Field and Greenhouse Bioassays to Determine Mesotrione Residues in Soil

Whole-plant bioassays using sugar beet, lettuce, cucumber, green bean, pea, and soybean as test crops were used to detect mesotrione residues in the soil. The test crops were planted in soil treated with mesotrione in the field the previous year at rates of 0 to 560 g ai ha(-1) and in nontreated soil from the same field, with mesotrione added at concentrations of 0 to 320 mu g kg(-1). Experiments were conducted in the greenhouse for a 21-d period. Values for the dose giving a 50% response (I-50) were predicted using a log-logistic nonlinear regression model. I-50 values (mean +/- SE) of 8.6 +/- 1.8, 14.9 +/- 2.0, 29.8 +/- 11.0, 41.6 +/- 7.3, 52.9 +/- 6.4, and 67.9 +/- 30.3 g ai ha(-1) for sugar beet, lettuce, green bean, cucumber, pea, and soybean, respectively, indicate that these crops were effective bioassay test species for quantifying mesotrione residues. A greenhouse bioassay was a simple and sensitive tool to detect mesotrione at concentrations of less than 1.0 mu g kg(-1) with sugar beet and lettuce being the most sensitive test species. The I-50 values for soil treated with known concentrations of mesotrione were lower than for field soil treated with mesotrione the previous year. Knowing the level of mesotrione residues in the soil, growers have flexibility in crop rotations following mesotrione use on corn. Growers can use this information to minimize risk of crop injury by choosing appropriate rotation crops that suffer little or no yield reduction.