Agronomy Update Features Soybeans Weeds
Agronomy update: Volunteer canola competition in soybean is intense

Volunteer glyphosate-resistant (GR) canola is a challenge for Roundup Ready soybean growers since a glyphosate application does not control the volunteers. The objectives of this research study, led by Robert Gulden in the department of plant science at the University of Manitoba, were to determine the action and economic thresholds for volunteer canola (B. napus) in soybean grown at narrow- and wide-row spacing, and to evaluate the impacts of increasing volunteer canola densities on both soybean and volunteer canola plant development and seed yield. 

The research was conducted at three sites in Manitoba over two years at the Ian N. Morrison Research Farm near Carman, at Kelburn Farms near St-Adolphe, and on an independent research farm near Melita. Roundup Ready soybeans were seeded into wheat stubble in mid- to late-May at 180,000 seeds/ft² (445,000 seeds/m²) in narrow (10-inch; 25-cm) and wide (30-inch; 75-cm) row spacing. Roundup Ready canola was broadcast on the plots immediately before seeding soybean at densities of zero, one, two, four, eight, 16, and 32 seeds/ft² (zero, 10, 20, 40, 80, 160, and 320 seeds/m²) in 2012, with an additional treatment of 64 seeds/ft² (640 seeds/m²) included in 2013. 

Other weeds were controlled with two in-crop applications of glyphosate at 0.67 L/ac rate (900 g a.e./ha).

Volunteer canola competition was intense and caused over 50 per cent yield loss in soybean in most cases, and as much as 79 percent soybean yield loss under wide row production at Carman in one year. 

Average maximum soybean yield loss of about 60 per cent was similar in narrow- and wide-row production systems. This was unexpected as other weed interference research has found greater yield loss in wide-row soybean compared to narrow row spacing.

Soybean yield loss at low and high volunteer canola densities were similar. This indicates that volunteer canola competition with soybean is generally intense, leading to substantial yield loss in soybean at most volunteer canola densities – even at less than one volunteer canola plant/ft² (10 plants/m²). 

Action and economic thresholds were generally low due to the highly competitive ability of volunteer canola with soybean. Overall, action thresholds were less than 0.9 plants/ft² (<9 plants/m²) and economic thresholds were less than 0.5 plants/ft² (<5 plants/m²). At these thresholds, volunteer canola seed return to the weed seedbank was on average 1,440 seeds/ft² (14,400 seeds/m²) in narrow-row soybean, and 1,040 seeds/ft² (10,400 seeds/m²) in wide-row soybean. 

The low economic and action thresholds were attributed to differences in canola and soybean development. For example, by the time soybean reached the first trifoliate, volunteer canola plants already had an average of three leaves; once soybean reached its fourth trifoliate, volunteer canola had reached the reproductive phase. Additionally, canola has the ability to aggressively branch out and compete for soil and water resources when plant densities are low, resulting in intense competition even at low densities.

Thresholds developed in this study are an important decision-making tool for effective management of volunteer canola in soybean, and highlight the need for volunteer canola control in soybean even at low plant densities. 

Several herbicide Groups are available to control volunteer Roundup Ready canola (Group 9). These include herbicides in Groups 2, 5, 6, 14 and 15 – some pre-emerge and others post-emerge applications. 

Volunteer canola seed returns at the action thresholds were greater than typical seedbank additions caused by harvest losses of a canola crop. Therefore, soybean in rotation can be a significant contributor to the replenishment of volunteer canola in weed seedbanks if left uncontrolled. 

Bruce Barker divides his time between CanadianAgronomist.ca and as Western Field Editor for Top Crop Manager. CanadianAgronomist.ca translates research into agronomic knowledge that agronomists and farmers can use to grow better crops. Read the full Research Insight at CanadianAgronomist.ca.