<P> With the modern rise of corporate agribusiness and the decline of localized family farms, many breeds of sheep are in danger of extinction . The Rare Breeds Survival Trust of the UK lists 22 native breeds as having only 3,000 registered animals (each), and The Livestock Conservancy lists 14 as either "critical" or "threatened". Preferences for breeds with uniform characteristics and fast growth have pushed heritage (or heirloom) breeds to the margins of the sheep industry . Those that remain are maintained through the efforts of conservation organizations, breed registries, and individual farmers dedicated to their preservation . </P> <P> Sheep are exclusively herbivorous mammals . Most breeds prefer to graze on grass and other short roughage, avoiding the taller woody parts of plants that goats readily consume . Both sheep and goats use their lips and tongues to select parts of the plant that are easier to digest or higher in nutrition . Sheep, however, graze well in monoculture pastures where most goats fare poorly . </P> <P> Like all ruminants, sheep have a complex digestive system composed of four chambers, allowing them to break down cellulose from stems, leaves, and seed hulls into simpler carbohydrates . When sheep graze, vegetation is chewed into a mass called a bolus, which is then passed into the rumen, via the reticulum . The rumen is a 19 - to 38 - liter (5 to 10 gal) organ in which feed is fermented . The fermenting organisms include bacteria, fungi, and protozoa . (Other important rumen organisms include some archaea, which produce methane from carbon dioxide .) The bolus is periodically regurgitated back to the mouth as cud for additional chewing and salivation . After fermentation in the rumen, feed passes into the reticulum and the omasum; special feeds such as grains may bypass the rumen altogether . After the first three chambers, food moves into the abomasum for final digestion before processing by the intestines . The abomasum is the only one of the four chambers analogous to the human stomach, and is sometimes called the "true stomach". </P> <P> Other than forage, the other staple feed for sheep is hay, often during the winter months . The ability to thrive solely on pasture (even without hay) varies with breed, but all sheep can survive on this diet . Also included in some sheep's diets are minerals, either in a trace mix or in licks . Feed provided to sheep must be specially formulated, as most cattle, poultry, pig, and even some goat feeds contain levels of copper that are lethal to sheep . The same danger applies to mineral supplements such as salt licks . </P>

The digestive system of a sheep has evolved to