Abstract: Sulfonylurea herbicide residues in soil can affect rotational crops even at low concentrations. Although analytical methods are efficient enough to measure them, the lack of an efficient herbicide extraction technique makes bioassays useful for determining the presence of phytotoxic levels of sulfonylurea residues in soil. A growth chamber bioassay using sunflower was developed to detect MON-37500 and triasulfuron residues in two different soils. Root length was measured 15 d after the treatment. A sigmoid equation described plant root length response as a function of herbicide concentration. The nonlinear regression established a range of I₅₀ values from 0.9 to 2.9 ppb ai for both sulfonylureas.

Nomenclature: MON-37500 (proposed common name, sulfosulfuron), 1-(4,6-dimethoxypyrimidin-2-yl)-3-[(2-ethanesulfonyl-imidazo[1,2-a]pyridine)sulfonyl]urea; triasulfuron; sunflower, Helianthus annuus L. ‘Albani’.

Additional index words: Dose–response curves, I₅₀, phytotoxicity, rotational crops, sulfonylurea herbicides.

Abbreviations: I₂₀, dose required for 20% inhibition; I₅₀, dose required for 50% inhibition; OM, organic matter.