Effects of cultural management and other factors on the amount of Echinochloa oryzicola dry biomass in directly seeded forage rice grown in north Tohoku

Abstract

The effects of management and other factors on the amount of Echinochloa oryzicola dry biomass at harvest time of directly seeded rice was analyzed by an additive model using smoothing splines. Among the factors studied, the length of the weed-free period after rice sowing had the greatest effect on the amount of weed biomass. Extension of the weed-free period from 20 to 24 days reduced greatly the biomass of E. oryzicola, and weed biomass barely changed when the weed-free period was more than 42 days. Increasing the forage rice seedling density from 105 to 210 plants m⁻² slightly reduced weed biomass, and there was little change in E. oryzicola biomass when the forage rice density was more than 210 plants m⁻². Increasing the number of E. oryzicola seeds in soil up to 10000 grains m⁻² slightly increased weed biomass with little change in E. oryzicola biomass when there were more than 10000 grains m⁻². The amount of E. oryzicola biomass after broadcast seeding forage rice was smaller than that with row seeding. For making full use of the weed suppression ability of forage rice in directly seeded culture, the seed of lodging-tolerant cultivars should be broadcast at a high density to establish 210 seedlings m⁻² in northern Japan. In that case, the amount of E. oryzicola biomass will be kept below the threshold by maintaining the field free of weeds for 30 days after rice sowing, although the field should be kept weed free for 40 days as a general rule.