EFFECT OF REDUCED RATE OF APPLICATION OF A HERBICIDE, PROMETRYN +THIOBENCARB DURING DRAINAGE PERIOD ON IMPROVEMENT OF WEED MANAGEMENT IN DIRECT SEEDED RICE IN AKITA, NORTHERN JAPAN

A combination product of a granule herbicide of prometryn 0.80% and thiobencarb 8.0% (hereafter PT) which is not a one-shot herbicide, has been recommended in direct seeded rice fields in order to suppress Echinochloa spp. from seeding to emergence stage during the drained period just after seeding rice . The herbicides have a recommended application rate range of 0.32 kg + 3.2 kg to 0.48 kg + 4.8 kg a.i. ha^-1. Changes in efficacy and crop injury on direct seeded rice in a wet condition with reduced application rate of PT were evaluated under field and greenhouse conditions in Akita Prefecture, northern Japan. The aim of the study was to identify the herbicide rates required to avoid herbicidal injury to rice seedlings caused by unexpected rain, and to assess the sequential herbicidal efficacy of one-shot herbicides, which are major tools for weed management in direct seeded rice, applied under re-flooded condition after establishment of rice seedlings. With application of PT reduced to 0.16 kg + 1.6 kg a.i.ha^-1, sufficient herbicidal efficacy was obtained without injury to rice seedlings during the drained period after seeding. Effective control of weeds during the drained period using reduced rate of PT will ensure a sequential treatment of one-shot herbicide which can be applied after reflooding to provide good control of a range of weed species.