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The rumen is a stomach-like structure of the digestive system of certain animals that is characterized as a pre-digestive chamber in which critically symbiotic microorganisms live to initiate the breakdown of the animal’s specific diet. Commonly called a paunch, animals possessing this anatomy are called ruminants, and most are herbivores whose dietary carbohydrate requirement is supplied by plants that are difficult to digest. Much is well known about the various organisms that reside within a rumen and their chemical roles in the digestive process, in part because many ruminant animals, such as cows and sheep, are important commercial livestock in many parts of the world.
"Reticulorumen" is the term given to the first internal organ of a ruminant’s digestive tract. It typically is very large — the rumen of a cow might be more than 25 gallons (94.6 liters) in capacity — and its contiguous reticulum chamber is about one-tenth as large. Although the inner lining of the two differ, they have a singular function — to store chewed plant matter while trillions of bacteria, single-celled protozoa and other microbes break it down, both for their own consumption as well as for the host.
When grass and other plants are partially chewed with saliva and swallowed down the esophagus throat tube, muscular wave contractions of the rumen push the matter further into the reticulorumen, which continues to rhythmically contract and thus churns the food. With a full gut, the animal typically will rest, regurgitating, re-chewing and swallowing again the ingested material in a process called rumination, commonly called “chewing the cud.” This is repeated at length, with some cattle spending as much as six hours a day continuously chewing. When sufficiently broken down, the food is passed on to a chamber called the omasum, which pumps it to the animal’s true stomach, a small chamber called the abomasum.
The rumen functions in a way quite analogous to a gardener’s compost bin. Within it is a mat of fibrous plant clippings composed of a large quantity of cellulose, a long chain of sugar molecules that is broken apart by an enzyme called cellulase, which is secreted by bacteria. Some of these are consumed by the bacteria, and additional bacteria use the simple sugars to initiate fermentation, breaking plant proteins down into fatty acids, such as the amino acid lactate necessary for the host animal’s milk production. Some of these essential nutrients are absorbed by the capillary lining of the reticulorumen directly into the bloodstream.
Several species of bacteria are involved, categorized as fibrolytic, amylolytic and proteolytic, based on their digestion of complex carbohydrates, simple sugars and proteins, respectively. Single-celled protozoa digest all three, primarily by consuming bacteria. Fungi are less numerous but are important for breaking the chemical bonds between cellulose and the non-carbohydrate substrates of plants. About 3 percent of the microbial mass are archaea, a type of anaerobic bacteria that metabolizes the hydrogen and carbon dioxide waste of the other organisms into methane. Along with the eventually liquefied plant material, many of these microorganisms are also inevitably digested by the ruminant host for their vitamins, minerals and other nutrient content.
Rumen metabolism is an efficient way to extract the sugar energy in the carbohydrates of a cellulose diet. Ruminant animals harbor symbiotic gastric microbes that produce the enzymes required and are supplied with the nutrients and environment necessary for them to grow and multiply. The microbial anaerobic respiration and fermentation of the diet, however, has an undesired byproduct. A single cow is estimated to exhale 74 gallons (280 liters) of the greenhouse gas methane each day through a process called eructation, otherwise known as burping.
You can't feed a cow without feeding the microbes in that cow's rumen!
What's interesting also is that much of a cow's protein is obtained from the microbes in that rumen. The microbes have about a 10 to 15 minute lifespan, and lots of them get passed from the reticulorumen through to the small intestine where they are absorbed and utilized as protein. But of course a cow still needs to be fed protein in order to get protein, because those microbes depend on protein to live and reproduce. But this is not animal protein; it's protein from plants like soybeans or alfalfa.