To ascertain the effects of difference of paddy field management from winter-spring on the fate of Aeschynomene indica L. seeds, we investigated the dormancy of seeds that wintered in paddy fields at different depths and under different ground cover conditions and the germination of those seeds in the summer as they floated on the surface of water. From winter to mid-late May, seeds buried at a depth of 10cm in autumn did not break dormancy for the most part, regardless of the surface conditions, while seeds on the soil surface did break dormancy. However, ground cover with winter vegetation or rice straw delayed the breaking of dormancy by seeds on the soil surface. In mid-late May, >50% of the seeds that wintered under straw cover were dormant while >95% seeds on a bare soil surface broke dormancy. More than 80% of the seeds that wintered on a bare soil surface germinated within 5 days of being floated on the surface of water in mid-May, and none of seeds survived to early July. Only a few of the seeds that wintered under soil or straw cover germinated while floating on the surface of water, and >50% of the seeds remained dormant and survived to early July. These results suggest that most of the seeds dispersed in autumn will break dormancy from winter-spring in fallow/untilled fields and that most of these will be lost due to germination during rice cultivation.

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Herbicidal injury is a significant problem in weed management using one-shot herbicides for direct-seeded rice in flooded paddy fields, because at the time of herbicide application, rice plants (Oryza sativa L.) are less developed in comparison to transplanted rice. Total numbers of established seedlings, tillers at the tillering stage and grain yield have been used as indicators to evaluate herbicidal injury in direct-seeded rice. However, the influence on tiller development in terms of nodal position and order has not been considered. Tillers from lower nodal positions produce higher quality rice in direct-seeded rice cultivation, and hence, procuring the same is emphasized, as practiced in Akita prefecture. Therefore, information on the nodal position and the order of tillers susceptible to one-shot herbicides is indispensable for evaluating their adaptability to direct-seeded rice. For the above reason, the development of tillers at every nodal position and the order and ratio of productive tillers in direct-seeded rice plants treated with one-shot herbicides at the 2.0 or 2.5 leaf-stage, the earliest recommended application stage, were investigated over 2005-2007 under field and glasshouse conditions. A flowable formulation of pyriminobac-methyl, bromobutide, bensulfuron-methyl, pentoxazone was applied and the rates of development of primary and secondary tillers from the 2nd node were suppressed to 60% and 55%, respectively, and that of primary tillers from the 7th node increased compared to the non-treated control. There was no significant difference in the total number of tillers that developed in treated and untreated plots. The response depended on the active ingredients and the formulation of one-shot herbicides, and was not observed under higher temperature condition. These results suggest the importance of evaluating the effect of one-shot herbicides on the development of tiller from the 2nd node, which produces a vigorous panicle, in weed-management methods for direct-seeded rice cultivation in Akita prefecture, northern Japan.

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[in Japanese]

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