Influence of Nitrogen and Weed Interference on Grain Yield, Yield Components and Leaf Chlorophyll Value of Soybean

In order to evaluate the influence of nitrogen (N) rate and weed interference durations on grain yield, yield components and chlorophyll (SPAD) value of soybean, a set of experiments were conducted in the Research Station of the University of Tabriz, Iran, during 2005 and 2006. The experiments were carried out as split-plot, based on randomized complete block design with three replications. Nitrogen application rates (0, 20, 40 and 60 kg/ha) were included in the main plots and 10 weed interference periods (two sets) in subplots. Soybean was either kept free of weeds for Ve, V1, V4, R1 and R3 growth stages (GS) or weeds were allowed to grow for the corresponding periods, and subsequently kept weed free. Weed infested conditions for the entire growing season led to 14.68%, 17.33%, 21.42%, 49.52%, 54.55%, 13.01%, 18.05% and 48.42% reduction in SPAD value of V4, R1 and R3 GS of soybean, pods and grains weight and grain yield, respectively, as compared with full season weed-free control treatments. These traits were positively influenced by increasing N levels and consequently better vegetative growth was observed. However, SPAD value showed a decreasing trend with the progress of soybean GS for all levels of N. SPAD value gradually increased in response to increasing weed-free period compared with the weedy control and gradually decreased in response to increasing weed-infested period compared with the weed-free control. The results suggested that the adverse effect of weed competition on soybean is more apparent at low N levels and increasing the amont N provides a competitive advantage for soybean over to weeds thus high N rates are more beneficial to the crop than to the weeds.