In order to evaluate multispecies competition of weeds with winter wheat and to evaluate the economic damage threshold of the effective weeds, an experiment was conducted in 2002 at Agricultural Research Station, Ferdowsi University of Mashhad. Using a quadrate (50 cmx 50 cm), 30 points (non destroyed samples) and 10 points (destroyed samples) were randomly selected. Weeds present in the wheat field were: Avena ludoviciana, Chenopodium album, Stellaria holostea, Convolvulus spp., Fumaria sp., Sonchus sp., Polygonum avicular. LnW regression model was shown to better fit to weed densities. The economic damage threshold of effective weeds was determined by Swinton method. LnW regression model indicated that Avena ludoviciana, Convolvulus spp. and Chenopodium album had a significant effect on wheat yield reduction. By Swinton method and by using the extended hyperbolic model, economic damage threshold was determined at about 5.23 TCL (Total Competitive Load); this is equivalent to 5.23 plants of Avena ludoviciana or 12.20 plants of Convolvulus spp. or 19.65 plants of Chenopodium album per m2.
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