Accumulation of cadmium with time in Romney sheep grazing ryegrass white clover pasture: Effect of cadmium from pasture and soil intake

Cadmium (Cd) concentrations in various tissues of Romney sheep and rates of accumulation, as affected by Cd concentrations in pasture and soil, DM intake, and animal age, were determined in a 25-month study. After weaning, 6 groups of 10 wether lambs were grazed on a low Cd (0.18+/-0.08 mu g Cd/g DM) or high Cd (0.52+/-0.17 mu g Cd/g DM) pasture in 3 replicates. The rate of accumulation of Cd (mu g/day) into the liver and kidney was greatest in sheep aged about 6 months (for the low and high Cd pastures: kidney 0.27 and 0.56; liver 0.35 and 1.1), after which it declined with age (kidney 0.02 and 0.03: liver <0.01 and 0.05; at 28 months of age). Total content of Cd in the organs continued to increase. In g-month-old animals, 0.25% of total Cd ingested was retained by the kidney and liver, but this proportion decreased to about 0.05% of Cd intake with 28-month-old animals. Net retention of Cd in the fleece-free body as a percentage of daily Cd intake for both the control and treatment animals was estimated between 0.35 and 0.5%. Except for muscle tissue from sheep in the treatment group, for which the mean Cd concentration was lower than that of the control group (2.0+/-0.24 and 3.9+/-0.63 eta g Cd/g fresh tissue, respectively), Cd concentrations in kidney, liver, thymus, muscle, and lung tissue increased over the first 3 months for both groups of animals. After 2 years, animals on the treatment pastures had approximately 3-fold greater Cd concentrations in liver, kidney, and duodenal tissue tissue than those grazing the control pastures (liver, 361+/-58 v. 97+/-11; kidney, 1485+/-200 v. 352+/-48; duodenal tissue, 32+/-4 v. 18+/-2.5 eta g; Cd/g FW, respectively). At higher Cd intakes, the adaptive effect of increased metallothionein synthesis enabled the animal to be more efficient in binding Cd, which restricted accumulation of Cd in muscle. The regression( [Cd](kidney) = -205+0.981Cd(intake)+/-0.726Time; r=0.82, P<0.001) gave the best fit for the observed Cd concentration in the kidney tissue. Daily Cd intake was also a better predictor of the Cd concentration in liver tissue than the concentration of Cd in pasture by itself ([Cd](liver)=24.7+0.353Cd(intake); r=0.81, P<0.001). Accurate predictions of the potential for Cd accumulation in young grazing animals will enable more effective management strategies to be implemented to reduce Cd accumulation and therefore minimise the potential impact relating to both environment and market.