Volunteer canola is difficult to control in soybean because the two crops are often resistant to the same herbicides, leaving few chemical control options available to growers in Western Canada. That causes more than a few headaches for soybean growers, especially as research shows that Prairie canola seed harvest losses on average are around six per cent of yield (around 4,000 to 5,000 seeds/m²). Because soybean is much less competitive than canola, reducing the canola seed bank left behind becomes critical when soybeans are grown after canola.
Tillage and seeding rates
Dr. Rob Gulden at the University of Manitoba recently completed a three-year study, funded by Manitoba Pulse & Soybean Growers (MPSG) to evaluate the effect of different management practices to control volunteer canola in soybean.
In Gulden’s studies, timely fall tillage shortly after the canola harvest encouraged volunteer canola seedling germination, which reduced the volunteer canola seed bank over winter. Timing was more important than tillage type — even a low-disturbance tillage pass (tine harrow) was effective at encouraging early fall and spring emergence of volunteer canola.
Making soybeans more competitive with volunteer canola is not easy. Through his research, Gulden found the main factors that contributed to increased soybean competitive ability were seeding rate and residual soil nitrogen levels. Row spacing had little impact on soybean yield in response to volunteer canola.
Action thresholds for volunteer canola in soybean are low (three to five plants m²), when growers can expect at least a five per cent yield loss. Narrow-row soybean (7.5 inches) was more competitive and had higher action thresholds before a five per cent yield loss was observed than wide-row (30 inches) soybean.
With today’s high seed costs, soybean growers are showing a lot of interest in reducing seeding rates to increase net profit margins. General recommendations for seeding rates in Manitoba are 190,000 to 200,000 seeds/acre for air seeders (based on 70 to 75 per cent expected seed survival), and 170,000 to 180,000 seeds/acre for planters (based on 80 to 85 per cent expected survival. The aim is to establish a live plant stand of 140,000 to 160,000 plants/acre.
In Gulden’s study, soybean seeded in narrow rows at around 262,550 seeds/acre (about 1.5 times the recommended seeding rate) resulted in a 44 per cent yield increase compared to a seeding rate of 184,210 seeds/acre (the recommended rate) regardless of volunteer canola pressure. That’s consistent with other research that found a positive connection between higher soybean plant density and yield under weedy conditions. What isn’t known is whether the increase in yield is sufficient to compensate for the higher soybean seed costs.
Recent research in Western Canada and Manitoba has found that lower plant populations can result in 95 and 100 per cent optimum soybean yield under weed-free conditions.
Gulden’s research, however, shows that recommended seeding rates were not enough to maximize yield potential under weed competition. Lower seeding rates contribute to promoting the development of herbicide-resistant weeds in less competitive crops.
Soybean seeding rate did not affect volunteer canola seed production, which demonstrates how volunteer canola can rapidly outgrow and compete with soybean.
Nitrogen and IWM
Gulden manipulated soil nitrogen levels through supplemental nitrogen fertilization; this affected the competitive balance between soybean and volunteer canola. As soil mineral nitrogen increased, volunteer canola became more competitive and soybean yield decreased. Soybean yield was not affected by supplemental nitrogen in the absence of volunteer canola.
Integrated Weed Management (IWM) techniques, including inter-row tillage, living mulches and terminated mulches were all effective in reducing volunteer seed production in soybean, especially when there is a high level of canola seedling emergence.
Inter-row tillage in wide-row soybean resulted in 37 per cent greater soybean yield and 55 per cent greater soybean biomass at the sites with high volunteer canola recruitment densities compared to wide-row soybean without inter-row tillage.
Inter-row tillage in the wider, 30-inch rows, however, increased volunteer canola seed weights by up to 11 per cent compared to the absence of inter-row tillage because the stress of inter-row tillage caused the volunteer canola plants to compensate.
Gulden also compared inter-row mulches of wheat and fall rye between soybeans on 30-inch row spacings with inter-row tillage. “The mulches did not appear to affect yield or the seed return very much, but the inter-row tillage looks promising from an IWM perspective because it increased soybean yield and a lowered volunteer canola seed return.”
Herbicides and the CWFP
Both Xtend and Enlist soybean varieties required an in-crop herbicide effective on volunteer canola to maximize volunteer canola control and soybean yield, and in both cases the best herbicide choices were not consistent among locations and years, appearing to be influenced by the growth rates of the volunteer canola and soybean.
Gulden’s research found that a number of herbicides with various modes of action are effective for in-crop management of volunteer canola in soybean. Herbicides with faster-acting modes of action were more effective at preventing soybean yield loss, particularly when volunteer canola was developing quickly at the beginning of the critical period of weed control in soybean.
“How quickly the herbicide works will determine how much soybean yield is preserved because growers need to hit the critical weed-free period (CWFP) to get good control,” said Gulden. Group 2s take two to three weeks to work, during which time the volunteer canola competes with slower-growing soybean crops. Products that work very quickly and are applied when the CWFP starts may do a much better job at preserving soybean yield.
The critical weed-free period in Manitoba is not easy to pin down. In related research Gulden has discovered that the CWFP can be influenced by row spacing, target plant density and variety choice.
Of the three practices, narrowing row spacing was most effective to shorten the CWFP, up to three developmental stages compared with wide-row spacings. The narrow-row spacings never required more than one glyphosate application at any of the study sites to limit yield losses to 10 per cent, and in four of the six sites no herbicide was necessary. This was especially effective under medium to heavy weed pressure situations.
Decreasing soybean target plant densities below 180,000 plants/acre lengthened the CWFP in many cases, and often required additional herbicide applications. Increasing the plant density to 270,000/acre had little effect on the end of the CFWP.
Choosing the right soybean variety is important to reduce the number of herbicide passes needed to protect yield. Taller and mid-height soybean varieties used in the study (DeKalb 23-40 and 24-10 respectively) had shorter CWFPs than a less competitive shorter variety (DeKalb 22-60). Choice of variety could lengthen the CWFP by up to two developmental stages, requiring at least one additional herbicide application. The variety differences, adds Gulden in his report on the study, were hard to predict because of differences in growing seasons and weed pressure.
All of these practices helped to reduce the CWFP in soybean and reduced herbicide applications needed to prevent yield loss due to weeds. The report concludes: “Individually, using these tools reduced the risk of developing herbicide-resistant weeds in soybean and reduced herbicide costs. Their efficacy could be improved even more when used in combination, as these tools seem to act synergistically.”