The effect of time weeding and different weed densities of pigweed on growth properties and cotton fiber quality in Varamin variety

In order to evaluate the effects of pigweed (Amaranthus retroflexusL.)on growth properties of cotton (gossypium hirsutum), this research was conducted in Agriculture and Natural Resources Research Center of Tehran province in 2014. Field trial was based on randomized complete block design with 9 treatments (consisting of three Amaranthus retroflexus emergency times coincide with cotton cultivation, two-leaves and four-leaves of cotton and three plant densities including (3, 6 and 12 Amaranthus retroflexus per square meter) in three replications. The results showed that all cotton quantity properties consisting of plant height, number of sympodia and monopodia, height and lenght of the first sympodia, leaf area index, leaf and stem biomass, number and weight of boll, and cotton yield in all treatments reduced compared to control treatment (cotton cultivation). The difference among treatments was significant in all traits except quality traits at the level of 1% so that emergence of pigweed coincide with cotton emergence by density of 12 plant pigweed, Morphological traits and cotton yield decreased. Due to the significant reduction in cotton yield in the presence of weed compared to conrol treatment,consideration of Amaranthus retroflexus time weeding is mandatory. Considering that the highest cotton yield reduction was related to plant density of 12 pigweed and coinciding germination of pigweed with cotton cultivation time, it is necessary weed control of pigweed from the beginning growth season and timely weeding at sowing time before formation of boll