International Research Journal of Applied and Basic Sciences. Vol., 3 (6), 1274-1278, 2012 Available online at http:// www. irjabs.com ISSN 2251-838X ©2012 Evaluation of some of competitiveness indexes in competition between cotton varieties and common cocklebur (Xanthium strumarium L.) Asghar Rezakhanlou 1* , Mahdi Aghabeigi 2 and Hossein Bagheri 3 1- Department of Agronomy, Saveh Branch, Islamic Azad University, Saveh, Iran. 2- Department of Plant Protection, Agricultural & Natural Resources Researches Center of Tehran, Tehran, Iran. 3- Department of Agriculture, Chaloos Branch, Islamic Azad University, Chaloos, Iran. *Corresponding Author Email: Rezakhanlou@Yahoo.Com Abstract In order to examine the competitive strength of cotton varieties experimental farm in 2010 in a factorial randomized complete block design was in 3 replication that consisting of 12 variety and three density levels (0,2,4 of plant-1) common cocklebur weed (Xanthium strumarium L.) was in conditions of farm. With this aim, for competitive potency, the two indices were used; AW (Ability with stand competition) and Cl (Competition index). Results showed that the index of AW is the ability to better estimate the competitive power. And there are varieties that have the greatest value AW, lowest yield in terms of varieties compete with weeds are than to Conditions of weeds free. Keywords: Competitive index, weed, cotton, common cocklebur. Introduction Weeds damage the crop growth, yield and quality considerably. These damages depend on the weed species and weed density occurring in a crop community. Appearance and persistence of a weed in a locality largely depends on and influenced by the crop, season and method of cultivation, climate, edaphic and biotic factors. Most often competition between crops and weeds occur for moisture, nutrients, space, and light and possibly for carbon dioxide. This is because of overlapping of demand from weeds and crops simultaneously which exceeds the supply. Competition increases when these inputs are in short supply. Many factors like nature of the crop, variety, agronomic factors followed to control weeds etc. decide the competition. Quick growing better canopy crops like cowpea, greengram etc., suppress the weed growth while, short statured slow growing crops like onion and garlic tend to be suppressed by weeds (Yaduraju and Mishra, 2003). In most part of Iran, cotton is planted during the middle spring when soil temperatures is too high for optimum growth; however, there are many weed species that germinate and thrive under condition that is unfavorable for cotton growth. Cotton yield and quality losses may ensue following competition by weeds for water, nutrients, sunlight, and possibly by allelopathic weed–crop interaction (Frans and Chandler 1989). Cocklebur (Xanthium strumarium) is an annual broadleaf weed capable of growing more than 2 m tall. Because of its large and aggressive growth, cocklebur detrimentally affects crop growth and yield by competing effectively for light, water, space, and nutrients. It can produce hundreds of thousands of seed per plant, which may remain dormant in the soil for years (Keeley et al. 1987). Extensive research has been reported on several weeds that grow in the same environment as cocklebur; however, research particular to that weed's interference with cotton is limited. Many researchers have characterized the interference potential of various weed species in cotton (Steckel and Gwathmey, 2009). There are many examples of the impact of weed interference on cotton yield. Palmer amaranth (Amaranthus palmeri S. Wats.), jimsonweed (Datura stramonium L.), common cocklebur (Xanthium strumarium L.), and tall morningglory [Ipomoea purpurea (L). Roth] were some of the most competitive weeds in cotton (Steckel and Gwathmey, 2009). At high common cocklebur densities (6.6 plants m21 of cotton row), which reflected both interspecific interference of common cocklebur with cotton as well as