Marker concentration patterns of labelled leaf and stem particles in the rumen of cattle grazing bermuda grass (Cynodon dactylon) analysed by reference to a raft model

Large (>1600 mum), ingestively masticated particles of bermuda grass (Cynod on dactylon L. Pers.) leaf and stem labelled with Yb-169 and Ce-144 respect ively were inserted into the rumen digesta raft of heifers grazing bermuda grass. The concentration of markers in digesta sampled from the raft and ve ntral rumen were monitored at regular intervals over approximately 144 h. T he data from the two sampling sites were simultaneously fitted to two pool (raft and ventral rumen-reticulum) models with either reversible or sequent ial flow between the two pools. The sequential flow model fitted the data e qually as well as the reversible flow model but the reversible flow model w as used because of its greater application. The reversible flow model, here after called the raft model, had the following features: a relatively slow age-dependent transfer rate from the raft (means for a gamma 2 distributed rate parameter for leaf 0.0740 v. stem 0.0478 h(-1)), a very slow first ord er reversible flow from the ventral rumen to the raft (mean for leaf and st em 0.010 h(-1)) and a very rapid first order exit from the ventral rumen (m ean of leaf and stem 0.44 h(-1)). The raft was calculated to occupy approxi mately 0.82 total rumen DM of the raft and ventral rumen pools. Fitting a s equential two pool model or a single exponential model individually to valu es from each of the two sampling sites yielded similar parameter values for both sites and faster rate parameters for leaf as compared with stem, in a greement with the raft model. These results were interpreted as indicating that the raft forms a large relatively inert pool within the rumen. Particl es generated within the raft have difficulty escaping but once into the ven tral rumen pool they escape quickly with a low probability of return to the raft. It was concluded that the raft model gave a good interpretation of t he data and emphasized escape from and movement within the raft as importan t components of the residence time of leaf and stem particles within the ru men digesta of cattle.