The fate of water in the rumen

A mathematical treatment is given of the marker technique for studying water balances in the rumen. The treatment is extended to cover non-steady-state conditions and emphasis is placed on the underlying assumptions and practical limitations of the technique. 2. The 51 Cr complex of ethylenediamine tetra-acetic acid ( 51 Cr EDTA) was used as a marker in the rumen of sheep. 3. The 51 Cr EDTA entered 95–98% of the water in the rumen. Mixing in the rumen, though adequate, was usually not quite complete even 1·5 h after injection of the dose. The techniques of injecting the dose and of withdrawing the sample were found to be important; some techniques often led to poor mixing. 4. Provided that the time of injection of the dose was suitably selected, reliable estimates of the volume of water in the rumen could readily be made, but reliable estimates of flow rates needed stringent attention to details of technique.