the effect of nitrogen rate and weeding time on weed communities dynamic and yield of canola (brassica napus l.)

in order to evaluate the effect of nitrogen rate and weeding time on weed communities dynamic, a field study conducted in a research farm of chamran university, ahwaz. the experimental design was split plot based on rcb with three replications. nitrogen rates (77, 110, 143 kg/ha) were considered in the main plot and time of weeding (at 5, 8, 11, leaf stage with control and weedy plot) as sub plot. the effects of treatments on weeds were evaluated at two periods: the first period was each time of weeding and the second one was before canola harvesting. the results indicated that nitrogen rate and weeding time had significant effect on weed dry matter and density. the highest weed dry matter with 482 g.m-2, was obtained from 143 kg n.ha-1 and weedy check and the highest weed density with 292 plant.m-2 was obtained from 77 kgn.ha-1 and weeding at 8-leaf stage. in the most cases, weed dominant species, frequency and phenological stages were affected by treatments. however, the effect of nitrogen rate and weeding time at the first period was higher than other treatments. the highest grain yield with 4740 kg.ha-1 was obtained from 143 kg n.ha-1and weed free plots. therefore, canola gain yield was increased with increasing of nitrogen rate.