Comparison of the behaviour of three herbicides in a field experiment under bare soil conditions

A case-study was design in order to rank three alternative herbicides according to their exposure to surface waters. Herbicide persistence and losses to surface waters were studied in an uncultivated Finnish field (ca. 3500 m2, slope <1%). Glyphosate, glufosinate-ammonium and ethofumesate were sprayed onto bare soil in July. Surface runoff and subsurface drainage volumes were automatically recorded and sampled for herbicide analysis on volumetric basis until the following May. Soil was cored six times (0-131 days after the spray) for herbicide residue analyses. Herbicides were detected mainly in the topsoil (0-3 cm). The field dissipation half-life time of glufosinate-ammonium in the topsoil was about 1 week, whereas that of ethofumesate was over 10 weeks. Glyphosate analyses from soil media failed because organic fertilizer caused similar peaks in chromatography. All three herbicides were detected in surface runoff in summer and in spring. Moreover, ethofumesate and glufosinate-ammonium were found in subsurface drainage water in April. Although herbicide concentrations in runoff water were at their highest in summer, the main part of herbicide losses was generated by surface runoff during snow melt and soil thaw. Ethofumesate loss was 1.0%, glufosinate-ammonium 0.2%, and glyphosate 0.1% of the applied amounts within 302 days after application. These figures, obtained without a crop, may be higher than those in cultivated fields. However, the leaching potential of ethofumesate appears to be the highest of these three herbicides on sandy silt loam under boreal climate conditions. Furthermore, the results illustrate the importance of spring runoff for herbicide loadings. The produced data set will be useful in model testing.