To study the mechanisms of competition between barnyardgrass and redroot pigweed with dry bean, an additive experiment was conducted at the Mashhad experimental station of Ferdowsi University. The experiment was laid out as a completely randomized block, with three replications. Treatments included three levels of redroot pigweed densities (4,8,12 plant/m2) and three levels of barnyardgrass (10,20,30 plant/m2) planted in a dry bean crop with a density of 20 plant/m2. A weed free control was included in each block. Results showed that as a consequence of competition by the weeds total dry matter (TDM), growth rate, height and leaf area of the bean crop decreased significantly. For all above parameters, redroot pigweed had a stronger competitive effect on dry bean than barnyardgrass. Except for the early season growth, redroot pigweed growth index curves exceeded that of bean. For barnyardgrass, except for the end of the season (during the physiologycal ripening period of bean) growth index curves were lower than for bean. Greater height and a better leaf position of redroot pigweed in the mixed canopy from early season on, resulted in redroot pigweed being more competitive for light than bean. Barnyardgrass had a much weaker competitive ability for light, mainly due to a lower height and a less favourable leaf position. Apparently, barnyardgrass imposed its competitive effect mainly through root competition, whereas redroot pigweed imposed its competitive force through roots and mainly foliage.

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