Decreased Activity of EPTC + R-25788 Following Repeated use in Some New Zealand Soils

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Abstract
Greenhouse and laboratory experiments showed that the biological activity of EPTC (S-ethyl dipropylthiocarbamate) + R-25788 (N,N-diallyl-2,2-dichloroacetamide) was lost more rapidly in soils if they had been previously treated with this herbicide. Treatments which reduced or eliminated the microbial activity in the soil restored the herbicide activity in ‘problem’ soils. A combination of 2 kg/ha of the insecticide fensulfothion (diethyl-4-methylsulphinyl phenyl phosphorothionate) with EPTC + R-25788 also restored the biological activity of the herbicide. Field trials confirmed the decreased biological activity with repeated use of EPTC + R-25788; other herbicides recommended for grass weed control in corn (Zea mays L.) performed satisfactorily in these ‘problem’ soils. The performance of EPTC + R-25788 was improved by a rotation away from it for one season (using another herbicide) but the level of activity was still below that obtained in a soil treated with EPTC + R-25788 for the first time. A new formulation of EPTC + R-25788 containing a herbicide extender R-33865 (O,O-diethyl-O-phenyl phosphorothioate) was not affected by the enhanced microbiological breakdown and provided effective season-long weed control in the ‘problem’ soils.