Abstract
From 1991 to 1994, a rice cultivar, Koshihikari, was cultured under dense (25 hills/m2) and sparse (17 hills/m2) planting with a low supply of agricultural chemicals and with continuos application of farmyard manure (FM) or chemical fertilizer (CF) . Grain yield under dense planting was not different from that under sparse planting when FM was applied without CF, but was higher in the former than in the latter when FM was applied together with the small amount of CF or only a small amount of CF was applied. This is because the former had a larger number of ears than the latter. When a large amount of CF was applied, grain yield in the plot with dense planting was lower because of the fewer florets, lower percentage of ripened grains and lighter grains though having many ears. Panicle blast occurred more frequently under dense planting, especially when a large amount of CF was applied. No difference was observed in the occurrence of insects between the plots with dense and sparse planting. Grain yield under dense planting was higher than in the plot with sparse planting regardless of the weather condition when FM was applied together with a small amount of CF. However, it w as lower when a large amount of CF was applied even in a good weather year. It is concluded that dense planting sometimes gives a high yield when FM is applied continuously, but often causes over-luxuriant growth, less dry-matter production and frequent occurrence of panicle blast when CF is applied continuously. Therefore, the method for high yield must be explored with sparse planting, which also costs less and stabilizes rice production.
