Safener is a group of adjuvants used in commercial herbicide formulations to selectively protect crop plants from the active herbicide ingredient. Safeners often have similar structures to the active ingredients of herbicide, however, studies on the environmental fate of safeners are scarce. Benoxacor is a safener paired with the high-use herbicide <i>S</i>-metolachlor. The mixture is recommended for pre-emergent application (i.e., spraying onto soil). In this study, we evaluated the photolysis of benoxacor and metolachlor, individually and as mixtures, in three different reaction environments: in water and on quartz and kaolinite surfaces. When irradiated with simulated sunlight (&gt; 290 nm) individually, benoxacor exhibited rapid decay in all three reaction environments, with apparent first-order rate constants in the range of 0.4–1.6 h<sup>-1</sup>. In comparison, metolachlor photolysis was 9–5000 times slower regardless of the reaction environment. When irradiated as mixtures, benoxacor decayed at similar rates as when it was irradiated individually, but metolachlor decayed 1–3 fold faster, particularly on quartz. The enhancement of metolachlor photodegradation increased as the benoxacor-to-metolachlor molar ratio increased. These results suggest that benoxacor can serve as a photosensitizer and induce metolachlor photodegradation. It was noteworthy that although benoxacor rapidly photodegraded (e.g., by 3 h), the sensitizing effect on metolachlor degradation was observed over 48 h. Correspondingly, we observed that the absorptivity of the reaction matrix in the sunlight range increased initially despite the decay of benoxacor; the absorptivity then persisted until 8.5 h before it started to decrease slowly. Analysis by high performance liquid chromatography with diode array detection showed that the photoproducts capable of absorbing sunlight are more hydrophilic than either benoxacor or metolachlor. Work is underway to further characterize the photoproducts from the benoxacor/metolachlor reaction mixtures.

Safener benoxacor induces herbicide metolachlor photolysis on simulated soil surface

2019 Spring National Meeting

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