The accumulation of potentially toxic elements in edible body tissues of lambs grazing after a single application of sewage sludge.

The application of sewage sludge to grassland can lead to the accumulation of potentially toxic elements (PTEs) at the soil surface that may be ingested, together with soil and herbage, by grazing ruminants. To investigate the extent to which the method of application of sludge to grassland might influence the accumulation of PTEs in body tissues of grazing sheep, two experiments were undertaken using grassland overlying two contrasting types of soil. At both sites, plots were either untreated or given liquid digested sludge in a single application in either autumn or spring by surface spreading or by injection. Weaned lambs were grazed on the experimental sites for 149 days (Experiment 1) or 160 days (Experiment 2). Muscle, kidney and liver tissues were assayed for concentrations of PTEs at each slaughter date. Applications of sludge were reflected in elevated concentrations of Pb and Cu in soil in Experiment 1 only (P < 0.05). Total loadings of PTEs for the treated grasslands did not exceed UK statutory limits to annual additions to soil. No significant differences were observed in the concentrations of PTEs in herbage between untreated and treated sites, with the exception of the concentration of Cu in herbage after surface application in the spring (Experiment 2). At the end of Experiment 1 the concentration of Cd tended to be higher in kidney tissues of lambs grazed on the pastures amended with sludge in the autumn compared to lambs grazed on untreated pastures. No other treatment effects were evident. Concentrations of PTEs in edible tissues of lambs grazed on grassland given a single dose of sludge were generally low and unlikely to pose a significant risk to the food chain. However, reduced food intake was observed in lambs grazed on grassland which had received sludge recently, indicating that the 21-day UK statutory no-grazing period should be reviewed in relation to grassland given sewage sludge in the spring.