Karimmojeni H, Rahimian Mashhadi H, Alizadeh HM, Cousens RD & Beheshtian Mesgaran M (2010). Interference between maize and Xanthium strumarium or Datura stramonium. Weed Research50, 253–261.Summary Xanthium strumarium and Datura stramonium are two of the most important weeds of maize in Iran. Their growth was compared in a field study conducted over 2 years. Relationships were derived between weed density and crop yield, yield components and various growth parameters. Yield, grain number ear−1 and grain weight were affected more by X. strumarium than by D. stramonium. Xanthium strumarium followed a pattern of growth resembling that of the maize, whereas D. stramonium continued growing for a longer period, mainly affecting maize during the grain‐filling period. A large portion of the loss in maize yield caused by X. strumarium could be explained by a reduction in grain number ear−1, whereas a greater portion of the maize yield loss from D. stramonium was explained by a reduction in grain weight. Weed biomass was greater for X. strumarium than for D. stramonium, whereas seed production was higher for D. stramonium. It is concluded that although X. strumarium should be given the highest priority for control in maize on the basis of its superior competitive ability, control of D. stramonium is also essential because of both its considerable competitiveness in the current crop and its long‐term effects resulting from its greater seed production.