Cud

Cud is a portion of food that returns from a ruminant's stomach to the mouth to be chewed for the second time. More accurately, it is a bolus of semi-degraded food regurgitated from the reticulorumen of a ruminant. Cud is produced during the physical digestive process of rumination. The idiomatic expression chewing one's cud means meditating or pondering; similar expressions such as "he chewed that over for a bit", or "chew on that!" likely have the same derivation.

The alimentary canal of ruminants, such as cattle, goats, sheep, alpacas, and antelope, are unable to produce the enzymes required to break down the cellulose and hemicellulose of plant matter.Accordingly, these animals have developed a symbiotic relationship with a wide range of microbes, which largely reside in the reticulorumen, and which are able to synthesize the requisite enzymes. The reticulorumen thus hosts a microbial fermentation which yields products (mainly volatile fatty acids and microbial protein), which the ruminant is able to digest and absorb. This allows the animals to extract nutritional value from cellulose which is usually undigested.

The process of rumination is stimulated by the presence of roughage in the upper part of the reticulorumen. The chest cavity is stretched, forming a vacuum in the gullet that sucks the semi-liquid stomach content into the esophagus. From the esophagus it is taken back to the mouth with retro peristaltic movements. When the stomach content, or the cud, arrives in the mouth of the ruminant, it is pushed against the palate with the tongue to remove excess liquid, the latter is swallowed and the solid material is chewed thoroughly. The function of rumination is that food is physically refined to expose more surface area for bacteria working in the reticulorumen, as well as stimulation of saliva secretion to buffer the rumen pH.

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