In order to determine the critical periods of weed control in dry bean, an experiment was conducted in 2011 at Agricultural and Natural Research Center of Hamadan, Iran. The experiment was a randomized complete blocks design with 12 treatments and 3 replications. Treatments were divided into two groups. The first group included weed interference periods until 10, 20, 30, 40 and 50 days after emergence of dry bean. The second group included weed control periods until aforementioned periods. Control plots consisted of two treatments that kept weed free and weed infested throughout the growing season. The planted bean cultivar was Naz, which is a creeping cultivar. Seeds were hand-planted in June 2011. Results showed that among the weeds, the maximum importance value of weed (IVW) was related to lamb's quarters and red-root amaranth (36.66 and 30.11%, respectively). Increase in duration of weed-infested periods caused an ascending trend for dry weight of weeds, while their density increased at first and decreased when it reached at maximum point. Taking 5% and 10% acceptable yield loss for dry beans, the critical period of weed control was determined as 20 to 68 days after emergence (equivalent to V2 to R1 stages) and from 22 to 55 days after emergence (equivalent to V2 to V6 stages), respectively. Therefore, weeds of dry bean must be controlled in these periods

Keywords: Weed dry weight, Yield Loss, Plant density, Interference.

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