Determination of Critical Period of Weed Control in Sunflower (Helianthus annuus cv. Sandra) in Mobarekeh Esfahan Region

In order to determine the critical period of weed control in sunflower (Helianthus annuus L. cv. Sandra)a field study was conducted in the Mobarakeh region during 2013. The experiment was carried out in a randomized complete blocks design with three replications. Treatments were based on the days after emergence of sunflower in two series. The first series consisted of weed removal, 10, 20, 30, 40, 50 and 60 days after sunflower emergence and then weed maintained until harvest. The second series consisted of weed maintaining till mentioned stages and then weed removal until harvest. Also, two control treatments (weed free and weed infested for all of the growth season) were applied. Results showed that with increasing period of weed interference, grain yield, oil yield, seed number per head and 1000 seed weight decreased significantly. Increasing period of weed control, above characteristics increased significantly. Increasing period of weed interference, weed dry weight significantly increased, but weed density first increased and then decreased. With increasing period of weed control, weed dry weight and density significantly reduced. Considering 10% acceptable grain yield losses, the critical period of weed control of sunflower was determined to be 12-40 days after emergence (between 4 leaves and flowering stage).