Weeds compete with crops for nutrients, water and space. To determine the interaction of cropping density and weed population in rice transplanting system, a two-year experiment was conducted in Guilan Rice Research Institute. Two dominant genotypes of rice (the improved cultivar of Khazar and local cultivar of Binam) in three transplanting intrarow distances of 13, 15, and 17.5 cm of machine transplanter with fixed 30 cm interspacing (which equals 256400, 222200 and 190500 rice bushes per hectare, respectively) were used in a randomized complete block with 3 replications in a split plot design. The genotypes of rice are considered as main factors, and the planting intrarow spaces as subunits. The most dominant and abundant weeds of the plot areas were barnyardgrass (Echinochloa crus-galli), bog-bulrushes (Scripus mucronatus, S. juncoides), umbrella-nutsedge (Cyperus difformis) and water-plantain (Alisma plantago), respectively. The results showed a direct correlation between the intrarow spacing and the density of barnyardgrass at all growth stages of rice. In contrast, the significant difference of intrarow spacing effects on weed density of broadleaves and sedges were limited to early stages of rice growth. Sprouting more tillers from the bases of rice and increasing the plant heights reduced the differences on weed density of broadleaves and sedges within each intrarow. The different responses of broadleaves and sedges with barnyardgrass in competition with rice are due to earlier germination, faster growth and higher final height of barnyardgrass. The improved cultivar of Khazar at row intraspace of 15 cm produced the highest yields, while the yield of local variety of Binam on 13 cm intraspace was more than those for the other distances.
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