Ecological effects of different nitrogen rates and weed interference on oil and protein percentage of Soybean

Ecological effects of different nitrogen rates and weed interference on oil and protein percentage of soybean was studied in a two-year experiment at the Agricultural Research Station of Tabriz University, Iran. In each year, experiment was carried out as split-plot based on a randomized complete block design with three replications. Four nitrogen levels (0, 20, 40 and 60kgN/ha) were the main plots and 10 weed interference periods (two sets) were the subplots. In the first set, the plots were kept weed-free initially until VE (emergence of cotyledons), V1 (1st trifoliate), V4 (4th trifoliate), R1 (beginning bloom) and R3 (beginning pod) growth stages (GS) by hand. In the second set, plots were left weed infested initially for the corresponding periods and subsequently kept weed free. Results of analysis of variance showed that in the first year, protein and oil percent were significantly affected by different levels of nitrogen and period of weed interference. However, in the second year of testing and combined analysis of the two years, protein and oil percentage were not affected significantly by different levels of nitrogen and period of weed interference. In general, protein was reduced with increase in duration of weed contamination. When weed interference duration increased, oil percentage of soybean seed increased. Soybeen seed in weedy control had 5.71% less protein than full season weed-free control treatment. The increase in the nitrogen levels enhanced soybean tolerance to the presence of weeds. Although, vegetative phase was shortened, the growth and grain yield were increased by nitrogen application. The reductions in nitrogen use may warrant more intensive weed management. Furthermore, reductions in the nitrogen level before soybean planting may cause the need for more immediate weed control and for longer times.