Variation in the metabolism of radiocaesium between individual sheep

Considerable variability has been recorded in the radiocaesium activity con centration of muscle between individual sheep in the same flocks in upland areas that received fallout from the Chernobyl accident. In a previous pape r we demonstrated that there is a propensity for certain sheep within a flo ck to be always amongst the most contaminated and others to be consistently the least contaminated. Here we report a study to determine the extent to which variation in the metabolism of radiocaesium by individual sheep may c ontribute to the observed variability within sheep flocks. The transfer coe fficient and biological half-life of orally administered ionic radiocaesium in muscle were determined under controlled conditions in 22 ewes from an u pland farm in an area of the UK which received comparatively high levels of Chernobyl fallout. There was considerable variation between individuals in both the transfer coefficient (0.19-0.56 day kg(-1); mean 0.34 day kg(-1)) and biological half-life in muscle (5.2-18.7 days; mean 9.8 days). Changes in liveweight during the study and feed intake together accounted for 72% of the variation in the derived transfer coefficients; liveweight change al so accounted for 56% of the observed variation in biological half-life. In a subsequent study, the true absorption coefficient of radiocaesium was det ermined in 12 of the ewes. There was a positive correlation between transfe r and true absorption coefficients (R=0.57). We conclude that differences i n the metabolism of radiocaesium will contribute to the observed variabilit y in radiocaesium activity concentrations within sheep flocks in areas whic h were contaminated by Chernobyl fallout. We also suggest that for growing animals, the influence of liveweight change and feed intake on radiocaesium transfer may be greater than observed here. Similarly, in dairy cattle, fo r which feed intake changes considerably during the course of a lactation, large temporal variation in radiocaesium transfer to milk could be expected .