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Agronomy update: Less weed competition for soybeans grown on low-nitrogen soils

April 6, 2023  By Bruce Barker, P.Ag

This was a cage match for all time. Soybean versus volunteer canola in a competition for resources to see which would survive – thrive.

Research was conducted in Manitoba by Robert Gulden of the University of Manitoba, and Charles Geddes with Agriculture and Agri-Food Canada at Lethbridge, Alta. The research looked at the resource-ratio (R*) hypothesis, and how it could be applied to crop production. This hypothesis looks at the competitive balance between two plant species. The most common application is that when two species compete for the same resource, the species that can survive at the lowest level will out-compete the other species. 

To test how this hypothesis would work in soybean, the research looked at how soil mineral N levels would affect volunteer canola competition. In one corner was volunteer canola that was dependent on soil nitrogen (N) levels for growth. In the other was soybean, which can fix its own N. The hypothesis, in this case, was that under lower-nitrogen (N) levels in the soil, volunteer canola would be less competitive, but soybean would fix its own N to compete more vigorously. 


The research was conducted at the Ian N Morrison Research Station near Carman, Man., with a total of three site years. Four different glyphosate-resistant canola populations consisted of F1 (crop) and F2 (volunteer) generations and two open-pollinated canola varieties to represent varying levels of competitiveness of different volunteer canola populations. A weed-free control was also included. The trials were grown on wheat stubble at each site-year. Canola was seeded at eight viable seeds/ft2 (80 seeds/m2) at 0.4 inch (1 cm) depth just prior to seeding soybean.

Urea fertilizer was applied at 0, 20, 40, 80 and 160 pounds N per acre (0, 22.5, 45, 90 and 180 kg N/ha). Soil nitrate-N test samples were taken at 0-6 inch and 6-24 inch (0-15 cm and 15-60 cm) depths. Total soil mineral N was considered to be applied urea plus soil nitrate-N in the top two feet of soil. 

A single application of glyphosate was applied at recommended label rate at soybean stage BBCH 13 (V1) to remove all weeds except volunteer canola. At this stage (4-5 canola leaf stage), soybean and volunteer plant density were also measured. Above-ground soybean and canola biomass were measured at BBCH 82 (beginning of pod ripening) and soybean stage BBCH 77 (approximately R5). Soybean and canola seed yield were also measured.

Soybean yield was unaffected by soil N levels when volunteer canola was not present. However, in plots with volunteer canola, soybean yield declined linearly by 2.31 lbs/ac (2.6 kg/ha) for every 0.89 lbs (1 kg soil mineral N/ha) increase in soil mineral N per acre. 

At 26.7 lb/ac soil mineral N (30 kg N/ha), yield was 50 bu/ac (3,350 kg/ha). This yield decreased by 17 per cent when soil mineral N level was 222.5 lbs/ac (250 kg/ha). 

The research indicated that volunteer canola competition at very low levels of soil mineral N below 13.4 lb N/ac (15 kg N/ha) would not affect soybean yield. The researchers note, though, this conclusion was extrapolated from the data, as the lowest mineral N in this trial was 26.7 lb N/ac (30 kg N/ha). The results highlight that planting soybeans in fields with low soil minerals can help reduce weed competition and improve yield. 

In terms of managing the weed seedbank, volunteer canola seed production increased as soil mineral N levels increased up to 163 lb N/ac (183 kg N/ha). At this level of soil mineral N, volunteer canola seed production ranged from 1,980 to 5,160 seeds per square foot (19,800 to 51,600 seeds/m) among the three site-years. Under the lowest N conditions present at each site, volunteer canola seed production declined by 19 to 74 per cent. Above 163 lbs N, seed production also declined, likely due to lodging of canola plants.

Similarly, previous trials by the same researchers found that applying 20 lbs N/ac (23 kg N/ha) when seeding soybean doubled volunteer canola seed production compared with soil residual N between 10 to 30 lbs N/ac (11–34 kg N/ha).

The current research shows that low N conditions of 27 lbs N/ac can reduce volunteer canola seed production to one-quarter of the amount compared to 163 lbs N, as shown in 2016. With two-thirds of seeds produced by volunteer canola contributing to the seedbank, under high soil mineral N conditions of 163 lbs N, seedbank inputs likely ranged from 1,320 to 3,440 seeds/ft2 (13,200 to 34,400 seeds/m2).

The resource-ratio (R*) hypothesis was confirmed in this research with soybean the clear winner. From a weed competition perspective, it indicates that legume crops can benefit from being grown on low soil mineral N fields with limited N fertilization.  

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


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