Influence some agronomical factors on the Critical Period of Weed Control in Soybean

Background and objectives. The critical period of weed control (CPWC) refers to a part of crop growing season in which weeds should be removed in order to prevent maximum crop loss due to weed competition. Major factors which can influence the CPWC are planting date, cultivar and crop density. Planting date is impressive on the weed-crop interference. Soybean cultivars are different from the viewpoint of competition with each other. This subject maybe related to competitive or allopathic characteristics. Crop plant density also has an inhibitory effect on the weed density. About these factors have not done any study.
Material and Methods. This research was done in two separates experiment in a field that located in Sari Dashte Naz agronomical company. The first experiment done in split plot factorial and RCB design with three replications in 2013. Treatments were planting date in two levels (5 June and 24 June) as main factor and cultivar in two levels (033 and Agili) and weeding (with 12 weeding time) in factorial interactions as sub factor. Second experiment with split plot form, two factors in RCB format and three replications operated. First factor was planting density in two levels (31 and 42 p m-2) and second factor was different management (removal and weedy treatments). Treatments were placed in two groups based on soybean growth stages in both study. In the first set, the crop was kept weed-free until the growth stages of emergence, 1-trifoliate, 3-trifoliate, 5-trifoliate and flowering. The beginning and end of the CPWC based on 5% loss of the marketable seed loss, was determined by fitting Logistic and Gompertz equations to the relative yield data representing increasing duration of weed interference and weed-free period for three factors.
Results. According to 5% acceptable yield loss beginning of CPWC for early and late planting date estimated 29 and 359 Degree Day After Planting(DDAP) and ending of CPWC were 1678 and 1258 DDAP. Appropriate sooner opportunity such as irrigation and fertilization in early planting , gave more time to weeds in order to germination and competition with soybean and resulted in become longer CPWC. Mean weed density was 77 p m-2 and dominant species were Velvetleaf, Black nightshade and Redroot Pigweed. The beginning of CPWC was uniform for both cultivars but duration CPWC was longer in Agili. 033 had more growth rate and could close sooner canopy. For these reasons its CPWC was shorter. With increasing crop density there was not any CPWC. This finding was described through weed density reduction especially redroot pigweed which required to light for germinating.
Conclusion. Differences in CPWC due to planting date documented in this study highlight the need for a greater understanding of environmental factors affecting competition for limited resources. A practical implication is that EARLY soybean requires more intensive weed management than LATE soybean, at least in the north-east Iran. On the whole late planting with more density and using 033 cultivar, not only can reduce herbicide dose but also deduct risk injury to the next crop in rotation.