The ability of the ruminant animal to utilize low-quality fibrous feedstuffs (e.g., grasses and forages) to produce a high-quality end-product (i.e., meat, milk, and wool) is the result of a mutually beneficial relationship between the mammalian host and the fermentative microbial population inhabiting the rumen (forestomach). Animals equipped with a rumen include cattle, buffalo, sheep, antelope, gazelle, duiker, reindeer, deer, giraffe, and goats; other animals that consume grass (e.g., horses and donkeys) are not considered true ruminants, but rather utilize a postgastric fermentation. Mammals do not produce enzymes that degrade cellulose (a primary fibrous component of plant materials), but ruminants are able to degrade cellulose via fermentation because of the presence of the rumen and its resident microbial population.
Ruminant animals are characterized as having teeth on the bottom jaw, and a hard dental pad on the top. This arrangement of teeth results in incomplete mastication (chewing) of ingested feed. Feed is swallowed and deposited into a large pouch (the rumen) at the end of the esophagus. The rumen is a large chamber (can compose up to 30% of the mass of the animal) that is anaerobic (does not contain oxygen) and populated by a very large, diverse population of microorganisms (bacteria, protozoa, fungi, and viruses). These microorganisms degrade feeds through the process of fermentation (described subsequently). Feedstuffs in the rumen are continuously broken down into smaller and smaller pieces by microbial activity as well as regurgitation and remastication (a process known variously as ruminantion, or chewing the cud). As feed is broken down to pieces less than 1 mm in size, it passes out of the rumen and then to the abomasum (or true stomach) for further degradation and to the intestine for digestion by mammalian enzymes.
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