Mixing and propulsion of the contents of the reticulo-rumen

Roughage intake and digestion by ruminants involve complex interactions between the roughage constituents, the microorganisms in the reticulo-rumen (RR) and its motility. Ruminal digestion requires intense activity, ie comminution of feed particles and mixing and propulsion of digesta. The regular repetition of the contraction sequences in the RR every 50 to 70 s subjects the digesta to a consistent pattern of movements. The particles are distributed according to their functional density which depends on the density of the plant structure of the particle, the liquid inside the particle and also the gas, ie on the degree of particle fermentation. An interwoven mat of large low-density particles fills the dorsal sac and the top of the ventral sac of rumen. This mat traps part of the small high density particles. Squeezed by the contractions, the interwoven mat acts like a filter and lets a liquid containing small particles of high density pass into the bottom of the ventral sac. This liquid then flows into the reticulum and passes through the reticulo-omasal orifice (ROO). Chewing during rumination reduces particle size, eliminates particle gas and aids in separating the low-density particles, which are less fermented, from the heavy residues. The outflow of digesta, made possible by the opening of the ROO during the second phase of the reticular contraction, is highly selective. The effluent does not contain particles greater than 2 mm in size in sheep and 4 mm in cattle. This is due to the buoyancy of the large particles in the reticulum, to the self-filtration of the digesta during the passage through the ROO and possibly to backflow from the omasum to the reticulum. Finally, RR motor activity, ie continuously mixing the digesta and monitoring the evacuation of gas and outflow of digesta, allows the homeostasis in the rumen necessary to microbial fermentation. The characteristics of the ingested particles, their rates of size reduction and density increase, the consistency of digesta and the intensity of the rumen wall stimulations are all factors which depend on the nature of feed and intake level. Via mechanisms which are not yet all well known and/or quantified, these factors act upon the efficiency of the mixing and propulsion of the reticulo-ruminal content and thus upon the retention time of the feed in the RR as well as its digestive utilisation.