Managing Seed Return of Volunteer Canola in Soybean Production.

Based on 2014 seeded acreage, canola was the most abundant crop grown in Manitoba.  Volunteer canola is the 14^th most abundant weed (after in-crop management) in western Canada due to high harvest losses, herbicide resistance and prolonged seedbank persistence.  Similar to canola, soybean (currently the third most abundant crop grown in Manitoba) is also genetically engineered to be resistant to glyphosate (GR).  Due to the limited options for chemical management of GR volunteer canola in GR soybean, an integrated management approach is necessary.  Throughout 2013 and 2014, four replicated field trials were conducted in Manitoba, Canada, to evaluate integrated cultural, chemical, and mechanical strategies for management of volunteer canola seed return and yield in soybean production.  Under in-crop glyphosate chemical management, high residual soil nitrogen, via supplementation of 26 kg N ha^-1, caused a 44% increase (equal to 14,460 seeds m^-2) in volunteer canola seed return compared to the unsupplemented soil treatment.  In a combined analysis over all site-years, soybean row spacing, soybean seeding rate and inter-row management had no effect on volunteer canola seed return under in-crop glyphosate chemical management.  Under in-crop chemical management with pinoxaden or clodinafop-propargyl, inter-row tillage and spring seeded wheat or fall-rye intercrops caused an approximate two-fold decrease in seed return (15,410; 12,480; and 13,960 seeds m^-2, respectively) compared to the wide-row soybean treatment with no inter-row management.  Soybean row spacing, soybean seeding rate, and residual soil nitrogen had no effect on volunteer canola seed return in this herbicide regime.  Results indicate that soybean did not compete well with volunteer canola even when soybean density and spatial arrangement were manipulated.  The effect of residual soil nitrogen on volunteer canola seed return in soybean production warrants further investigation.