effect of nitrogen on critical period of weed control in sunflower (helianthus annuus l.) in birjand region

in order to study of nitrogen effect on critical period of weed control in sunflower (helianthus annuus l.) in birjand region, 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 three levels of nitrogen (0, 100 and 200 kg.ha-1) and the sub factor was different weed competition periods containing of three levels of weed free, 3 levels of weed infested until phonological stages of v2, v4 and vr with two control (all season weed free and weed infested) treatments. the critical periods of weed control based on 5%, 10% and 15% of acceptable yield loss, were estimated by fitting of gompertz and logestic functions to driven data of respective yield of sunflower in weed free and weed infested treatments respectively. results showed that increasing of weed infested period and decreasing of weed free periods, increased weed dry weight but decreased their density. the most weed dry weight and competition and so the least yield of sunflower was observed in 100 kg ha-1 nitrogen. so, the longest period of weed control was obtained at this level of nitrogen fertilizer. the critical periods of weed control based on 5% of acceptable yield loss were 1-66, 3-91 and 2-76 dae for 0, 100 and 200 kg ha-1nitrogen, respectively