Early season, The Bean Report

The Bean Report – May 8, 2024

May 8, 2024

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Crop & Seeding Update

Field pea seeding is underway in Manitoba as field conditions allow, with most fields not reaching emergence yet. Some earlier sown fields are emerging at VE staging, where the epicotyl has reached through the soil surface.

Soybean seeding is limited in Manitoba, with some fields seeded. More soybean seeding tips to consider below from Manitoba focused research.

MPSG’s On-Farm Network trials are getting established as soil and weather conditions allow. So far, pea seeding rate, pea inoculant and pea seed treatment trials have been established.

Soybean Seeding Tips 

Seeding Date – Research Round-up

The window to seed soybeans is flexible throughout the month of May in Manitoba. Aim to seed during the second to third weeks of May to maximize soybean yield while reducing the risks associated with cold soil and late spring frost.

From 2017-2019, Kristen MacMillan, UM-MPSG Agronomist-in-Residence evaluated four seeding windows at Arborg, Carman, Dauphin and Melita. Soybean yields did not differ among seeding dates from May 1 to May 24, but yields were reduced by 15%, on average, when seeding was delayed until May 31 – June 4. Very early seeding, during the week of April 28 – May 6 was risky. Late spring frosts occurred in two years and soil temperatures during this window were as low as 0°C, causing plant stand reductions and reducing yield by up to 19% at some sites. Report →

From 2017-2021, Dr. Ramona Mohr, AAFC-Brandon evaluated two seeding date windows, among other factors like residue management at Carberry, Brandon and Indian Head. Seeding dates of May 8 to 14 increased soybean yield by 11%, on average, compared to later dates of May 24 – 30.

Seeding Rate and Target Plant Stand

Target a living plant stand of 140 – 160,000 plants/ac to maximize yield. Take into account expected germination and survivability to calculate seeding rate. In on-farm trials, soybean seed survival has been 81% on average (82% with planters and 80% with air seeders).

MPSG’s On-Farm Network has been evaluating different soybean seeding rates on-farm since 2012. There have been 120 trials comparing soybean seeding rates to date, with another 17 trials planned for 2024. Most of the time (88%), lowering soybean seeding rates by 30,000 seeds/ac from a farmer’s traditional seeding rate has not changed soybean yield. From these trials, seeding above 190,000 seeds/ac has not provided a consistent return on investment. Generally, seeding rates of 150,000 to 180,000 seeds/ac has maintained soybean yield while minimizing seed costs. Dropping below 150,000 seeds/ac is not recommended since it introduces much more risk, especially in terms of weed control.

Row Spacing

Narrow row spacing results in soybean yields that are similar to or greater than wide rows according to research conducted in Manitoba. In small-plot research, where narrow rows of 10″ were compared to wide 30″ rows, narrow rows had a consistent yield advantage (+1.5 to +11.6 bu/ac). Where intermediate 16-24″ rows were compared to narrow 10″ rows, yields were similar between row spacings.

MPSG’s On-Farm Network has conducted 21 row spacing trials on-farm since 2019.

  • 10 trials compared – narrow (7.5-10″) vs. intermediate (15-20″) rows
  • 11 trials compared – intermediate (15″) vs. wide (30″) rows

Seeding rates were held constant between row spacings. Soybean seed survival was 6% lower in 30″ rows, on average. Narrow rows improved yield over intermediate rows 40% of the time (on average by 1.8 bu/ac). Intermediate 15″ rows improved yield over 30″ rows 27% of the time, increasing yield by 2.5 bu/ac on average.

Row spacing influences more than yield. Seeding soybeans on narrow row spacings improves their competitive ability against weeds. In research conducted by Dr. Charles Geddes, AAFC – Lethbridge, growing soybeans on narrow rows (<10″) improved yield by 11%, reduced weed biomass by 20% and reduced soybean yield loss from weeds by 4%.

Seeding Depth

When seeding into good moisture conditions, it can be tempting to seed shallow. The risk with shallow seeding is conditions drying out on the soil surface. Research has shown that regardless of moisture, the optimum depth to seed soybeans is 0.75″ to 1.75″ deep. Adjust within this range depending on available soil moisture, soil type and equipment.

Inoculation Strategies

Choosing a soybean inoculation strategy will depend on the history of soybeans in a field. In first-time soybean fields double inoculation (two forms or placements) is recommended and is typically the combination of liquid on-seed inoculant with granular in-furrow. When soybean history is established in a field (two previous well nodulated soybean crops), a single inoculation strategy is sufficient to maintain competitive N-fixing Bradyrhizobia strains. Most often, this is liquid on-seed alone. To facilitate effective nodulation, seed soybeans on fields testing low (<50 lbs N/ac) in residual N.

MPSG’s On-Farm Network has conducted 98 soybean inoculant trials comparing:

  • Double vs. single inoculant strategies: 56 trials from 2013 – 2023
  • Single vs. no inoculant strategies: 42 trials from 2016 – 2023

On fields with two previous soybean crops, and the last soybean crop within the last four years, double and single inoculant have resulted in the same nodulation and yield. On fields with three previous soybean crops, there has been no difference in nodulation or soybean yield when comparing single inoculant to none at all. However, Bradyrhizobia strains need to be alive in the soil for biological nitrogen fixation to occur. Living organisms can be affected by weather extremes such as flooding or drought, previous nodulation on your soybean crop, residual nitrogen levels along with other factors. A liquid on-seed inoculant is an economical way to ensure nitrogen fixation, especially in comparison to a rescue N fertilizer application in-season in the event of a nodulation failure.

Fertility (P, K, Starter N)

Soybeans remove relatively large amounts of phosphorus (P) in harvested seed (~0.85 lbs P2O5 /bu). Field trials at 10 locations across Manitoba (2013-2015) evaluated soybean yield response and seedling toxicity of P fertilizer rates (0, 20, 40 and 80 lbs P2O5 /ac) applied in side band, seed row or broadcast. Soil test P levels ranged 0-10 ppm Olsen P.

Seed-placed P reduced plant stands at 6 of 28 site-years when applied at rates of 80 lbs/ac. Phosphorus fertilizer only increased yield at one of 28 site-years, where 40 and 80 lbs/ac increased yield by 15%, regardless of placement. Despite the low likelihood of seeing a yield response to P fertilizer in soybeans, balancing P levels across the rotation is encouraged. To maintain P levels, apply P during the soybean year and the best placement would be side banding to minimize fertilizer toxicity risk, facilitate higher rates and place fertilizer below the soil surface.

Soybeans also remove relatively large amounts of potassium (K) (1.1-1.4 lbs K2O / bu). There were no yield responses to K fertilizer treatments in field trials conducted across Manitoba (2017-2018) on low K soils (49-117 ppm ammonium acetate soil test K). Potassium rate and placement was evaluated on soils ranging in soil test K levels.

Yield response to K fertilizer may be unlikely in Manitoba, but it is encouraged to maintain K fertility throughout the rotation to support yields of other crops. Use expected K removal rates and yields to estimate the amount of K fertility needed to support other crops in the rotation.

Soybean yield response to starter N fertilizer rates of 0, 15, 30, 45, 60 and 75 lbs N/ac (broadcast and incorporated urea) were evaluated at Carman (2015-2017). There were no differences in soybean biomass, yield, protein or oil.

Seed Treatments

If seeding into saturated soils, or into fields with a history of root rots, consider a fungicide seed treatment to help soybeans establish in the face of seedling and root rots. Additionally, seeding into warm, well-drained soils at 0.75-1.75 inch depths will allow plants to emerge quickly. A strong, healthy plant is better equipped to defend itself against disease.

Insecticide-containing seed treatments will provide protection from wireworms and seedcorn maggots. If your risk of theses insects is low, consider a fungicide-only treatment, or bare seed.

Rolling

First, consider if you need to roll your soybeans. Do you have rocks, soil clods or corn root balls to manage? If not, rolling may not be necessary in that field.

Soybeans can be rolled immediately after seeding up to two days after seeding, or post-emergence targeting the V1 (first trifoliate) stage on a warm day (+25°C) when plants are flexible.

Early-Season Weed Control

Soybeans need to remain free of weeds until V2 – V4, on average to avoid yield loss during the critical weed free period. This window of weed control may be extended to R1 when weed pressure is great.

Achieving plant stands of 140,000 – 160,000 plants/ac on narrow rows results in a competitive crop that can better combat weed pressure. When considering volunteer canola specifically, increasing soybean seeding rates and managing residual soil N were the best tools to minimize yield loss.

Meet Our Agronomists

Have pulse or soybean agronomy questions this growing season? We’re here to help! Contact Laura (204.751.0538, laura@manitobapulse.ca) or Jennifer (204.751.0737, jennifer@manitobapulse.ca).

Interested in an on-farm trial or results from the On-Farm Network? Contact Chris (204.751.0439, chris@manitobapulse.ca).