CADMIUM IN NEW-ZEALAND AGRICULTURE

The accumulation of cadmium (Cd) in New Zealand agricultural systems as a result of the extensive use of phosphatic fertilisers has been recognised as a potential problem following the appearance of unacceptable levels of Cd in some animal products. In this review, the accumulation of Cd in soil, its uptake by plants, and consequent intake by grazing animals are considered and a simple model for the transfer of Cd through a New Zealand agricultural system is presented. The model indicates that for pastures which have received 20 kg P/ha as superphosphate made from a 50:50 mix of Nauru and Christmas Island phosphate rocks, for 21 years, careful grazing management avoiding excessive animal intake of soil and fertiliser, which may account for a significant proportion of the total Cd intake, will be required if the Cd content of the kidneys of lambs older than about 8 months is not to exceed the maximum permissible concentration of 1 ppm fresh weight. In some areas of New Zealand, this level is probably already being exceeded. With older lambs, culled ewes, hoggets (1-2-years-old), and cattle, the model indicated that excessive kidney Cd may currently be achieved under normal grazing management, and thus the value of the offal to both the producer and the processor will be lost. However, the model relies heavily on the estimates of the Cd content of soil and herbage, and on the retention of Cd by grazing animals. There is therefore a need to obtain reliable data for these. Other areas of essential future research which may lead to the correction of this problem are identified.