Environmental fate and aquatic risk assessment of oxyfluorfen in California rice fields

The herbicide oxyfluorfen [OXY; 2-chloro-1-(3-ethoxy-4-nitrophenoxy)-4-(trifluoromethyl)benzene] recently emerged as a potential solution to combat herbicide resistance in California rice. Proposed as a preemergent applied preflood to soil, products are in development for use with OXY-tolerant rice strains. Currently, OXY is not registered for use with rice and its use in or near aquatic resources is restricted due to its high aquatic toxicity. Before OXY may be registered for use in California rice fields, its potential fate and aquatic risk must be evaluated. Particularly important is the identification of the minimum period water must be held on the field (water holding period) necessary for OXY to dissipate below levels of concern. In this assessment, the environmental fate of OXY and its risk to aquatic organisms under simulated California rice field conditions are characterized. The Pesticides in Flooded Applications Model (PFAM) was used to estimate environmental concentrations based on anticipated use patterns and water management practices in California (e.g., winter flooding, turnover, water holding, etc.). Two California rice field soil conditions were simulated in addition to standard soil conditions used in ecological risk assessment for rice. Results suggest OXY is likely to concentrate in sediment, dissipate slowly, and persist. Water holding period had little effect on paddy and release water concentrations. Risks from water column exposure were generally below levels of concern (LOC) for aquatic animals, whereas risks to aquatic plants, algae, and benthic invertebrates exceeded LOCs under all conditions evaluated. California rice field soil conditions were also associated with less risk compared with standard conditions. Reduced application rates were sufficient to reduce risk to acceptable levels in some situations. However, holding times up to 30 days had no effect on risk outcomes, suggesting water management needs of growers should be strongly considered when stipulating water holding periods for OXY.