- Previous studies have shown loss of ethofumesate activity when
the herbicide was applied to dry soil (2% w/w moisture content) in
both field and greenhouse studies, even when rainfall or irrigation
was received within a few days. Laboratory studies were conducted to
determine the mechanism for this activity loss. Dry soils (Woodburn
silt loam, 2% moisture) were treated with 20 ppmw radiolabeled ethofumesate
and were either wetted immediately to 35% moisture or remained
dry for 4 days before wetting. After equilibration, the soil
samples were centrifuged to extract the soil solution for analysis.
Radiolabeled ethofumesate and degradation products in the soil
solution were separated by thin-layer and column chromatography and
assayed by liquid scintillation. Ethofumesate and metabolites also
were extracted with methanol 4 days after herbicide application from
both dry and wetted soils. After extraction, the soil samples were
oxidized to determine the quantity of radioactivity remaining in the
Ethofumesate degradation, as affected by soil moisture, pH,
temperature, and the length of time soils remained dry after herbicide
application, was studied in a Woodburn soil. Several soil
moisture levels were maintained between 0.7 and 6.7% (w/w) by placing
soil samples in desiccators at various relative humidities. To
ascertain the effect of pH, the soil reaction was adjusted to 3.9,
4.9, 7.0, 8.0, and 9.0 prior to drying and treatment. The influence
of soil temperature on degradation was determined by incubating dry
soil samples at temperatures from 20 to 50 C and constant relative
humidity. The time period between ethofumesate application and soil
wetting was varied between 0 and 8 days to study the rate of ethofumesate
degradation. To determine the effect of soil type, the
herbicide was applied to air-dried Woodburn, Dayton, Madras, and
Agency soils. All soil samples were treated with 20 ppmw ethofumesate.
Ethofumesate applied to soil that remained dry for 4 days
degraded in significant quantities to two metabolites, while little
ethofumesate degraded in soils wetted immediately. The major metabolite
accounted for more than 80% of the degradation products and
was identified by gas chromatography and mass spectroscopy to be an
oxidation product, 2, 3- dihydro- 3,3- dimethyl- 2- oxo -5- benzofuranyl
methanesulfonate. The second metabolite was not identified.
The percentage of applied ethofumesate that was tightly
adsorbed, not extracted with methanol, was at least 5% greater for
applications to soils that remained dry than for soils that were
Soil moisture levels substantially influenced metabolite formation.
Increasing moisture contents to greater than 3% (w/w) reduced
degradation to negligible levels. Rapid ethofumesate degradation
occurred at soil moisture levels between 1 and 3%. Metabolites were detected 1 day after application to soils at 2% moisture content and
as much as 18% of the ethofumesate was degraded in 4 days.
Degradation was four to five times greater in soils of pH 4.9 or 7.0
than in more acid or more alkaline soils and ethofumesate metabolite
formation increased 6-fold as soil temperature increased from 20 to
50 C. Of the soil characteristics correlated with ethofumesate
degradation, percent clay and the level of soluble salts were best
correlated with degradation (r = .89 and -.95, respectively).
Ethofumesate adsorption studies were conducted in four soils
comparing a batch equilibrium technique with a centrifuge extraction
technique. Madras and Agency soils and two Woodburn soils were
treated with 1 to 20 ppmw ethofumesate. The batch technique had a
water-to-soil ratio of 5:1 while the ratio for the centrifuge technique
was 0.35:1. The quantity of ethofumesate adsorbed with the
centrifuge method was approximately twice that adsorbed with the
batch method for a given amount of applied ethofumesate. For a given
equilibrium concentration, the batch technique adsorbed more ethofumesate;
i.e., the Freundlich K values were higher for the batch
method. At least 24 hours were required for equilibrium to be
established with the batch technique while the equilibrium concentration
was established in a few hours with the centrifuge technique.
Ethofumesate degrades rapidly in dry soils to two metabolites.
The herbicide also is tightly adsorbed in greater amounts in dry soil
than in wet soil. Ethofumesate degradation and increased adsorption
results in the activity loss of ethofumesate on dry soil. Soil
moisture content is critical in determining the extent of degradation.
Moisture levels below 1% or above 3% reduce degradation
substantially. Metabolite formation is optimal for soilt of moderately
acid to neutral reaction (4.9 to 7.0) and the degradation rate
also depends on temperature, increasing linearly as temperature
increases. Metabolite formation is influenced by the chemical and
physical properties of the soil; clay content and soluble salts are
most highly correlated with degradation.