Weed Occurrence, Growth and Yield of Rice Transplanted with 30-Day-Old Seedlings in No-Tillage Paddy Fields

No-tillage cultivation of rice was evaluated in the transplanting cultivation of 30-day-old seedlings from 1993 to 1997. This study was conducted to examine the changes in weed occurrence, rice growth and rice yield during the no-tillage cultivation compared with those during tillage cultivation, and to clarify a critical year limit for the no-tillage cultivation. Five weed species, namely Echinochloa crus-galli, Monochoria vaginalis, Aneilema keisak, Eleocharis kuroguwai, and Sagittaria trifolia emerged in tillage paddy fields, whereas nine weed species, namely Echinochloa crus-galli, Monochoria vaginalis, Aneilema keisak, Ottelia alismoides, Persicaria hydropiper, Eleocharis kuroguwai, Sagittaria pygmaea, Sagittaria trifolia, and Zizania latifolia occurred in no-tillage paddy fields. As years of cultivation passed by, shoot dry weight of weed species tended to increase in both tillage and no-tillage cultivation. The dominant weed species were Eleocharis kuroguwai after 1 year and Echinochloa crus-galli after 2 to 5 years in tillage paddy fields. However, Eleocharis kuroguwai was the dominant weed species all the time in no-tillage paddy fields. Annual weeds occurred with much higher frequency than perennial weeds during the entire period aside from 1 year in tillage paddy fields, whereas perennial weeds occurred more often than annual weeds all the time in no-tillage paddy fields. Plant height, number of tiller, shoot dry weight, and root dry weight of rice in no-tillage cultivation were all much lower than those in tillage cultivation. Especially, these investigative parameters decreased conspicuously 20 to 30 days after transplanting. Rice plants in no-tillage paddy fields exhibited about an 8-15% lower yield than those in tillage paddy fields. Considering that yield of rice plants, a critical year limit for no-tillage rice cultivation was within 2 years for transplanting cultivation of 30-day-old seedling. Rice yield in no-tillage cultivation was affected directly or indirectly by factors such as lower missing and floating hill and poor root distribution. Therefore, to prevent rice yield from being reduced in no-tillage paddy fields, we disseminated a nitrogen fertilizer at 150 kg per ha in no-tillage rice fields and this increased rice yield by 14% compared with the recommended rate (110 kg per ha)