The Effect of Competitive Power of Oil Sunflower Cultivars on Growth and Development of Redroot Pigweed (Amaranthus retroflexus L.)

In order to evaluate redroot pigweed (Amaranthus retroflexus L.) growth and development at interference with sunflower (Helianthus annus L.) oil producing cultivars, a factorial experiment was conducted during 2005-2006 with three sunflower cultivars (Azarghol, Hysun and Allstar) and three redroot pigweed densities (8.3, 25 and 41.7 plants m-2) at 3 relative times of weed emergence (with sunflower, 15 and 30 days after sunflower emergence) using additive design. The results indicated that each one-day delay in weed emergence, the number of its leaves in three sunflower cultivar treatments reduced 2.7, 3.2% and 3.1%, respectively. By enhancement of weed density and earlier emergence time, weed cover increased in all 3 cultivars, especially in Allstar. Emerging 41.7 weeds/m2 with sunflower, increased weed leaf area index up to 45%, 62% and 70% compared with control, in three sunflower cultivars, respectively. With increasing of weed density from 8.3 to 25 plants/m2, weed leaf area per plant reduced from 1170, 1630 and 1910 cm2 to 410, 571 and 690 cm2 in three sunflower cultivars, respectively. In short, the results showed that the growth traits of redroot pigweed were influenced not only by the emergence time and density, but also by the cultivar traits in sunflower as well. It is suggested that redroot pigweed controlling in sunflower should be considered as soon as possible, especially in short height cultivars with less LAI such as Allstar.