Let's Ruminate On It

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.

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 cud-chewing. 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.

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.

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

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

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.