Rumination

Rumination is a complex process and consists of the following:

• Regurgitation
• Remastication
• Insalivation
• Deglutition

Rumination is initiated by the rumination center close to the gastric center in the medulla oblongata. Rumination allows further physical breakdown of feed with the addition of large quantities of saliva and is an integral part of ruminal activity. The time devoted to rumination is determined by the coarseness of ruminal contents and the nature of the diet. Rumination usually commences 30 to 90 minutes after feeding and proceeds for 10 to 60 minutes at a time, resulting in up to 7 hours per day spent on this activity.

The epithelial receptors located in the reticulum, esophageal groove area, reticulorumen fold, and ruminal pillars detect coarse ingesta and initiate rumination. The receptors can be activated by increases in volatile fatty acid concentration, stretching, and mechanical rubbing.

An intact dorsal or ventral vagus nerve is necessary for regurgitation to proceed. Regurgitation is associated with an extra contraction of the reticulum immediately preceding the normal reticular biphasic contraction of the primary cycle. The glottis is closed, and an inspiratory movement lowers the intrathoracic pressure. The cardia then relaxes, and the distal esophagus fills with ingesta. Reverse peristalsis moves the bolus up to the mouth in which it undergoes further mastication.

Abnormal rumination can occasionally result in “dropped cuds,” during which the regurgitated bolus is dropped on the ground.

The usual causes for a reduction or absence of rumination are the following:

• Reticulorumen hypomotility or atony.
• Central nervous system depression.
• Excitement, pain, or both.
• Liquid ruminal contents such as a high-concentrate diet with no coarse fiber.
• Mechanical injury to the reticulum (peritonitis).

Other less common causes include chronic emphysema (difficulty in creating a negative thoracic pressure) and extensive damage to the epithelial receptors that incite the reflex, as occurs in rumenitis.

Reticulorumen motility is required for rumination to proceed. The extra reticular contraction is not essential for regurgitation because fixation or removal of the reticulum does not prevent rumination from occurring. Rumination can be easily inhibited by higher brain centers, as disturbance of a ruminating cow often stops the process and is absent when animals are stressed or in pain. Milking commonly elicits rumination in cows and goats.