The critical period of weed control is the portion of the life cycle of a crop during which it must be kept weed-free to prevent yield loss due to weed interference. The advent of herbicide-resistant canola (Brassica napus L.) varieties in western Canada has meant that there are now more options for postemergence weed control in canola, and this has prompted increased interest in identifying the optimum timing for weed control in this crop. A critical period experiment was conducted at three locations in southern Manitoba in 1998 and 1999, and it consisted of two sets of treatments. In the first set of treatments, the crop was kept weed-free for increasing lengths of time to determine when emerging weeds would no longer reduce crop yield. In the second set of treatments, weeds were permitted to grow in the crop for increasing lengths of time to determine when weeds emerging with the crop began irrevocably to reduce crop yield. Results of the experiments indicated that canola must be kept weed-free in most cases until the four-leaf stage of the crop (17–38 days after crop emergence [DAE]) and, in one early-seeded experiment, until the six-leaf stage of the crop (41 DAE), in order to prevent >10% yield loss. After the four- to six-leaf stage of the canola crop, few weeds emerged, and late-emerging weeds accumulated little shoot biomass. Weeds needed to be removed by the four-leaf stage of the crop (17–38 DAE) to prevent >10% yield loss due to weed interference. In all but the early-seeded experiment, the critical weed-free period and the critical time of weed removal overlapped, such that a single weed removal at the four-leaf stage of the crop would have been sufficient to prevent >10% yield loss. This information will be useful for providing weed control recommendations to canola producers.

Nomenclature: Diquat; glufosinate; glyphosate; terbufos; barnyardgrass, Echinochloa crus-galli (L.) Beauv. ECHCG; canola, Brassica napus L. BRSNS, ‘Innovator’; common hempnettle, Galeopsis tetrahit L. GAETE; common lambsquaters, Chenopodium album L. CHEAL; common purslane, Portulaca oleraceae L. POROL; dandelion, Taraxacum officinale Weber in Wiggers TAROF; field pennycress, Thlaspi arvense L. THLAR; green foxtail, Setaria viridis (L.) Beauv. SETVI; quackgrass, Elytrigia repens (L.) Nevski AGRRE; redroot pigweed, Amaranthus retroflexus L. AMARE; roundleaved mallow, Malva pusilla Sm. MALPU; smartweed, Polygonum persecaria L. POLPE; volunteer barley, Hordeum vulgare L. HORVX; volunteer flax, Linum usitatissimum L. LINUS; volunteer wheat, Triticum aestivum L. TRZAS; wild buckwheat, Polygonum convolvulus L. POLCO; wild mustard, Brassica kaber (DC.) L. C. Wheeler SINAR; wild oat, Avena fatua L. AVEFA; yellow foxtail, Setaria pumila (Poir.) Roem & Schult. SETLU.