COMPETITION OF BARNYARDGRASS ON SOYBEAN C ULTIVATED IN A HYDROMORPHICSOIL .1. EFFECT ON WEED DRY-MATTER PRODUCTION AND SOYBEAN GRAIN-YIELD

The aim of this experiment was to study the interaction between compet ition period of barnyardgrass (Echinochloa spp.) and row spacing on dr y matter of the weed and soybean grain yield grown in a hydromorphic s oil. The results indicated that barnyardgrass dry matter increased lin early with the competition period whereas soybean grain yield decrease d with duration of competition. Reduction of row spacing increased the competition of soybean against the weed. The critical competition per iod varied between 20 and 50 days after soybean emergence. It was obse rved that continuous soybean cropping during three years significantly reduced the infestation of barnyardgrass and that this reduction has an influence on the critical competition period.