Cadmium and zinc in kidney, liver, muscle and mammary tissue from dairy cows in conventional and organic farming.

Input of Cd to arable soils occurs mainly through atmospheric deposition and mineral fertilisers. Phosphate fertilisers are often contaminated with Cd. In organic farming the use of mineral fertilisers is restricted. The impact of conventional and organic farming on Cd and Zn levels in tissues from dairy cows was studied. Kidney, liver, muscle and mammary tissue samples were collected at slaughter from 67 cows, aged 30-95 months, in a project with conventional and organic production at the same farm. Samples were analysed by electrothermal atomic absorption spectrometry with a quality control programme. Significantly lower levels of Cd were found in cows from the organic system (n = 29) than from the conventional cows (n = 38) in kidney [330 +/- 100 (mean +/- s) micrograms kg-1 vs. 410 +/- 140], liver (33 +/- 15 vs. 44 +/- 19) and mammary tissue (0.38 +/- 0.14 vs. 0.59 +/- 0.37), while there were no differences in muscle (0.48 +/- 0.13 vs. 0.49 +/- 0.14). Organic cow kidneys had lower Zn levels than conventional cows (19 +/- 1.4 mg kg-1 vs. 20 +/- 2), whereas muscles had higher Zn levels than conventional cows (67 +/- 16 vs. 51 +/- 12). Cd and Zn in mammary tissue were positively related to age and milk production. There was a positive relationship between levels in kidney of Cd and metallothionein (MT) and a Cd/MT concentration ratio indicating protection from Cd-induced renal dysfunction. When older animals, that entered the project as milk-producing cows, were included the differences in kidney and liver Cd levels between the systems were no longer significant, while Cd in kidney became related to age- and production-related parameters. The change of significant relationships when older animals were included shows the importance of controlled conditions for environmental monitoring.