Radiocaesium variability within sheep flocks:: determination of 137Cs intake in free-ranging sheep

A study is described in which Cs-137 intake by free-ranging sheep was estimated at two farms in the area of west Cumbria (northwest England) which received some of the highest amounts of fallout from the Chernobyl accident within the United Kingdom. The faecal excretion of Cs-137 was estimated from faecal Cs-137 activity concentrations and the use of intraruminal controlled release devices containing Cr2O3 to determine faecal dry matter output. The intake of Cs-137 was estimated by assuming an apparent absorption coefficient appropriate to the herbage grazed. The methodology has the advantage that sampling of herbage representative of that ingested by study animals is not required. Caesium-137 dietary intake explained >60% of the observed variability in the Cs-137 activity concentration determined in the muscle of sheep. Resultant transfer coefficient (F-f) values to describe the transfer of Cs-137 from the diet to muscle were in agreement with previously reported values. At one farm, there was a positive correlation between the Cs-137 activity concentration in muscle and F-f,whilst at the other farm there was a negative correlation between F-f and Cs-137 dietary intake. Potential reasons for these observations are discussed.