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.