Density and species composition of weeds are determining factors of weed control. To investigate the critical period of weed control in soybean an expriment was conducted in Eraghi-Mahaleh Research Station 7 Km north Gorgan in Golestan provience in 1998. The expriment comprised 14 treatments which were compared in a Compelete Block Design with 4 replication. In this expriment species composition. density, weed dry matter and their relation to reduction of soybean seed yield was investigated. The treatment were divided in two groups: The first group were kept totally weed free and the second one were kept weed infested up to some defined phonological growth stages of soybean. In weed infested treatments pigweed (Amaranthus retrotlexus) was the dominant weed in early growth season while by the end of the season velvetleaf (Abution sp.) Look over. Weed density followed an undetermined reduction trend. By continuation of weed competition from the begining of growing season, the weed biomass
significantly increased, while their density reduced. Evironmental conditions. soil and competition of neighboring plants (weeds with one another with crop plants) contributed in achieving such results. In contrast as the weed period was longer the weed density as well as weed dry weight significantly deareased. The Soybean seed yield significantly decreased as competition period with weeds lasted longer. An exponential relationship between soybean seed yield and weed dry weight was observed. The peak competition period of weeds in soybean crop was between formantion of the thiro node (V 3) to seventh node (V 7). If weeds are controlled in this period, Soybean would be protected from a significant in se_d yield.
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