オキサジアゾン乳剤による新しい水田除草法の確立に関する研究

Abstract

The present study was undertaken with a view to clarifying the properties of oxadiazon as a herbicide for transplanted rice and to developing effective application methods best suited to its properties.Weed control effect of the herbicide on barnyardgrass was evaluated and the following results were obtained.1) Oxadiazon application was most effective as a pre-emergence treatment. Weed control effect of the herbicide was reduced with the progression of the growth stages of barnyardgrass.2) Oxadiazon was able to control barnyardgrass effectively in flooded water even at a concentration as low as 0.05ppm.3) The deeper the incorporation of oxadiazon in soil, the lower was the herbicidal activity.4) In the soil incorporation treatment, oxadiazon was effective in the control of barnyardgrass at the pre-emergence stage up to a depth of 2-3cm from the soil surface. However oxadiazon lost its effectiveness in the control of barnyardgrass when it was incorporated in deeper soil layers.As for the phytotoxicity of oxadiazon to rice plants, the following characteristics were observed.1) No toxic symptoms were recorded in rice plants at a concentration of oxadiazon below 0.2ppm in flooded water.2) The deeper the incorporation in soil, the less was the phyototoxicity to transplanted rice plants.3) The highest degree of phytotoxicity in rice plants was observed when the seedlings were transplanted with the entire root zone located in the oxadiazon treated layer.4) A negligible degree of phytotoxicity was observed in rice seedlings with newly developed roots coming in to contact with the oxadiazon-treated layer.5) The characteristic symptom of oxadiazon toxicity was the appearance of browning in newly developed leaves.6) Leaf browning was the external manifestation of browning of the assimilatory tissues. No adverse effect of oxadiazon was observed in other tissues.7) In general, the degree of leaf browning (browning in both leaf sheath and lamina) tended to be directly proportional to rice growth inhibition. A minimal degree of leaf sheath browning did not exert a deleterious effect on the growth of rice plants.8) Oxadiazon absorbed by the roots was readily translocated to the shoots.Based on the properties of oxadiazon as a herbicide for transplanted rice, to achieve effective weed control with minimum or no phytotoxicity to rice plants, oxadiazon should be applied in layers located at a 2-3cm depth from the soil surface. A flooded condition should be maintained after the treatment and the final concentration should not exceed 0.2ppm in the flooded water to avoid damage to rice plants.To satisfy the above conditions at the time of application of oxadiazon an effective, economic and easy application method suited to different types of soil and acceptable to a large number of farmers was developed as follows: the hebicide in the form of an emulsifiable concentrate is scattered directly over the water surface before or immediately after final puddling of the field.