Environmental risk assessment of the use of zinc oxide medicated feeds for weaning piglets in the UK

Concerns over environmental impacts resulting from the use of zinc oxide containing medicines for weaning piglets led to the withdrawal of the authorisations for these products in the EU. In order to better understand these issues more detailed assessments were conducted for the UK, taking account of the fate of zinc in the environment and its bioavailability to ecological receptors. Four regional scenarios covered the main pig farming areas in the UK and the emission scenario was based on current agricultural practices in the UK. The fate and transport of zinc in the environment was modelled using the Intermediate Dynamic Model for Metals, and the toxicity of zinc in the environment was assessed based on current UK regulatory practices. The model takes account of historic additions of metals to the soils to calculate current and future metal levels in the environment. Whilst three of the four regional scenarios predicted a marginal risk, or no risk, to soils after 50 years of use one of the scenarios indicated a risk to surface waters prior to the use of zinc oxide medicated treatments for weaning piglets, and risks to local soils within 10 years of use. Further site-specific assessments were conducted for this region and one of the other regions, based on site specific emission scenarios, soil and surface waters characteristics. These two site-specific assessments revealed that the modelling results were accurate or conservative depending on the assumptions made about historic inputs of metals to agricultural soils from manure spreading, and that the regional scenario that resulted in significant predicted risks to surface waters did not reflect the actual conditions at the local pig farming sites considered. Comparisons between measured concentrations of copper and zinc at pig farming sites suggest that historic agricultural inputs have been an important source of these metals to agricultural soils at some sites. The limited data available for validation suggest that the IDMM is able to provide accurate predictions of metal levels in both soils and surface waters, but that there is significant uncertainty associated with historic inputs of metals to the soils.