Effects of Rye Sowing Dates on Weed Occurrence in Organic Soybean Field

This study was conducted to determine the effect of rye sowing date on weed suppression in the organic soybean field located at Ansung, Gyeonggi province. In 2006, rye was sown on March 20th, April 1st, April 20th, and May 10th. Among the these sowing dates, March 20th resulted in highest biomass of rye and weed control value as 99.3%. Weed occurrence was negatively correlated with grain yield of soybean. Consequently, the rye sowing date of March 20th provided highest soybean yield among the spring sowing dates. In 2007, effects of autumn sowing and spring sowing of rye on weed suppression and soybean yield were compared. The biomass of rye in autumn sowing was higher than spring sowing, however, weed suppression rate was not significantly different from each other. Considering negative correlation of weed occurrence and soybean yield, yield of autumn sowing plot was expected similar to or higher than spring sowing plot. However, severe drought stress of the year decreased the soybean yield of autumn sowing plot, possibly because of water competition between rye and soybean. Results indicated that early spring sowing of rye is practical to suppress weed in organic soybean field when sufficient biomass of rye is assured.