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

BACKGROUND: 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. RESULTS: We analyzed data from La Jailliere, an ARVALIS experimental site (nine plots, 1993 to 2017), representative of scenario D5 from the EU FOCUS Group, for four herbicides (isoproturon, aclonifen, diflufenican, flufenacet). Our study demonstrates the relevance of the time application management measure by showing the decreasing trend in the transfer of pesticides in drained plots. In addition, it validates, still on the La Jailliere site, the hypothesis of a management measure based on an indicator of soil profile saturation before drainage flow (soil wetness index, SWI). CONCLUSIONS: A conservative measure consisting of 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 and values of maximum or flow weight average concentrations by 70- and 27-fold, ratio of exported pesticide 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 can be easily calculated by considering the local weather data and soil properties for any drained field. (c) 2023 Society of Chemical Industry.

Automated summary

Due to changes in aquatic risk assessment in the marketing authorization process in France, the contamination of surface water through subsurface drainage is better addressed. The prohibition of selected pesticides on drained plots is a risk management measure. However, there is a scarcity of herbicide solutions for subsurface-drained plots, leading to challenges in autumn weed management for winter cropping systems on drained plots. The study emphasizes the relevance of time application management and proposes a conservative measure based on an indicator of soil profile saturation before drainage flow.