Critical period of weed control in soybean (Glycine max) as influenced by starter fertilizer

The critical period of weed control (CPWC) in crops depends on many factors including supply and availability of nutrients. In order to investigate the effect of starter fertilizer on the CPWC in soybean a field experiment in a randomized complete block design with four replications in a factorial arrangement was conducted at the Agricultural Research Farm of Razi University, Kermanshah, Iran. Two factors were examined: the first factor was the starter fertilizer levels (0 and 25 kg ha) applied in the form of monoammonium phosphate and the second factor was the different weed interference periods. Results indicated that soybean seed yield and yield components were significantly decreased when the weed interference period was increased. Weedy condition for the entire growing season reduced seed yield by 36% as compared with full season weed free condition. In order to prevent > 5% seed yield loss soybean field must be kept weed-free from the V1 to R1 (9-47 days after crop emergence (DAE) or 111.20 to 743.05 accumulated thermal units (ATU) and between the V7 and R3 (26-52 DAE or 374.55 to 832.75 ATU) at 0 and 25 kg.ha of starter fertilizer level, respectively. Generally, the use of starter fertilizer slightly delayed the end of the CPWC (by 5 days), but this condition shortened the CPWC by 12 days because of the later beginning of the CPWC (by 17 days). Practical implication of this finding is that application of monoammonium phosphate as starter fertilizer can reduce the time needed for weed control in soybean field and consequently lead to less herbicide application and cultivation.