Glyphosate-resistant (GR) crops are produced over large areas in North America. A study was conducted at six western Canada research sites to determine seed date and tillage system effects on weed populations in GR spring wheat and canola cropping systems from 2000 to 2003. Four-year wheat–canola–wheat–pea rotations were devised with varying levels of GR crops in the rotation. Weed populations were determined at pre– and post–in-crop herbicide application intervals in 2000 and 2003. Early seeding led to higher and more variable in-crop wild oat and wild buckwheat populations. High frequencies of in-crop glyphosate wheat in the rotation usually improved weed management and reduced weed density and variability. Canonical discriminant analysis (CDA) across all locations revealed that by 2003, green foxtail, redroot pigweed, sowthistle spp., wild buckwheat, and wild oat, all associated with the rotation lacking in-crop glyphosate. Similar CDA analyses for individual locations indicated specific weeds were associated with 3 yr of in-crop glyphosate (Canada thistle at Brandon, henbit at Lacombe, and volunteer wheat, volunteer canola, and round-leaved mallow at Lethbridge). However, only henbit at Lacombe and volunteer wheat at Lethbridge occurred at significant densities. Although excellent weed control was attained in rotations containing a high frequency of GR crops, the merits of more integrated approaches to weed management and crop production should also be considered. Overall, rotations including GR spring wheat did not significantly increase short-term weed management risks in conventional tillage or low soil-disturbance direct-seeding systems.

Nomenclature: Glyphosate; annual sowthistle, Sonchus oleraceus L. SONOL; Canada thistle, Cirsium arvense (L.) Scop. CIRAR; green foxtail, Setaria viridis (L.) Beauv. SETVI; henbit, Lamium amplexicaule L. LAMAM; perennial sowthistle, Sonchus arvensis L. SONAR; redroot pigweed, Amaranthus retroflexus L. AMARE; round-leaved mallow, Malva pusilla Sm.; spiny sowthistle, Sonchus asper (L.) Hill SONAS; wild buckwheat, Polygonum convolvulous L. POLCO; wild oat, Avena fatua L. AVEFA; canola, Brassica napus L.; pea, Pisum sativum L.; wheat, Triticum aestivum L.