In order to investigate the interaction between weed competition times and at different levels of nitrogen fertilizer on growth characteristics of sunflower a split plot experiment based on randomized complete block design was conducted with 24 treatments and 3 replications in research farm of college of agriculture of Birjand University in 2010. The main factor was 3 levels of nitrogen (0, 100 and 200 Kg/ha-1). The sub factor was different weed competition periods containing of 3 levels of weed free, 3 levels of weed infested until phonological stages of two leaf (V2), four leaf (V4) and head visible stage(VR) of sunflower and with 2 levels (all season weed free and weed infested) treatments. The results showed that the power of weeds competition with sunflower with the application of nitrogen fertilizer increase until the level of 100 Kg/ha-1of nitrogen fertilizer consumption, but increases more nitrogen is favor of sunflower. The main weeds were barnyard grass, bindweed, common mallow, red root pigweed, common lamb’s quarters, and hoary cress. Application of nitrogen fertilizer also have a significant increase of biological yield, height and head diameter of sunflower, but the severity was of the increased under the influence of competing weeds.
