Weed density. Results indicated that cultivar significantly affected A. retroflexus and S. arvensis. This
means the two cultivars had different competitive ability with weeds; Naz was more competitive than
Derakhshan (Figure 1). Other researchers have also reported the variable ability of different cultivars to
complete with weeds [7].
Time of herbicide application had also significant effect on A. retroflexus and S. arvensis, but had no effect
on S. nigrum. S. nigrum has probably shown same response to the time that herbicide was applied. Spraying
bentazon at the third trifoliate stage was more effective than spraying 15 days after the third trifoliate (Figure
2). This verifies that bentazon is more effective when applied at early growth stages that weeds are more
sensitive. Motley et al. (2001) also concluded that growth stage greatly affects the efficiency of herbicide [9].
Dose of bentazon significantly affected A. retroflexus and S. arvensis; S. nigrum showed no response to
dose. Increasing the dose of herbicide application from 1.5 to 3 li/ha increased weed control and 3.5 li/ha
controlled weeds by 100% (Figure 3). In another experiment [15], it was concluded that although the full dose
was better, reduced doses were also effective and controlled weeds desirably. They suggested that to increase
the efficiency of reduced doses, herbicide must be applied at early growth stages of weeds.