Iron loading and secondary multi-trace element deficiency in a dairy herd fed silage grass grown on land fertilized with sewage sludge

Recycling sewage sludge by applying it to agricultural land is strategically important in the European Union and is regulated by Directive 86/278/EEC, aimed at protecting the soil and humans from the presence of unwanted substances. However, because of the ruminant feeding habits, there is a risk that animals grazed on pasture or fed crops grown on land treated in this way may ingest biosolids adhered to foliage and/or on the top soil. This paper describes an episode of toxicity in a dairy herd consuming silage from a field fertilized with sewage sludge produced in a wastewater treatment plant. The affected cows were recumbent, unable to rise and suffered diarrhoea. Analysis of tissues (fresh weight) from a cow that subsequently died revealed severe hepatic iron loading (6720 mg/kg) and secondary multi-trace element hepatic deficiency, particularly of copper (0.812 mg/kg) and manganese (0.436 mg/kg), but also selenium (0.164 mg/kg) and zinc (19.9 mg/kg). The study findings indicate that the use of sewage sludge in agriculture can cause secondary multi-trace element deficiencies in ruminants. Careful attention should be taken when crops are ensilaged avoiding top-soil sewage sludge contamination, since the acidification process may greatly increase Fe bioavailability.