effects of crop rotation and tillage depth on weed competition and yield of rice in the paddy fields of northern iran

crop rotation is one of the essential practices in sustainable agricultural systems, be-cause of its effects on soil fertility and other benefits including a reduction in weed compe-tition. a field experiment was conducted at chaparsar rice research station, tonekabon, iran, from 2002 to 2003 to assess the influence of crop rotation, time of planting, and till-age depth on rice (oryza sativa l.) yield and density and the biomass of three important paddy weeds. treatments included the crop rotations of continuous rice and rice-soybean-rice, in combinations with planting times and no tillage, minimum tillage, and chisel plow-ing. crop rotation had a significant effect on the growth of barnyardgrass (echinochloa crus-galli), arrowhead (sagittaria sagittifolia), and water plantain (alisma plantago-aquatica) weeds. continuous rice planting produced the highest density of barnyardgrass, arrowhead, and water plantain weeds, compared with a rice–soybean-rice rotation. weed biomass and weed densities were lower in the rice-soybean-rice rotation than in continu-ous rice. there were a 62.5 and 80% reductions in the weed density and weed biomass re-spectively, in the rice-soybean-rice rotation compared with continuous rice planting. fewer barnyardgrass, arrowhead and water plantain growth were observed in no-tillage treatments than in minimum tillage and chisel. therefore, rice yield in rotation treat-ments increased by 17 and 21% in 2002 and 2003, respectively, compared to continuous rice planting.