Effect of redroot pigweed (Amaranthus retroflexus L.) in various densities and emergence times on yield and yield components of sunflower (Helianthus annus L.)

Influence of redroot pigweed in various densities and emergence times on sunflower yield was studied in Miyaneh region of Iran in 007. The experiment was carried out as randomized complete block design in a factorial arrangement of three pigweed density (.5, and 6 plants m- of row) and five time of emergence (emergence with sunflower,, 4, 6 and 8 weeks after sunflower emergence) in three replications. The effect of weed density and time of emergence on sunflower yield, yield component and head diameter were analyzed and described by rectangular hyperbola model. Results revealed that seed number per head, seed thousand weight, head diameter and seed yield significantly decreased with increasing weed density and earlier pigweed emergence. Pigweed emerging up to six weeks later than sunflower did not cause any significant reduction in sunflower seed yield, but even at this time, increased pigweed density reduced the yield.