric) system of digestion capable of digesting
cellulose.
Cud - food of a ruminant regurgitated to be chewed again.
Rumen - The first and largest of the four stomachs of ruminant animals in which initial digestion occurs
by anaerobic fermentation by bacteria and protozoa.
Reticulum - the second compartment of the stomach of a ruminant.
Omasum - the third compartment of the stomach of a ruminant.
Abomasum - the fourth compartment of the stomach of a ruminant.
Microbes - Minute living organisms, including bacteria, viruses, fungi and protozoa. The microbes in the
rumen are beneficial because they enable ruminants to digest fibrous plant materials.
Fermentation - process in which an agent causes an organic substance to break down into simpler
substances.
Bloat - swelling of the rumen or intestinal tract of domestic animals caused by excessive gas. Can be
fatal.
Creep feed - feed given to young animals isolated in a creep. A creep is a pen that is fenced so that
young animals can enter, but adults cannot.
Greenhouse gas - an atmospheric gas that tends to prevent heat from radiating back into space, thus
having a warming effect on the atmosphere. Examples: water vapor, carbon dioxide, methane.
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