Diclofop in runoff from agricultural watersheds in a high winter rainfall zone Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/9306t233f

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  • Diclofop-methyl [me yl 2-(4-(2,4-dichlorophenoxy) phenoxy) propanoate] hydrolysis, degradation, and leaching were studied in field and laboratory experiments. Greater than 75% of the applied diclofop-methyl hydrolyzed to diclofop in 2 days at 4°C, the prevailing soil temperature during western Oregon winters. Degradation beyond the diclofop molecule, also an active herbicide, was much slower with a half life of 42 days. Microbial activity was identified as the primary mechanism in both the hydrolysis and degradation processes. Diclofop residues were detected in the 1-5 and 5-10 cm depths, indicating that leaching had occurred. Runoff losses of diclofop [2-(4-(2,4-dichlorophenoxy) phenoxy) propionic acid] were also measured following January applications to small (6.0 and 1.4 ha) agricultural watersheds in the Willamette Valley, Oregon. Diclofop runoff losses were higher in 1980 (7.1 and 3.9% of the applied herbicide at the 6.0 and 1.4 ha watersheds, respectively) than in 1981 (0.7 and <0.1%, respectively). In 1980, between 80 and 90% of the diclofop seasonal loss was partitioned into the aqueous phase of runoff. The maximum diclofop concentrations in runoff were 370 and 790 mg/m³ at the 6.0 and 1.4 ha watersheds, respectively. Larger diclofop losses in 1980 were attributed to (1) the close proximity of large runoff events to the application date and (2) the high antecedent soil moisture status prior to the first runoff event which resulted in a high percentage runoff. The presence of a soil surface crust limited the depth to which overland flow extracted diclofop residues from the soil profile and resulted in very shallow calculated mixing depths (0.13-0.16 cm). A subsurface drainage system installed at the 1.4 ha watershed effectively lowered antecedent soil moisture as compared to the 6.0 ha watershed. Consequently, percentage runoff and diclofop runoff losses were reduced at the 1.4 ha watershed. Diclofop losses in the subsurface drainage effluent accounted for 1.2 and <0.1% of the applied herbicide in 1980 and 1981, respectively. Subsurface drainage and changing the herbicide application date were recommended as potential management practices.
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