To investigate the effect of interference natural population weeds and plant density on physiological characteristics of soybean (Glycine max L.) in different sowing dates, a field experiment was carried out during the spring 2009 at University of Zanjan as a factorial split plot on the basis of randomized complete block design, with three replications. Treatments were including factorial of sowing date in two levels (May, 27; and June, 9) and plant density in three levels (25, 33 and 50 plants. m-2) as main plot and weed interference at two levels (control and no control) as sub plot. Crop canopy was closed earlier with the early sowing and increasing the plant density, that lead to increasing leaf area index and the total dry matter. Also the maximum crop growth rate obtained from 50 plant.m-2 plant density and 27 May sowing date. The results showed that growth indices and grain yield decreasing in weed interference

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