The Fate and Risk of Oxyfluorfen Under Simulated California Rice Field Conditions

Rice production in California is a thriving, multi-billion dollar industry, with the Sacramento Valley alone producing over 95% of the nation’s short- and medium-grained rice. The sustainability of this industry is threatened, however, by the rapid evolution of herbicide resistant weeds spurred by long-term monoculture and a lack of herbicides with alternative modes of action. One agent proposed to ameliorate the lack of chemical control options is oxyfluorfen (OXY). OXY is a broad-spectrum, diphenyl ether herbicide that disrupts chlorophyll synthesis through inhibition of the protoporphyrinogen oxidase (protox) enzyme. It has demonstrated effectiveness against rice weeds, including weedy rice (Oryza sativa f. spontanea), a pest for which no herbicide is currently registered, and resistance to its mode of action has not been reported by rice growers. However, OXY is not currently registered for use with rice and its use in- or near-aquatic resources is currently prohibited due to its high toxicity to aquatic organisms. As rice field floodwater is ultimately released into nearby waterways, use of OXY with rice introduces the potential for its transport to the Sacramento River Basin where sensitive aquatic organisms may be harmed. Thus, it is imperative that registered uses be informed by a clear understanding of its transport and dissipation processes and potential for environmental impacts when it’s used as a rice herbicide. To that end, it is the objective of this work to elucidate the environmental fate and aquatic risk of OXY when used as an herbicide in California rice fields.Partitioning processes greatly influence overall fate by determining where a pesticide is found and which processes contribute most to dissipation. Thus, the soil-water partitioning behavior of OXY under simulated California rice conditions was characterized using a batch equilibrium method. Soil-water partitioning was investigated in two soils collected from Sacramento Valley rice fields