| The critical period of weed control is the time during the lifecycle of a crop during which it must be kept weed-free to prevent yield loss from weed interference. The advent of soil-applied herbicides and herbicide-tolerant canola varieties in western Canada has increased interest in research to find the proper timing for weed control in canola. A critical period experiment was performed at three sites in southern Manitoba in 1998 and 1999 and consisted of two sets of treatments. In the first set of treatments the crop was kept weed-free for increasing lengths of time to find the minimum weed-free period required to maintain maximum yield. In the second set of treatments, weeds were permitted to grow in the crop for increasing lengths of time to find the maximum tolerable weed-infested period. It was found that canola must be kept weed-free until the 6th leaf stage (20–39 DAE) to consistently prevent greater than 10% yield loss. In addition, the crop required the removal of weeds by the 4th leaf stage (14–32 DAE) to prevent greater than 5% yield reduction from interference. It was also found that after the 4th leaf stage not many weeds emerged, and those which did emerge did not accumulate significant biomass to compete with the crop. Comparative growth analysis of weed-free and weed-infested plots revealed that total dry weight, crop growth rate, leaf area index of the canola crop was reduced by weed interference. The stem weight ratio was increased, while the leaf weight ratio was reduced by weed interference. The presence of weeds also decreased the amount of branching observed in the crop and increased the proportion of biomass allocated to reproductive parts. This information will be useful for making weed control recommendations to canola producers, in developing weed-crop interference models, and for breeding more competitive canola varieties. |
