Abstract
In order to determine the critical period of weed control in Euro-flour variety of sunflower (Helianthus annus, L.) an experimental was conducted with a randomized complete block design with 10 treatments and three replications in Islamic Azad University Astara-branch-Agricultural College in 2006 growing season. Experimental treatments were conducted in two sets, the first set included weed free periods of 20, 40, 60, 80 and 100 days after crop emergence and the other set was weed infested in the same periods. In the first set, weeds removed in mentioned periods and then were allowed to compete with corp. In the second set of treatments, weeds competed with sunflower in those periods, and then removed up to harvest. In basis of percentage of sunflower seed yield in various treatments compare with completely weed free treatment, the two sets of treatments fitted with Gompertz and Logistic equations, respectively. The result showed that the critical period for sunflower. using %5 and %10 reductions, were 10-79 and 15-59 days after emergence, respectively. The highest yield was found in weed free control (3544 kg/ha), and the lowest yield was seen in weedy check treatment (321 kg/ha). Redroot pigweed (Amaranthus retroflexus), Knot grass (Paspalum disticum), Crab grass (Digitaria sanguinalis) were the most dominant weeds of the experimental site.

Keywords: Sunflower (Helianthus annus L.), Critical period, Weed, Interference, Control