Current understanding of the role of the reticulum and reticulo-omasal orifice in the control of digesta passage from the ruminoreticulum of sheep and cattle

Simultaneous abolition of the motility of the ruminoreticulum (RR) and digesta passage from the RR by bilateral vagotomy indicate that the motility of the RR coordinated with quiescence of the reticulo-omasal orifice (ROO) are prerequisite for caudal movement of digesta from the RR and forestomach. Rate of passage is of importance in ruminant animals since it is related to maximal voluntary feed intake, extent of digestion of the diet, amount of protein which bypasses the rumen, efficiency of microbial growth, extent of methane loss, and susceptibility of animals to bloat. In a series of studies, we clarified the roles of frequency, pressure and duration of reticular contractions and the function of the ROO in controlling digesta flow from the rumen in response to changes in diets, environmental conditions and physiological status. Variability in amount of NDF and OM transferred per contraction suggests that frequency of reticular contraction is not the rate-limiting step in digesta passage from the rumen. This concept is supported by our finding that the relaxation of the ROO is not always coincidental with reticular electromyographic activity in solid meal-fed relative to milk fed sheep. The magnitude of reticular pressure may not precisely control digesta passage through the ROO as the amount of OM and NDF associated with each centimetre of pressure during contraction varies from 1.2 to 1.4 and from 0.9 to 2.5 g. On the other hand, the amount of OM and NDF passed per second of reticulum contraction varies only from 0.3 to 0.6 and from 0.2 to 0.3 g, respectively, in cattle. In addition, a 38% decrease in mean retention time of particulate matter in the rumen was associated with a 14% increase in duration of reticular contraction in cattle. Similarly, quiescence of the ROO increased from 2.9 to 3.3 s in sheep when feed intake increased from maintenance to twice maintenance. We suggest that duration of reticular contractions may be more closely related to digesta flow than is either frequency or pressure changes. We also suggest that the ruminant animal can influence reticular contraction characteristics and duration of the quiescence of the ROO in response to various stimuli and that reticular contractions are coordinated with other diet and physiological demands for optimal transfer efficiency of digesta from the RR. Key words: Digesta passage, reticular contractions, reticulo-omasal orifice, electromyographic activity