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This study was conducted to predict the rice yield loss and to determine the economic threshold levels for direct-seeding flooded rice cultivation from competition to the most serious weeds, Sagittaria trifolia L. and Bidens frondosa L. The rice yield loss models of S. trifolia and B. frondosa were predicted as Y=497.0/(1+0.003760x), R 2 =0.869 and Y=486.0/(1+0.007612x), R 2 =0.887, respectively. In comparison of the competitiveness represented by parameter β, it was 0.003760 in S. trifolia and 0.007612 in B. frondosa, respectively. Economic thresholds calculated using Cousens' equation was negatively related with the competitiveness of weed. The economic thresholds of S. trifolia and B. frondosa were 7.6 and 3.9 plants per m 2 , respectively.Key words: Bidens frondosa; competitiveness; direct-seeding flooded rice; economic threshold; Sagittaria trifolia.                                 (Kwon  2009).      ,

Prediction of Rice Yield Loss and Economic Threshold Level by Densities of Sagittaria trifolia and Bidens frondosa in Direct-seeding Flooded Rice