Fomesafen, saflufenacil, and sulfentrazone had similar parameters for their mass spectrometry analysis, all being analyzed in negative mode with similar ionization energies. Flumioxazin was analyzed in positive mode using different ionization temperatures and voltage energies, and a larger injection volume (10 µl compared with 2–5 µl) due to lower liquid chromatography–mass spectrometry (LC-MS) detector response. Quantitative limits of detection in soil were < 5 parts per billion by weight for all herbicides. The field study was conducted three times (2010, 2011, 2012) with four blocks of each treatment each year. Herbicide concentrations over time were based on field samples that were later extracted and quantified using the described LC-MS procedures. Data were examined using a simple first-order (SFO) equation with each year-by-herbicide treatment combination regressed using SigmaPlot version 12.5 to determine regression parameters. The SFO rate constant was used to determine a half life, or DT50 (in days) for each curve. All data were analyzed using a GLMMix ANOVA procedure using SAS version 9.3 and contrast statements were used to directly compare each herbicide comparison. Slopes for each herbicide use the SFO curve and were estimated using SAS. The order from shortest to longest DT50 was flumioxazin (21.1 d) = saflufenacil (21.4 d) < fomesafen (45.6 d) < sulfentrazone (70.8 d). These results concur with the labeled recrop recommendations after application for flumioxazin and saflufenacil, which have shorter cotton plant-back restrictions compared with sulfentrazone and fomesafen. In these studies, none of the herbicides was highly persistent (all half-lives < 100 d), so none would be expected to be persistent pollutants in the environment, although further research is needed in this area.
Nomenclature: Soybean, Glycine max (L.) Merr.