In order to study the effectiveness of different planting dates and crop densities on weed growth characteristics, a field experiment was conducted in Ferdowsi University of Mashhad, Research Station of College of Agriculture, during 2006. The experiment was arranged as a split plot based on a Completely Randomized Block Design with 3 replications. Different planting dates (April,25th, May,10th and May,24th) were placed in the main plots and various crop density (10, 20, and 30 plant/m2) in the sub plots. The results indicated that planting date of April 25th, caused minimum weed dry matter and weed growth rate. The maximum yield and dry matter of common bean were obtained in crop density of 30 plants/m2, However, weed growth rate and dry matter weights in plots with 30 plants/m2 were minimum. 687. The Influence of Nitrogen Application on Critical Period for Weed Control in Corn (Zea mays L.). Reza Ghorbani1, Seid Hussieni1, Mohammad RashedMohasel1, The critical period for weed control (CPWC) is a period in the crop growth cycle during which weeds must be controlled to prevent unacceptable yield losses. A field experiment was conducted at Research Station of Ferdowsi University ofMashhad, Iran, in 2004 to evaluate the influence of nitrogen application on the CPWC in grain corn in competition with a naturally occurring weed population. Nitrogen fertilizer was applied at ratesequivalent to 184 and 368 kg N ha-1. A quantitative series of treatments of both increasing duration of weed interference and length of weed-free period were included in each nitrogen level. The beginning and end of the CPWC based on 10% acceptable yield loss level were determined by fitting the Logistic and Gompertz equations to relative yield data representing increasing duration of weed interference and weed-free period, respectively. The CPWC in 184 kg N ha-1 and 368 kg N ha-1 was determined 17-40 (V3 to V8) and 18-32 days after emergence (V4 to V7), respectively. The addition of 368 kg N ha-1 hastened the end of the CPWC when compared with 184 kg N ha-1. It seems, in higher Nitrogen level, rapid canopy closure resulting from higher crop leaf area index (LAI) resulted in less weed interference and the critical weed free period was ended earlier. Although not very much, difference in the CPWC due to nitrogen application documented in this study, highlight the importance of integrating decisions regarding nitrogen management and timing of weed contro

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