Weed Occurrence, Growth and Yield of Rice Transplanted with 10-Day-Old Seedlings in Tillage and No-Tillage Paddy Fields

No-tillage cultivation of rice was evaluated in the transplanting cultivation of 10-day-old seedlings from 1993 to 1997. This study was conducted to examine the changes in weed occurrence, rice growth and yield during the no-tillage cultivation compared with those during tillage cultivation, and to clarify a critical year limit for the no-tillage cultivation. Seven weed species, namely Echinochloa crus-galli, Monochoria vaginalis, Aneilema keisak, Eleocharis kuroguwai, Sagittaria pygmaea, Sagittaria trifolia, and Scirpus juncoides emerged in tillage paddy fields, whereas nine weed species, namely Echinochloa crus-galli, Monochoria vaginalis, Aneilema keisak, Eleocharis kuroguwai, Leersia japonica, Potamogeton distinctus, Sagittaria pygmaea, Sagittaria trifolia, and Zizania latifolia occurred in no-tillage paddy fields. As years of cultivation increased, shoot dry weight of weed species tended to increase in both tillage and no-tillage cultivation. The dominant weed species were Aneilema keisak and Eleocharis kuroguwai in no-tillage paddy fields while Echinochloa crus-galli and Eleocharis kuroguwai dominated in tillage paddy fields. When we looked into the ratio of annual and perennial weeds, annual weeds occurred with much greater frequency than perennial ones in paddy fields during both tillage and no-tillage cultivation. Plant height, number of tillers, shoot dry weight, and root dry weight of rice in no-tillage cultivation were much lower than those in tillage cultivation. Especially, these investigative parameters decreased conspicuously 20 days after transplanting. Rice plants in no-tillage paddy fields had a lower number of panicles per m2 compared than those in tillage paddy fields, but ripening rate, 1,000-grain weight and number of spikelets per panicle showed no difference. Rice plants in no-tillage paddy fields exhibited about an 8-13% lower yield than those in tillage paddy fields. Regression equations between yield (y) and duration of no-tillage cultivation (x) can be obtained as follows, y = -19.3x + 518.5 (r=0.746**) for transplanting cultivation of 10-day-old seedlings. Considering that yield of rice plants, a critical year limit for no-tillage rice cultivation was found to be 2 years for transplanting cultivation of 10-day-old seedlings. There are factors in reducing rice yield in no-tillage cultivation such as missing and floating hill, solid and gaseous phase ratio of paddy soil, dissolved oxygen, shoot dry weight, and root dry weight. We suggest that a decrease in rice yield in no-tillage was affected directly or indirectly by these factors