Field and laboratory dissipation of the herbicide fomesafen in the southern Atlantic Coastal Plain (USA)

Authors: Potter, Thomas; Bosch, David - Dave; Strickland, Timothy - Tim

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/7/2016
Publication Date: 6/15/2016
Citation: Potter, T.L., Bosch, D.D., Strickland, T.C. 2016. Field and laboratory dissipation of the herbicide fomesafen in the southern Atlantic Coastal Plain (USA). Journal of Agricultural and Food Chemistry. 64(25):5156-5163. doi:10.1021/acs.jafc.6b01649.

Interpretive Summary: Weed species with evolved resistance to the herbicide glyphosate have emerged as a complex and costly problem for production of cotton and other crops. Palmer amaranth (Amaranthus palmeri) is particularly troublesome in the Southeastern USA. Glyphosate continues to be used in the region but older more selective herbicides with alternate modes of action are often required for Palmer amaranth control. This includes fomesafen (5-(2-chloro-a,a,a-trifluoro-p-tolyloxy)-N-mesyl-2-nitrobenzamide). However, its properties indicate potential for runoff, leaching, and soil persistence, thus there are concerns about increased use. We examined field and laboratory dissipation from a single preemergence application to a cotton crop in the Atlantic Coastal Plain region of Georgia (USA). Findings showed that even under worst case conditions, rainfall and runoff on the day of application, fomesafen losses were <1%% of applied. The relatively low loss in runoff was linked to irrigation incorporation of the herbicide after application and implementation of a common conservation practice, strip tillage. In laboratory based incubations with surface soil (0-2 cm) aerobic soil half-life was about 100 days; however the compound was not found to persist after 1 year following field application. Persistence was indicated in the subsoil with about 3%% of applied detected in 1.2-m soil cores collected about 3.5 years after application. The compound did not leach below 60 cm due to slowly permeable subsoil horizons. Approximately 0.15% of the fomesafen that leached did move off-site in lateral subsurface flow. Finally, no degradates were detected in any soil or water samples. This includes the primary product reported in registration documents, fomesafen amine, and other products described in recently published reports. Taken together off-site fomesafen transport identified in this study is not expected to adversely impact water quality; however the compound leached and was found to persist in the subsoil. The fate of this material and whether more will accumulate in subsoil following repeated applications is unknown. Another uncertainty is whether degradates may accumulate and or be transported off-site in runoff or subsurface flow. None were detected in this study.

Technical Abstract: To control weeds with evolved resistance to glyphosate, cotton farmers in the Southeastern USA have rapidly increased fomesafen (5-(2-chloro-a, a, a-trifluoro-p-tolyloxy)-N-mesyl-2-nitrobenzamide) use. Its properties suggest potential for soil persistence, runoff, and leaching that may contribute to adverse impacts. This was assessed in this study. After application to 0.2-ha plots relatively low runoff rates <1 % were observed. This was linked to post-application irrigation incorporation and implementation of a common conservation tillage practice. Detection of 0.15% of fomesafen applied in lateral subsurface flow from these plots also indicated that leaching rates were low. Moderate persistence (t1/2 = 100 days) was indicated in laboratory incubations with surface soil however analysis of soil cores from treated plots showed that ˜3% of fomesafen applied persisted in subsoil >3 years after application. The long term fate of fomesafen that accumulated in subsoil and the identity of degradates remain uncertain. No degradates were detected in this study.