|Abstract or Summary
- Applications of ethofumesate in Oregon have been observed from
previous research and from commercial use to be less effective when
applied to dry soils, even if subsequent precipitation occurred
within a few days. Two field studies were established in the
summer of 1979 using sweet corn (Zea mays L. 'Jubilee') and winter
wheat (Triticum aestivum L. 'Stephens') as bioassay species to
determine the effect of dry soil on ethofumesate efficacy. Applications
of various rates of the herbicide were made to soils of
approximately 2 and 30% w/w soil moisture. After 2 to 4 days, all
soils were irrigated for stand establishment and maintained at a
high moisture level. Ethofumesate, at most rates, was significantly
less effective on both corn and wheat when applied to dry soil than
to wet soil.
Two greenhouse studies were conducted using spring wheat
(Triticum aestivum L. 'Fielder') bioassays. One employed five soil
moisture levels, 2, 5, 9, 15, and 36%, and all soils were wetted to
field capacity 4 days after herbicide application. The other study used 2 and 12% moisture contents and soils were wetted 0, 2, and 4
days following herbicide application. The first study showed a
general decrease in herbicide activity with decreasing water content
with a greater than expected drop in activity at 15% moisture.
This drop in herbicide activity is believed to be the result of the
soil surface drying after application.
The second study showed substantially greater herbicide activity
at 12% moisture than at 2% moisture. The ethofumesate was incorporated
in this study, eliminating the effect of surface drying.
An increase in herbicidal activity was apparent as the length of
time between herbicide application and wetting increased from 2 to
4 days for the 2% moisture soils. The reason for this effect is
Soil samples (50 g, air-dried equivalent) at 2 and 20% soil
moisture were treated with 484 pg of ethofumesate. The herbicide
was extracted from the soils with hot methanol 0, 2, 4, 6, and 12
days after application. A gas chromatography analysis for ethofumesate
revealed no loss in the amount of herbicide applied to
wet soil over the 12-day period while in the dry soil, the amount
extracted after 12 days was 10% of the amount extracted at 0 days.
These data suggest chemical degradation of ethofumesate as the
most likely mechanism for the activity loss in dry soil.