Barnyardgrass [Echinochloa crusgalli(L.) P. Beauv.] is an important weed of rice in irrigated and rainfed ecosystems in Cambodia. Rice yields decrease as barnyardgrass populations increase (Smith 1968). To minimize weed competition, Cambodian farmers transplant rice and practice hand-weeding (Jahn et al 1997). The success of this method depends on the amount of standing water after transplanting and the critical period of weeding. Unpredictable rainfall does not favor the use of water in weed management. In addition, labor for hand-weeding is becoming scarce because of competition from other industries. Although herbicides can be used as a laborsaving option, they are expensive and may cause health and environmental hazards if incorrectly applied. In low-input farming systems such as those in Cambodia, competitive rice cultivars may be an important part of weed management.