Abstract

Pastures growing on soils fertilised with phosphatic fertilisers accumulate cadmium (Cd) in their plant tops in proportion to substrate concentrations. Sheep grazing pastures absorb some of this Cd. To quantify the rate with which Cd is accumulated in sheep tissue, three replicated 0.4 ha blocks were fertilised with either 333 kg/ha of single superphosphate only (control) or amended with CdSO4 to give 274g Cd/ha (high-Cd), and set stocked with 10, three months old Romney wether lambs. Cadmium (Cd) concentrations were determined in liver, kidney, proximal duodenal and skeletal muscle tissue of sheep, ingesting either (means ± s.e.) 266±22 (control) or 746±58 (high-Cd) ug CD/day from three to six months or 290±21 and 721±72 ug Cd/day over six to eighteen months. Cadmium concentrations in kidney and liver tissue increased markedly over the first three months: 0.042±0.006 to 0.32±0.03 (Control) and 0.57±0.04 (high-Cd) ug Cd/g fresh tissue, and 0.04±0.006 to 0.12±0.02 (Control) and 0.31±0.06 (high-Cd) ug Cd/g fresh tissue, for kidney and liver respectively. Although total Cd content continued to increase, as organ size increased, over the next 12 months daily accumulation rates in both tissues were much lower compared with those over the first three months. For example, daily Cd accumulation in kidney decreased from 0.25±0.002 to 0.06±0.01 and 0.47±0.04 to 0.32±0.05 (ugCd/d) for sheep from the control and high-Cd pastures respectively. Total Cd content in kidney and liver combined accounted for approximately 0.25% of total Cd ingested by sheep age three to six months but this significantly decreased to 0.1% in sheep age six to eighteen months. At higher Cd intakes increased metallothionein protein synthesis in kidney and liver tissue enabled animals to be more efficient in sequestering Cd. This resulted in reduced accumulation of Cd in muscle tissue of sheep on the high- Cd pastures (2.0±0.04 and 5.0±0.08 ng Cd/g) compared with those grazing the control pasture (4.0±0.6 and 14±5 ng Cd/g) in animals from the first and second periods respectively. The results from this study show that it is the first few months after weaning that fastest uptake into kidney occurs and therefore strategies to reduce Cd absorption and subsequent accumulation, e.g. by increasing zinc in the diet, should focus on young animals.

J, Lee, ND Grace, and JR Rounce

Proceedings of the New Zealand Society of Animal Production, Volume 54, , 31-34, 1994