Effects of Water Deficit and Redroot Pigweed Interference Period at Different Growth Stages on Sunflower Yield and Oil Percentage

In order to investigate the effects of water deficit and redroot pigweed (Amaranthus retroflexus L.) interference period on sunflower yield, two field experiments were carried out during 2009-10 and 2010-11 growing seasons at the Agricultural College of Shiraz University. The experimental design was split plot based on randomized complete blocks with three replications. Main plots included water deficit at three levels (100%, 75% and 50% field capacity) and main plots consisted of two groups 1)Interference period of redroot pigweed up to 8-leaf, 12-leaf stages, head emergence, flowering and maturity(whole season interference) of sunflower and 2)Control of pigweed up to stages mentioned above. Results indicated that with increasing water deficit levels from 100% to 75% and 50% field capacity, seed oil percentage, seed yield and its components decreased. With increasing pigweed interference, sunflower yield, and yield components, seed oil percentage and plant height were decreased and these reductions were significant from 12-leaf stage until head emergence. Water stress decreased pigweed dry weight more than crop. Water stress increased pigweed interference, in other words impact of weeds on sunflower growth and yield, increased with water stress.