Yes there probably is, some work was done for Chernobyl and radiological intakes of grazing for sheep for example and doubtless Food Standards Agency runs models. The good bit is that blood and tissue analysis is normally readily available to help verify any model often in year 1 (delicious)!
However look after livestock or watch a few vet programs and you will quickly observe horses and cows routinely get sick and or even die eating ‘clean’ grass these are not at all robust test subjects.
Not sure I would want to argue for a 1 to 50000 ELCR for a sheep but maybe go somewhat higher for Frankel
Martin Wright