Less weed competition for soybeans grown on low-nitrogen soils

Growing soybean on high soil mineral nitrogen (N) fields, or when N fertilizer was applied in increasing rates, resulted in an increase in volunteer canola competition and a corresponding linear decrease in soybean yield.

Research was conducted in Manitoba looking at the resource-ratio (R*) hypothesis. 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, a research study was conducted in Manitoba to see how soil mineral N levels would affect volunteer canola competition. The rationale was under lower-nitrogen (N) levels in the soil, volunteer canola would be less competitive, but soybean could fix its own N to compete more vigorously.

The objectives of this study were to see if soil mineral N levels affect soybean and volunteer canola growth and seed production differently and consistently. The research was conducted at the Ian N Morrison Research Station near Carman, Manitoba. In 2015, two adjacent trials were conducted – one seeded on May 12 and the other on May 29 – and were considered two different environments. The trial was repeated in 2016 at one site in a different field.

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. The canola was seeded at 8 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 to 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 2 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.

Higher soybean yield at lower soil N levels

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 increase in soil mineral N per acre (1 kg soil mineral N/ha).

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% when soil mineral N level was 222.5 lbs/ac (250 kg/ha).

Soybean yield in response to soil mineral nitrogen in the presence and absence of volunteer canola

Source: Geddes and Gulden. 2022.

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), from the 2016 site. The results highlight that planting soybeans in fields with low soil mineral can help reduce weed competition.

Volunteer canola seed production increased with N levels

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 1980 to 5160 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%. 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 number of volunteer canola seeds produced per unit area in response to soil mineral nitrogen in soybean in the 2015i, 2015ii, and 2016 environments

Source: Geddes and Gulden. 2022.

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 1320 to 3440 seeds/ft2 (13,200 to 34,400 seeds/m2).

The resource-ratio (R*) hypothesis was confirmed in this research. 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. Conversely, fertilizing legume crops can increase weed competition from non-legume weeds, and result in yield losses directly in proportion to the increase in N fertilizer rate, while also increasing the weed seedbank.

This research was funded by the Canada and Manitoba governments through Growing Forward 2, a federal–provincial–territorial initiative, and the Manitoba Pulse and Soybean Growers. Geddes was supported by a Natural Sciences and Engineering Research Council of Canada Industrial Postgraduate Scholarship and a University of Manitoba Graduate Fellowship.

Geddes CM and Gulden RH (2022) Exploiting the resource-ratio (R*) hypothesis for weed management in legume crops: An example of volunteer Brassica napus in soybean. Front. Agron. 4:1040241. https://www.frontiersin.org/articles/10.3389/fagro.2022.1040241/full

Photo by Charles Geddes

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