Soybean [Glycine max (L.) Merr.] has recently become a popular rotational crop in the Canadian Northern Great Plains where herbicide-resistant (HR) soybean cultivars have been widely adopted. Intense reliance on herbicides has contributed to the development of HR weeds in soybean and other crops. Cultural weed management practices reduce the need for herbicides and lower the selection pressure for HR weed biotypes by improving the competitiveness of the crop. The effects of two row spacings, three target densities, and three cultivars on the critical weed-free period (CWFP) in soybean were evaluated as three separate experiments in southern Manitoba. In the row-spacing experiment, soybean grown in narrow rows shortened the CWFP by up to three soybean developmental stages at site-years with increased weed pressure. In the target density experiment, low-density soybean stands lengthened the CWFP by one soybean developmental stage compared with higher-density soybean stands. The effect of soybean cultivar varied among locations, yet tended to be consistent within location over the 2-yr study, suggesting that competitive ability in these soybean cultivars was linked to edaphic and/or environmental factors. Generally, the cultivar with the shortest days to maturity, which also had the shortest stature, consistently had a longer CWFP. Each of these cultural practices were effective at reducing the need for in-crop herbicide applications.

Canola (Brassica napus L.) and soybean [Glycine max (L.) Merr.] are currently two of the three most common crops grown in Manitoba which comprises the eastern regions of the Canadian Prairies. Volunteer B. napus is a prominent weed in soybean in Manitoba and glyphosate-resistant (GR) volunteer B. napus often is the only weed remaining after in-crop weed control with glyphosate in soybean. Additive-series field experiments were established at three locations in Manitoba in 2012 and 2013 to study volunteer B. napus interference with soybean and develop action and economic thresholds for this weed. Soybean were planted in narrow- (25-cm) or wide- (75-cm) row spacing and glyphosate-resistant B. napus seed was broadcast at densities of 0, 10, 20, 40, 80, 160, 320 and 640 seeds m-2 at the time of soybean planting. Development of soybean and volunteer B. napus were determined throughout the growing season and seed yield of both species was determined at their respective physiological maturity. Volunteer B. napus is highly competitive with soybean as action (< 9 plants m-2) and economic (< 5 plants m-2) thresholds were low. At these action thresholds, volunteer B. napus seed return to the weed seedbank was on average 14,400 seeds m-2 and 10,400 seeds m-2 in narrow and wide row soybean, respectively.

Canola (Brassica napus), as the world's second most important oilseed crop, is the most profitable farm crop for Canadian farmers, providing more than 207,000 Canadian jobs and $12 billion in wages across the country. Canada exports 90% of canola seeds, oil and meal produced annually to foreign markets (Canola Council of Canada, 2020).Blackleg disease caused by the actinomycete fungus Leptosphaeria maculans (anamorph: Phoma lingam) is one of the most destructive pathogens found in canola in North America, Australia, Europe and many other regions around the world .This pathogen can reduce yield by up to 80%, depending on the susceptibility of the cultivar grown (Marcroft et al., 2004). This fatal disease can be managed by an integrated approach using crop rotation, resistant cultivars, resistance (R)-gene rotation and fungicide application, while genetic resistance of the host is the mainstay