Abstract
Between 2007 and 2010, field experiments were conducted in Chuen area, Shizuoka to determine the effect of planting date on the suppression of Italian ryegrass, which causes serious problems in winter wheat crops. Wheat was seeded in late November, middle December, and early January, by using the till soybean/no-till wheat double-cropping system. It was found that delayed seeding decreased the emergence of Italian ryegrass from November to late January, because of shortening of the duration of emergence. However, after February, the effect of delayed seeding on seedling emergence in Italian ryegrass varied, depending on the weather. It was confirmed that, in the fields with a high density of Italian ryegrass seeds and seedling, delayed seeding did not decrease the Italian ryegrass number at harvest, even in the cropping system with effective tillage and non-tillage and residual herbicide application. Thus, in Chuen area, Shizuoka, to produce the same grain yield as that obtained after the November seeding, wheat should be seeded by late December and the seeding rate should be 1.5 times as much as that used in conventional cultivation.
