The Chromium-51 Complex of Ethylenediamine Tetraacetic Acid as a Soluble Rumen Marker.

In experiments on digestive function in ruminants, markers can be used to estimate the rate of movement of digesta, the volume of a viscus and the rates of absorption of solutes from the gut. The most widely used water-soluble marker is polyethylene glycol (PEG), which was introduced by Sperber, HydCn & Ekman (1953). They found that the polymer with mean molecular weight 4000 was not degraded in the gut or absorbed, and described the use of PEG for the three purposes set out above. HydCn (1961) has given a full mathematical treatment of the principles involved in the use of soluble markers to measure the rate of flow and volume of rumen fluid, and has indicated the inherent limitations of the method. He found that PEG gave a satisfactory estimate of rumen fluid volume (PEG space was approximately 95 % of rumen water volume); in experiments of long duration (i.e. days or weeks), the flow rate could be calculated from the rate of fall in concentration of the marker; and in short-term experiments, the flow could be estimated from the amount of reference substance that had disappeared from the rumen during the experiment and the mean concentration of the marker in the rumen fluid. However, a serious limitation in the use of PEG is the lack of a specific, sensitive and accurate method of analysis. All published methods have been based on precipitation from aqueous solution with subsequent gravimetric or turbidimetric measurement. Analytical difficulties have been recorded by Smith (1958), and Corbett, Greenhalgh, Gwym & Walker (19 j8) obtained incomplete recovery of PEG from the faeces of grazing cattle and sometimes from stall-fed cattle. An alternative reference substance was sought and radioactivity was selected as a ineans for sensitive and completely specific analysis. The complex of 51Cr with ethylenediamine tetraacetic acid (j1Cr EDTA) was tested and compared with PEG. The results which are reported here showed j3Cr EDTA to be a highly satisfactory soluble marker in sheep.