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Persistence of Carbofuran and Molinate in Flooded Rice Culture

Dep. of Agronomy, Altheimer Laboratory, Univ. of Arkansas, Fayetteville, AR 72701.

^* Corresponding author.

ABSTRACT

Field studies were conducted on silt loam and silty clay soils to determine the persistence of carbofuran (2,3-dihydro-2,2-dimethyl benzofuran-7-yl methylcarbamate) and molinate (S-ethyl hexahydro-1H-azepine-1-carbothioate) in the soil and water associated with rice (Oryza sativa L.) production in Arkansas. Paddy-rice plots were established with earthen levees surrounding each plot and standard cultural practices were followed concerning rice production. Granular formulations of these pesticides were applied 4 d following establishment of the permanent flood at the tillering stage of rice growth. Samples of water and soil were taken at various intervals until 49 days after application (DAA) and analyzed with gas chromatography. Carbofuran was not detected in paddy water after 28 DAA. By 21 DAA, >90% of the applied molinate had dissipated, but detectable levels (0.0012 mg L^–1) of residues remained in paddy water through 49 DAA. Both pesticides dissipated rapidly from paddy soil for approximately the first 14 DAA, probably due to solubilization of the granules into the overlying paddy water. Both pesticides had estimated time required for dissipation of 50% of the initial concentration (DT₅₀s) of 10 d in paddy water and paddy soil. Molinate dissipation from paddy soil slowed dramatically after the initial rapid phase. The carbofuran metabolite 3-keto carbofuran (2,3-dihydro-2,2-dimethyl-3-ketobenzofuranyl-N-methyl carbamate) was not detected in any water samples, but was detected in 6% of the soil samples. The metabolite 3-hydroxy carbofuran (2,3-dihydro-2,2-dimethyl-3-hydroxy-benzofuranyl-N-methyl carbamate) was detected in 2% of the water samples and 7% of the soil samples. In both cases the concentrations found were slightly above the lower limits of detectability (0.005 mg L^–1 for water and 0.01 mg kg^–1 for soil).