yield and yield components of winter canola (brassica napus l.) affected by

in order to determine the critical period of weed control and investigation the effect of periodical control and interference of weeds natural population on yield and yield components of winter canola (brassica napus l. cv. okapi) in west region of tehran an experiment was carried out at research field of tarbiat modarres university, tehran, iran on 2004-5 growing season. fourteen experimental treatments which divided into two sets were arranged in randomized complete blocks design with three replications. in the first set, the crop was kept weedfree from canola emergence time to two-leaf stage (v2), four-leaf stage (v4), six-leaf stage (v6), eight-leaf stage (v8), initiation of flowering (if), %50 of pod set (%50ps) and final harvest (h). in the second set of treatments, weeds were permitted to grow with the crop until above mentioned stages and then related plots kept weed free till end of season. furthermore two additional treatments known as whole season control and whole season weed infested were established. at mentioned phonological stages in interference treatments weeds were removed, separated to species and measured for dry weight. also during canola growth season trend of plant height and dry matter distribution were studied. at the end of season canola grain yield and yield components were determined. results showed that extending interference duration and limiting weed control duration significantly decreased all canola yield components except 1000 grain weight .furthermore extended weed interference duration up to canola 4-leaf stage decreased %20-70 of grain yield in compare to whole season control. delayed weed control up to early rosette stage creates decreasing trend in canola grain yield. according to gompertz and logistic equations, critical period of weed control in canola was estimated between 25-70 days after emergence of canola.