Selection of Rice Varieties Suitable for Direct Seeding under a Submerged Soil Condition by Using the Effective Thermal Index in Hokkaido, in Northern Japan

Abstract

The seeding date and heading date (HD) in direct seeding cultivation under a submerged soil condition were examined with three rice varieties at 27-40 sites of Hokkaido in 1999-2003. The effective thermal index (ETI) from the day after seeding to HD was determined using daily hours of sunshine, wind speed and air temperature in 19 paddy rice production areas. The HD estimated from EFI was nearly the same as the actual HD. In 1997-2002, grain yield and number of spikelets per panicle of direct-seeded rice (DSR) were 13-23% and 30% lower than those of transplanted rice (TR), respectively, in four areas with a similar amount of nitrogen fertilizer applied. The grain-filling period in DSR was also 3.2-6.7 days shorter than that in TR. From regression lines between the cumulative daily mean temperature during 35 days after heading and the grain-filling period, the grain-filling period estimated using the same cumulative daily mean temperature was six days shorter in DSR than in TR. From these results we assumed that the varieties whose HD estimated from ETI under normal weather conditions was earlier than the third day after the latest HD for safe cultivation under standard transplanting cultivation, are suitable for direct seeding. Although the medium-maturing variety was suitable only for the south arera, early variety was suitable for the area including the main rice cultivation area of Hokkaido.