Yield and harvest index of sunflower (Helianthus annus) grown by a in monocultur system e and in competition with redroot pigweed (Amaranthus retroflexus)

In order to Sstudy the effects of redroot pigweed (Amaranthus retroflexus) interference on yield and harvest index of sunflower (Helianthus annus), a factorial experiment based on a randomized complet block design in factorial arrangement werewas carried out conducted in the at Islamic Azad University of Tabriz Branch, induring 007 and- 008. Factors Treatments were three weed densities (5, 5 and 5 plants m- in of row) at three relative subsequent times of weed emergences (at the same timeemerged with sunflower;, 5 and 0 days after sunflower emergencegermination) and three sunflower cultivars (Azarghol, Hysun and Allstar). Indeed, a control plot was also considered for each variety in the experiment by roghing weeds. The rResults showed that biological yield at the in third level of weed emergence time appeared with a very had the lowest difference with the weed-free control (680 kg ha-). Sunflower biomass decreased of from 850 g g/m- ofin the control to lower than 50 g/ m- during the in full season interference of 5 weeds m- of in row. Grain Yyield production in Azarghol at weed emergence time of the 5 days ofter sunflower emergence decreased by 7% and % of that of the control compared with control with where increasing of weed density increased from a range of 5 to 5 and from 5 to 5 plants/ m- in of row, respectively., for the 5 days after sunflower emergence. at 5 days after sunflower emergence, respectively. In Regarding to the Hysun variety, all weed treatments produced yielded lower grain yield than the control, and the yield yield of three weed emergences time treatments (at the same time, 5 and 0 days after crop emergence) was reduced by 9%, 7% and 6% from of the control (858) kg/ ha- in control up to 9%, 7% and 6% in difference weed emergence times,, respectively. In Allstar variety, the yield loss in was more considerable than the other two varieties and it was reduced by 5% of the weakest lowest competition treatment with redroot pigweed that means this variety is highly susceptible to compete with weed plants like was calculated 5%, which was a reason for high competition power of redroot pigweed with this cultivar. In. The harvest index was also disclosed high variation between three cultivars since it’s value was decreased by 6.5%, % and 9% of the control yield in varieties Azarghol, Hysun and Allstar, respectively.comparison with control, harvest index in Azarghol, Hysun and Allstar cultivars reduced 6.5%, % and 9%, respectively.