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The dispersal of Avena spp. ( A. fatua and A. sterilis) by natural dissemination and by agricultural operations was studied in four experiments conducted in Spain and Britain. Natural dispersal was very limited, with a maximum dispersal distance of 1.5 m. Dispersal was higher in the geographic direction that was downwind than in any of the other three geographic directions. Although plant movement was very small under no-tillage, an annual patch displacement of 2–3 m in the tillage direction was observed under conventional soil tillage. Ploughing downhill resulted in much larger dispersal distances than ploughing uphill. In the crops studied, combine harvesters dispersed few Avena spp. seeds, because of the fact that the plants had shed most of their seeds (>90%) before harvest. The percentage of seeds available to be dispersed by the combine was dependent on the harvest time. Although combine harvesting may not contribute much to short-distance dispersal, it may play an important role in long-distance dispersal. In our studies, isolated plants were located up to 30 m from the original sources. This small proportion may have a significant effect on the distribution of the weed within a field, acting as foci for new patches. [ABSTRACT FROM AUTHOR]