Shifting application dates on cereal reduces pesticide transfer via subsurface drainage based on water flow forecasts during autumn applications.

Abstract

Thanks to the changes in aquatic risk assessment within the Marketing Authorization (MA) process in France, the contamination of surface water through the subsurface drainage network is better accounted for. The measure adopted by Risk Regulations is to prohibit any use of selected pesticides on drained plots. Herbicide solutions on subsurface-drained plots are becoming scarce due to a limited number of innovations combined with the re-approvals process. Autumn weed management then becomes a major issue for winter cropping systems on drained plots. Unlike runoff prevention, few risk management measures are available to prevent the risks associated with drained plots. We analyzed data from La Jaillière, an ARVALIS experimental site (9 plots, 1993 to 2017), representative of scenario D5 from EU FOCUS Group, for four herbicides (isoproturon, aclonifen, diflufenican, flufenacet). Our study first demonstrates the relevance of the time application management measure by showing the decreasing trend in the transfer of pesticides in drained plots. The second result is to validate, still on the La Jaillière site, the hypothesis of a management measure based on an indicator of soil profile saturation before drainage flow (Soil Wetness Index). A conservative measure consisting in restricting pesticide applications during autumn, when the SWI is < 85% of saturation, reduces the risk by a factor of 4-12 for quantification above the predicted no-effect concentration (PNEC) and values of maximum or flow weight average concentrations (Cmax and CMP) by 70- and 27-fold, ratio of exported pesticide (Rexp) by 20-fold, and total flux by 32. This measure based on SWI threshold appears to be more efficient than those using other restriction factors. SWI could be easily calculated considering local weather data and soil properties for any drained field. This article is protected by copyright. All rights reserved.