BY: Terry Buss – MPSG Production Agronomist – East
The 15th of May is in sight and there are a lot of acres of soybeans to go into the ground over the next while. A majority of my clients haven’t started yet as they push to finish up wheat, field peas and, in some cases, canola. Others have made a start since the weekend. There are a few individuals in the southern districts of Eastern and Central Manitoba who are well into planting with rumours of finishing up this week. Lots of variation as always but, overall, we are less than 5% done. However, I am expecting progress to ramp up exponentially if the weather decides to play fair.
In light of soybean planting taking centre stage in the next while, I thought a quick review of the agronomy of successful soybean planting was in order. Hopefully this saves you time as your fine tune your plans over the coming days.
Seeding Date
The window to seed soybeans is flexible throughout the month of May in Manitoba. Seeding during the second to third weeks of May maximizes soybean yield while reducing the risks associated with cold soil and late spring frost. Seeding in June is the most reliable way to ensure sub-par yield performance in soybeans.
From 2017-2019, Kristen MacMillan, MPSG agronomist-in-residence at the University of Manitoba, evaluated four seeding windows at Arborg, Carman, Dauphin and Melita. Soybean yields didn’t differ among seeding dates from May 1 to 24, but on average yields were reduced by 15% when seeding was delayed until May 31 to June 4. Very early seeding, during the week of April 28 to May 6 was risky. Late spring frosts occurred in two years during the study 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 to 2021, Dr. Ramona Mohr with 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 to 30. Click here for a summary of this research →
Seeding Rate and Target Plant Stand
Lowering seeding rates may maintain yield and be economical but lowering them too much is risky if establishment or survivability is poor. You should take into account expected germination and survivability to calculate seeding rate. In on-farm trials, soybean seed survival has been 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 135 trials comparing soybean seeding rates to date and, most of the time (84%), lowering soybean seeding rates by 30,000 seeds per acre from a farmer’s traditional seeding rate hasn’t changed soybean yield. From these trials, seeding above 190,000 seeds per acre hasn’t provided a consistent return on investment. Generally, seeding rates of 150,000 to 190,000 seeds per acre has maintained soybean yield while minimizing seed costs. Living plant stands of approximately 118,000 plants per acre or greater has maintained an average yield of 40 bushels per acre or higher. Report →
Row Spacing
Soybeans may be grown successfully on any row spacing, however, there’s greater yield potential with narrow row spacing than wide spacing 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″ to 24″ rows were compared to narrow 10″ rows, yields were similar between row spacings.
MPSG’s On-Farm Network has conducted 29 row spacing trials on-farm since 2019.
- 12 trials compared – narrow (7.5-10″) vs. intermediate (15-20″) rows
- 17 trials compared – intermediate (15″) vs. wide (30″) rows
Seeding rates were held constant between row spacings. On average soybean seed survival was 5% lower in 30″ rows. Narrow rows improved yield over intermediate rows 33% of the time (on average by 1.8 bu/ac). Intermediate 15″ rows improved yield over 30″ rows 24% of the time, increasing yield by 2.7 bushels per acre on average. Report →
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 at 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 drier soil moisture conditions, it can be tempting to seed deep. The risks with deep seeding include much slower and uneven crop emergence, increased seedling mortality and an increased incidence of root rots showing up later in the growing season due to early season infections. Research by Kristen MacMillan, MPSG agronomist-in-residence at the University of Manitoba, 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. Report →
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. MPSG’s On-Farm Network research has found that naturalized populations of Bradyrhizobium japonicum are effectively colonizing root nodules and fixing nitrogen in field with sufficient soybean history. On-Farm Network data has demonstrated that a single inoculation strategy, compared to no inoculant use, has never improved soybean yields in trials on fields in central Manitoba with more than three previous soybean crops. Although yield responses haven’t occurred to date on soybean fields with more than three previous soybean crops, liquid on-seed inoculant may provide peace of mind in knowing the crop’s Nitrogen (N) requirements are secured. To facilitate effective nodulation, seed soybeans on fields testing low (<50 lbs N/ac) in residual N.
MPSG’s On-Farm Network has conducted 102 soybean inoculant trials comparing:
- Double vs. single inoculant strategies: 58 trials from 2013 – 2024
- Single vs. no inoculant strategies: 44 trials from 2016 – 2024
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 95% of the time. 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. Report →
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 phosphorus (P) reduced plant stands at 6 of 28 site-years when applied at rates of 80 pounds per acre. Phosphorus fertilizer only increased yield at one of 28 site-years, where 40 and 80 pounds per acre 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. Report →
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 to 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. Report →
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. Report →
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. Report →
