determination of critical period for velvetleaf (abutilon theophrasti) control in summer seeded soybean, williams cultivar

velvetleaf (abutilon theophrasti) is one of the most important weeds of soybean fields in golestan province. a field experiment was conducted in gorgan agricultural research station in 2007 to determine the critical period for velvetleaf control in williams soybean (glycine max (l.) merr.) cultivar using a randomized complete block design with 4 replications. treatments were included two sets of weed-free (weed-control) and weed-interference (unweeded) periods. in weed-control treatments, plots kept weed-free until the following growth stages at which time velvetleaf was allowed to compete for the remainder of the season: 2-trifoliate leaf (gdd=320), 4-leaf (gdd=520), 6-leaf (gdd=750), flowering (gdd=950) and beginning pod (gdd=1325). unweeded treatments allowed velvetleaf to compete until soybean reached above-mentioned growth stages, when plots were maintained weed-free for the remainder of the season. in addition, study had interference and a weed-free control. non-linear gompertz and logestic equations were used to determine critical weed-free period and critical timing for weed removal, respectively. the effect of weed-free treatments on pod number per plant and grain yield was significant, statistically. however, weed removal treatments have no significant effects on number of seeds per pod and seed size. all yield components except seed size and grain yield were affected significantly by weed-interference durations. among yield components, pod number per plant had the highest correlation (r=0.97) with the grain yield. in contrast, the lowest change was that of seed size. based on 5 and 10 percent acceptable yield loss, critical period of velvetleaf control in soybean were 20 to 45 (from 3-leaf to flowering stage) and 25 to 35 (4- to 6-leaf stage) days after emergence, respectively