A simple model to describe the flow of large and small particles in the rumen

A brief outline of the structure of the reticulo-rumen is given and some of the important aspects of rumen function (movement, rumination, microbial activity, throughput of fibrous materials) are discussed in the light of published information. A model is proposed in which the fibrous digesta (large particle pool) is subject to a slow cyclic flow and in which the liquid and small particle pool is subject to reciprocating flows. The two pools interact during rumination and during cross-flows through the raft of digesta which acts as a renewable double-sided filter. The validity of important parts of the model was tested in a series of experiments in vitro, using the rumen simulation technique (Rusitec). These experiments showed that neither the linear velocity of flows of liquid through the fibrous mass, nor the physical action of rumination had any marked effect on the efficiency of microbial degradation of feeds. On the other hand, the increased removal of microbial matter by washing the solid digesta and removing the washings resulted in a marked redistribution of microbial matter within the system and in a steady increase in its efficiency. At constant 'gut fill', the digestibility of feed dry matter was a simple function of the residence time of feed, but the outputs of end-products of fermentation per unit dry matter digested and the efficiency of the microbial system increased with residence time and then decreased. The results are discussed in relation to the proposed model, which not only stresses the importance of particle size reduction but also underlines the controlling role of rumen movement and rumination. (author)