Awareness of critical period of weed control in Riceland is necessary in formulating appropriate and inexpensive weed control strategies. To determine the critical period of weed control in drought stress conditions, a split plot design was conducted with two factors including rice varieties (the improved cultivar of Khazar; and local cultivar of Binam) and hand weeding, in Rice Research Institute of Rasht in 1999. The experiment was conducted in a randomized complete block design with four replications and 10 treatments of hand weeding in two sets. The first set of treatments included weed free periods of 0, 10, 20, 30 and 40 days after rice transplantation (DAT), and the other set of treatments included weed infested periods of 0, 10, 20, 30 and 40 days. Plots were irrigated on specific times using a water-carrying truck. The two sets of treatments were fitted to Logistic and Gompertz functions to determine the critical periods of weed control. Crop height, number of tillers, crop yield, weed samples, and dry weight of a square meter of weeds in each plot before harvesting were also measured. The most dominant and abundant weeds of the station were Barnyard grass (Echinochloa crusgalli), Bog-Bulrushes (Scripus mucronatus, S. juncoides), Umbrella-Nutsedge (Cyperus djfformis) and Water - Plantain (Alisma plantago), with higher density in Khazar than Binam. Increasing the periods of weed free and decreasing periods of weed infestation reduced the number of weeds and created a better condition for rice. A critical period of weed control between 24.5 and 31.5 DAT was observed in Khazar using 5% acceptable yield reduction. In Binam, with 3% acceptable yield reduction, a critical point of time for weed control was obtained on 22 DAT. In evaluation of critical periods of weed control versus weed competition on plant height and tiller number, no correlation was obtained among periods of weed interference with crop height and tiller number, while, they mostly increased by increasing the periods of weed control.
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