Soil activity and persistence of sulcotrione and mesotrione.

Greenhouse bioassays were set up using a small pot test method to determine the intrinsic sensitivity of different plant species to sulcotrione and mesotrione applied in a sandy loam soil. Herbicides were applied over an appropriate concentration range. After a 2-3 week test period, foliage fresh weight was determined. Data were subjected to a non-lineair regression analysis. Using the regression equations, ED50-values (herbicide concentrations that cause 50 percent foliage fresh weight reduction) were calculated for each combination of crop species and herbicide. To determine which replacement crops might be grown in case of failure of a crop treated with one of these herbicides, field persistence experiments were conducted over the 1993-2003 period for sulcotrione and the 1998-2003 period for mesotrione at the Experimental Farm, Biocentre Agri-Vet, Ghent University at Melle. Herbicides were applied in spring (about mid-March) on a bare soil; untreated control strips were included. Replacement crops were sown or planted approximately five weeks after herbicide applications. Visual estimations of crop injury were recorded at several intervals from sowing and fresh matter yield of plant parts was determined. Based on these data, crops were ranked according to their degree of sensitivity to either sulcotrione or mesotrione. Maize is very tolerant to both herbicides, although in some years, temporary injury could be seen in the field experiments. Italian rye-grass and fibre flax are tolerant crops; in field experiments a slight, temporary injury could be noticed in some years. Winter wheat displayed a high degree of tolerance to mesotrione (in both experiment types): however this crop was less tolerant to sulcotrione especially in the bioassay experiment. Based on its ED50-value, black salsify is tolerant to sulcotrione but under field conditions, the selectivity of this herbicide is quite variable; tolerance to mesotrione is moderate. Turnip and witloof chicory are clearly sensitive to mesotrione and sulcotrione whereas sugar beet, red clover and lettuce are extremely sensitive to both herbicides in both experiment types. Bioassays and field experiments provide a detailed and complete information about soil activity and persistence of both herbicides.