1. Two rumen cannulated steers consuming 5·5 kg air-dry lucerne (Medicago sativa) hay/d given at two-hourly intervals were used to study the kinetics in the rumen of the two particulate markers, 103Ru-labelled Tris-(1,10-phenanthroline) ruthenium II chloride ([103Ru]P) and dysprosium (Dy). Provision of markers was achieved by allowing the steers to eat separatedstems of the hay on to which had been sprayed solutions of the markers.
2. The intake of large-particle (retained by a 3·2 mm mesh screen) dry matter (DM) in boluses and the rumen large-particle pool size measured by emptying the rumen wereused to calculate the turnover rate-constant of this pool (1·02 and 1·19/d for steers A and B respectively).
3. The decline with time of both [103Ru]P and Dy associated with large-particle DM in raft digesta was best described by two-compartmental kinetics. The first compartment apparently reflected a combination of the processes of mixing of labelled particles throughout the rumen contents, physical migration of marker from the labelled hay, and physical breakdown of large particles to small particles.
4. The disappearance with time of [103Ru]P and Dy associated with small particles from the raft, ventral digesta and faeces from 3 to 8 d was well described by a single kinetic compartment with a rate constant similar to that of the second compartment of the large particles from the raft.
5. [103Ru]P was rapidly distributed through both the raft and ventral digesta of the rumen. This observation, taken together with measurements of migration of [103Ru]P, suggested that on entering the rumen much of the [103Ru]P did not remain associated with the original feed material.
6. The majority (63–64%) of Dy entered the raft digesta and mixed only slowly through the rumen contents. Some Dy (18–27%) apparently migrated rapidly from large particles and to small particles immediately after ingestion and there was also evidence for some slow migration from small to large particles.
7. When used under the conditions described for this experiment neither [103Ru]P nor Dy was satisfactory as a marker to trace the passage through the rumen of a particular meal.