ピリブチカルブフロアブルおよび粒剤の除草効果と薬害に及ぼす変動要因

Pyributicarb, O-3-tert-butylphenyl 6-methoxy-2-pyridyl (methyl) thiocarbamate, is a paddy herbicide developed by Tosoh Corporation. Pyributicarb is formulated into a flowable form, a new directly-applicable formulation for paddy herbicides, and conventional granules for practical use. Using 5.7% flowable and 3.3% granules, factors affecting the herbicidal activity against barnyardgrass (Echinochloa oryzicola Vasing.) and phytotoxicity to transplanted rice (Oryza sativa L. cv. Yamahoushi) were investigated in a greenhouse. Standard application doses of the flowable was 100ml/a (60g a. i./a, specific gravity 1.05) and that of the granules was 300g/a (99g a. i./a). The results are as follows:<br>1. Pyributicarb flowable showed a higher activity against barnyardgrass than the granular form, though the rate of active ingredient of the flowable was lower than that of the granules (Fig. 1). When pyributicarb flowable and granules were applied at 4 days before transplanting (-4), no phytotoxicity was observed in rice at standard application doses, although a weak growth inhibition was revealed at high doses. In the case of application after transplanting, these two formulations did not lead to any phytotoxicity to rice even at high doses. Both pyributicarb flowable and granules could be applied not only after but also before rice transplanting at standard doses (Fig. 2).<br>2. Herbicidal activity of pyributicarb flowable and granules against barnyardgrass was not affected by the flooding water depth in the range of 1-6cm. The activity of both formulations, however, decreased when the flooding depth was 0cm (Fig. 3). Effect of flooding water depth in the range of 0-8cm on the phytotoxicity of the two formulations to transplanted rice was not detected (Fig. 4).<br>3. Herbicidal activity and phytotoxicity of pyributicarb flowable and granules were not affected by water leakage (0-4cm/day) (Fig. 5, Fig. 6).<br>4. When all the roots of the rice seedlings were located on the soil surface, the application of both pyributicarb flowable and granules led to a severe phytotoxicity. On the other hand, when the transplanting depth of rice was 1-2cm under the soil surface, neither pyributicarb flowable nor the granules inhibited the growth of rice seedlings whether the root tips were exposed on the soil surface or not. Under normal transplanting conditions, it is unlikely that these two pyributicarb formulations would cause any phytotoxicity to rice (Fig. 7).