Determining of critical period of weeds control in Soybean (Glycine max L.) in Sari region

In order to determine critical period of weeds control in soybean a field expriment was carried out in Agricultural research station, Dashtenaz, Sari in 2003 cropping season. The experimental design was a complete Randomized block with 3 replications with 16 treatments. The treatment were included different stages without weeds, weeds competition during different plant phenological stages. The control treatment were weeds free and with weeds competition during growing season. Among recognized weeds species, Amaranthus retrofelexsus and Convolvulus arvensis had the maximum and minimum dry matter, at the rate of 47.2% and 2.3%, respectively. Weeds dry matter also decreased significantly with increasing weeds free peroid. The weeds free peroid did not significantly affect on the stem nodules, seed dry weight and the number of grain per pods. The maximum number of pods per plant (85) was recorded in the control plot (weeds free). The minimum number of pods per plant (40) was recorded in the control plot (with weeds competition during growing season). The number of unfilled pods per plant, biological yield, seed yield and the number of sub branches per plant decreased significantly with increasing the period of weed interference. Therefore seed yield (570g/m2) in the control plot (weeds free) decreased (185g/m2) in the control plot (with weeds competition). It can be concluded that significant decrease in grain yield can be prevented by control of weeds between phenological stages V3 (25days after seeding) and R1 (52days after seeding). The trends of variation of seed yield in comparison to the controlds, in this period (V3 to R1) showed that it has decreased from 99.3 % in FR1 80.76 % in FV11 treatment and from 98.59% in CV3 to 72.2% in CV7.