Cadmium cycling in sheep-grazed hill-country pastures

Cadmium (Cd) cycling in a sheep-grazed hill-country fanning system was investigated to determine the important factors influencing accumulation in edible sheep tissues. A farmlet-scale trial was established consisting of a total of 10 paddocks that had a history of 15 years of 5 different rates of phosphatic (P) fertiliser applications. The trial comprised 2 flocks of Romney ewes managed at 2 different grazing pressures. Each flock was rotationally grazed over 1 block of 5 paddocks, each of which represented a different rate of P (10, 20, 30, 50, and 100 kg P ha(-1)). Over a 4-year period, the trial investigated seasonal variation of soil and herbage Cd; effect of rate of P and paddock stratum (sheep campsites, tracks, easy, or steep slopes) on soil and herbage Cd; and pasture intake or soil ingestion as possible modes of Cd entry into the grazing animal. Annual Cd liver and kidney tissue accumulation rates in ewes between 18 and 66 months of age were estimated at 19 and 43%, respectively, but were unaffected by grazing intensity. Soil ingestion contributed only 3-6% of total Cd ingestion by grazing sheep. The remainder arose from the pasture diet of the sheep and points to the need to reduce further Cd inputs to soils and minimise the uptake by plants of Cd already present.