Gravity and Reticulorumen Motility Combine to Create the Selective Flow of Particulate Matter out of the Rumen
Rumen ingesta are stratified and segregated by the effects of gravity and reticulorumen motility. Cattle receiving forage diets have distinct zones, or phases, of rumen ingesta. In the dorsal rumen a gas cap, or gas zone, is created by the fermentation gases.
Below this is a solid zone composed of intertwined particles of fermenting forage. The solid zone is sometimes referred to as the rumen mat because of the braided or woven nature of its particles. The solid zone is kept afloat by buoyancy created by air trapped in the feed particles, as well as by small bubbles of fermentation gases that form around bacteria that adhere to the plant material as fermentation takes place. At the bottom of the rumen there is a liquid zone with a waterlike consistency. The area between the solid and liquid zones is the slurry zone. The slurry zone has indistinct boundaries and forms a continuum of consistency from the liquid to the solid zones. These four major zones are created primarily by the effect of gravity; two additional functional zones are created by the motility patterns. These are the ejection zone and the zone of potential escape, which constitute the dorsal and ventral areas, respectively, of the reticulum and cranial sac (Figure 31-9).
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