Rumination, or Cud Chewing, Has an Important Effect on the Reduction of Particle Size and the Movement of Solid Material Through the Rumen
Rumination is the act of remasticating rumen ingesta. The initial act of rumination is regurgitation, which occurs just before the initiation of a primary rumen contraction. When regurgitation occurs, there is an extra contraction of the reticulum, which takes place just before the regular biphasic reticular contraction that initiates the primary cycle.
Simultaneous with the extra reticular contraction, the cardia relaxes, and there is an inspiratory excursion of the ribs with the glottis closed. The latter action creates a negative pressure within the thorax, favoring the movement of food into the esophagus. When food enters the esophagus, a reverse peristaltic wave propels the material Cranially into the mouth. As soon as the food bolus reaches the mouth, excess water is expressed by action of the tongue, the water is swallowed, and remastication of the material begins. The duration of remastication depends on the character of the diet, with coarse material requiring more time for remastication than finely ground or highly digestible feeds.Regurgitated material comes from the dorsal portion of the reticulum, where particle size and functional specific gravity are characteristic of the slurry zone. Thus the ingesta selected for remastication are not the coarsest material in the rumen, but rather the material that has already been through the digestive actions of the solid zone. This appears to be an efficient system in which some of the structural material of the plant is softened by soaking and removed or weakened by microbial action in the solid zone. I he partially fermented material then reaches the slurry zone and is subjected to remastication, causing further comminution and exposing additional fermentable substrate that may not have been directly exposed to previous microbial action.
Rumination may also aid the particle separation process: as the regurgitated bolus reaches the mouth, it is squeezed by the tongue and cheeks before mastication begins. Water and small particles are expressed from the bolus by this squeezing action and are swallowed before mastication of the remaining bolus. This action tends to separate small particles from large particles. The small particles, when reswallowed, sink into the zone of potential escape, whereas the larger particles, when swallowed after remastication, are ejected back into the slurry zone.
Rumination occurs when the animal is not actively eating, usually during times of rest, but not during deep sleep. The time spent ruminating depends on the type of diet and ranges from almost none for high-grain diets to a maximum of about 10 hours per day for high-forage diets. The feed intake level also influences the amount of rumination time, with high intakes stimulating greater rumination.