Timing of Velvetleaf Management in Soybean (Glycine Max (L) Merrill)

Velvetleaf (Abutilon theophrasti Medic) is one of the most important warm season weeds in soybean fields. To the aim of this study is to determine the critical period of velvetleaf control in two cultivars of soybean, Williams as indeterminate and Persian as determinate,with different growth patterns.. Theexperiment was conducted at the University of Agriculture and Natural Resources of Gorgan, Iran in 2007 using a randomized complete block design along with four replicates. The experiments consisted of two sets of treatments; one set was to keep the plot weed-free until the growth stages of 2- leaf (V2), 4- leaf (V4), 6- leaf (V6) stages, beginning of flowering (R1) and beginning of pod set (R3). The second set was interference treatments that velvetleaf was allowed to grow within the plot throughout the above-mentioned growth stages. Weedy and weed-free checks were also included in the study. The effect of control treatments were significant on final height, branch number per plant, leaf area index, dry matter, yield, and pod number per plant, while seed number in pod and 100-seed weights of soybean were not significantly affected. The effects of interference treatments were significant on all traits except 100-seed weights. Using the Gompertz and Logistic equations it was found that the critical period of controlling velvetleaf in both cultivars under study, considering 5% allowed decrease in yield, was between 260 to 943 CGDD or 14-51 DAE, which is approximately from 2-leaf stage to beginning of flowering. With a 10% allowed decrease, is the number would be between 320 to 752 CGDD or 18-41 DAE, which is approximately 3 to 6-leaf growth stage for both cultivars.