Impact of water deficit on critical period of pigweed (amaranthus retroflexus l.) in sunflower

ABSTRACT-The critical period of weed control is a part of crop life cycle during which weeds must be kept weed-free to avoid yield losses due to competition. In order to evaluate the effect of deficit irrigation on critical period of redroot pigweed (Amaranthus retroflexus L.) in sunflower (Helianthus annuus L.), an experiment was carried out  as split plot based on randomized complete block design with three replications at the experimental farm of  College of Agriculture, Shiraz University, during 2010 and 2011 growing seasons. Factors were water deficit at three levels (100%, 75% and 50% of field capacity) as main plots and weed interference periods in weedy and weed-free plots at five sunflower growth stages (8-leaf, 12-leaf, head emergence, flowering and maturity) as subplots. Results showed that water deficit decreased grain yield and grain yield components in both years. Critical period of redroot pigweed in normal irrigation with accepting 5% yield loss in sunflower was 35-86 DAP (days after planting) in the first year and 49-94 DAP in the second year. By decreasing irrigation water to 75%FC and 50%FC, the length of critical period increased to 34-100 and 32-105 DAP in the first year and 50-101 DAP and 44-98 DAP in the second year of the study, respectively. Generally, our results showed water deficit extended the length of critical period of redroot pigweed in sunflower. Nomenclature: Redroot pigweed, Amaranthus retroflexus L.; Sunflower, Helianthus annuus L.