ارزیابی قدرت رقابتی ارقام آفتابگردان (Helianthus annuus L.) در برابر علف‌هرز تاج خروس سفید (Amaranthus albus L.) در منطقه بیرجند

Introduction
Using crop species and cultivars with high competitive ability against weeds is one of the effective strategies for sustainable weed management. Emergence rate, rapid root growth, seed vigor, development rate of leaves, rapid root and shoot biomass accumulation, rapid canopy closure and plant height are important traits in relation to the competitiveness between different cultivars of crops. Competitive ability is measured using two indices including the weed growth prevention ability or weed biomass reduction index and crop tolerance to weed or yield reduction preventing index. The purpose of this study is to evaluate the competitive ability of six oilseed sunflower (Helianthus annuus L.) cultivars and also introducing the most important morpho-physiological attributes affecting their competitive ability with tumble pigweed (Amaranthus albus L.) in Birjand.
Materials and methods
This experiment was carried out as factorial layout based on randomized complete block design with three replications at the Agricultural Research Station, University of Birjand in 2012. Treatments were included six sunflower oilseed cultivars (Azargol, Jame esfehan, Farrokh, Syrna, Progress, Euroflor) and tumble pigweed densities in four levels (zero (control), 5, 10 and 15 plants per square meter). The number of days and cumulative degree days were recorded from sowing to emergence. Plant height, leaf area and dry matter were recorded at four stages from emergence to 75 days after it. Sunflower seeds were harvested after physiological maturity. Preventing indices were used to evaluate the competitive ability of cultivars, competitive tolerance (Watson et al., 2002) and weed biomass. Data were analyzed with the SAS software and cluster analysis was performed using SPSS software. FLSD test was employed for comparison of the means at the 5% significance level. The graphs were prepared by Excel.
Results and Discussion
Analysis of variance showed that there was a significant difference between the sunflower cultivars for Competitive Index (P≤0.01). While, different densities of tumble pigweed had no significant effect on Competitive Index. However, the interaction effect between sunflower cultivar and tumble pigweed density was significant on this index (P≤0.05). The highest competitive ability index (1.6) was obtained in the Jame Esfehan cultivar, Followed by Euroflour, Azargol, Progress, Farrukh and Syrna cultivars. Although weed biomass decreased index was not affected by weed density, it increased by 11% with enhancing weed density from 5 to 15 plants m-2. In this study, tolerance index was significant only by the effect of weed density (P≤0.01). In general tolerance index increased by 13.3 and 39.03 percent by increasing the weed density from 5 to 10 and 15 plants m-2. This index for 5, 10 and 15 plants m-2 of weed densities were 127, 144 and 177, respectively. The results showed that rapid emergence and height increasing rate at first 53 days after planting and leaf area development rate between 17 to 53 days after planting were the most important effective factors that increase sunflower competitiveness against tumble pigweed.
Conclusion
In general, Jame Esfehan cultivar had the greatest competitive ability against tumble pigweed due to a rapid emergence, height increasing and leaf area development rates at 24 days after planting. Progress, Azargol, Euroflor and Farrokh cultivars had medium competitive abilities. Syrna had a weak competitive ability due to late emergence, as well as low rates of height increasing and leaf area development

In order to evaluate the competitiveness of some sunflower cultivars against tumble pigweed, a factorial experiment based on completely randomized design was conducted at the research farm of College of Agriculture, University of Birjand in 2012. Experimental factors included four different weed densities (0, 5, 10 and 15 plants m-2) and six cultivars of sunflower (Azargol, Jame Esfahan, Syrna, Farrukh, Progress and Euroflor). The results showed that rapid emergence, the rate of plant height enhancement at 53 days after planting and the rate of leaf area expansion during 17 to 31 days after planting were the most important effective traits for improving the competitive ability of sunflower cultivars against the tumble pigweed plants. In general, Jame Esfahan was the resistant cultivar, Azargol, Euroflor, Farukh and Progress were semi-resistant cultivars, and Syrna was the non-resistant cultivar against tumble pigweed plants.