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

A study is described in which Cs-137 intake by free-ranging sheep was estim ated at two farms in the area of west Cumbria (northwest England) which rec eived some of the highest amounts of fallout from the Chernobyl accident wi thin the United Kingdom. The faecal excretion of Cs-137 was estimated from faecal Cs-137 activity concentrations and the use of intraruminal controlle d release devices containing Cr2O3 to determine faecal dry matter output. T he intake of Cs-137 was estimated by assuming an apparent absorption coeffi cient appropriate to the herbage grazed. The methodology has the advantage that sampling of herbage representative of that ingested by study animals i s not required. Caesium-137 dietary intake explained >60% of the observed v ariability in the Cs-137 activity concentration determined in the muscle of sheep. Resultant transfer coefficient (F-f) values to describe the transfe r of Cs-137 from the diet to muscle were in agreement with previously repor ted values. At one farm, there was a positive correlation between the Cs-13 7 activity concentration in muscle and F-f,whilst at the other farm there w as a negative correlation between F-f and Cs-137 dietary intake. Potential reasons for these observations are discussed.