Volunteer Brassica napus (L.) interference with soybean [Glycine max (L.) Merr.]: management thresholds, plant growth, and seed return

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.