Persistence of Glyphosate‐Resistant Canola in Western Canadian Cropping Systems

Abstract

Canola (Brassica napus L.) is the most important oilseed crop in western Canada. Its prevalence across the Canadian Prairies influences the occurrence and impact of canola volunteers as weeds. Here we determined the persistence of canola seed in cropping systems so effective volunteer management strategies can be developed. In mid‐ to late‐October of 2000, approximately 770 seeds m−2 of glyphosate [N‐(phosphonomethyl)glycine]–resistant (GR) canola were scattered on plot areas at seven western Canadian sites. From 2001 to 2003 the plots were seeded to a wheat (Triticum aestivum L.)–field pea (Pisum sativum L.)–barley (Hordeum vulgare L.) rotation or a fallow–field pea–fallow rotation in five different seeding systems involving seeding dates and soil disturbance levels, and monitored four times each year for canola plant density. Crop seeding date did not consistently influence volunteer canola density. With some exceptions, higher levels of soil disturbance led to higher volunteer canola densities. The vast majority of canola seedlings were recruited in the year following seed dispersal (2001). Across all locations, rotations, and seeding systems, and averaged over preplanting (PREP) and in‐crop prespray (PRES) intervals, canola densities were 6.2, 0.7, and 0.0 plants m−2 in 2001, 2002, and 2003, respectively. Canola volunteers were usually most abundant at PREP and PRES intervals; total recruitment at those intervals averaged across all seeding systems in the continuous cropping rotation was 3% (25 plants m−2). Preventing seed production in new canola volunteers in 2001 reduced canola densities in subsequent years (2002 and 2003) below those required to mitigate weed–crop competition influences in most crops.

The Biology of Canadian Weeds. 137. Brassica napus L. and B. rapa L.

“…In the 85 fields where volunteer canola was present, the mean density of 85.7 volunteer canola plants m (2 in the spring decreased to 13.7 plants m (2 by autumn. Similar observations were made by Harker et al (2006) in a 4-yr study across western Canada, which showed highly variable seedling densities among the locations (0 to 179 plants m (2 ), despite identical initial seed bank populations. Volunteer B. napus densities declined to B5 plants m (2 after the first year of the study.…”

“…Rotations including glyphosateresistant crops did not increase the risk from volunteer canola. Harker et al (2006) compared the density of canola volunteers in a wheat-field pea-barley rotation and a fallow-field pea-fallow rotation with five different seeding system combinations comprised of seeding date and soil disturbance level at seven sites across western Canada. Treatment effects were not consistent among sites and only one comparison was significant in 1 yr where greater volunteer canola densities tended to be associated with increasing soil disturbance in the continuous cropping rotation.…”

Canola rotation frequency impacts canola yield and associated pest species

“…However, it is notable that in 2011 and 2013, volunteer canola was the most prominent species even after 2 yr between canola crops (or 1 yr in 2011). Canola is known to exhibit secondary seed dormancy (Gulden et al 2003) and persist for several years in western Canadian cropping systems (Harker et al 2006).…”

Commercial generations of Brassica napus cause greater yield loss in Triticum aestivum, than volunteer B. napus generations

“…Brassica napus crop harvest losses of over 3000 seeds m (2 are common in western Canadian agriculture, and create a large seedbank (Gulden 2003). The greatest numbers of volunteers are seen in the first growing season following a B. napus crop (Harker et al 2006), which, in western Canada, is commonly a cereal crop. Harker et al (2006) estimated volunteer B. napus populations in wheat fields after broadleaf herbicide application range from 2.7 to 4.7 plants m (2 .…”

“…The greatest numbers of volunteers are seen in the first growing season following a B. napus crop (Harker et al 2006), which, in western Canada, is commonly a cereal crop. Harker et al (2006) estimated volunteer B. napus populations in wheat fields after broadleaf herbicide application range from 2.7 to 4.7 plants m (2 . Secondary seed dormancy allows volunteer B. napus to persist for 4 to 5 yr following a B. napus crop, potentially causing yield losses from weed competition (Simard et al 2002;Gulden 2003).…”