There are four main considerations for determining when to plant soybeans in Manitoba. These include calendar date, soil temperature, the 24-hour weather forecast and your personal risk profile, including frost risk and number of acres designated for soybeans compared to other crops on your farm.
Calendar date – Aim to plant during the second to third weeks in May for high yield potential and reduced frost risk. A rule of thumb for early planting is to seed within two weeks of the last spring frost date to ensure seedlings are not injured by frost after emergence. Emerged seedlings can tolerate air temperatures of -2.2°C for a brief period if plants have had a chance to harden, but cannot recover if the main growing point is killed. If the growing point is only injured slightly, growth may resume from the axillary buds. Avoid planting in June due to lower yield potential.
Soil temperature – Soybeans emerge within 24–35 days when seeded into 6–12°C soils and 4–16 days when seeded into 14–22°C soils (based on soil temperatures at a 5 cm depth at 10:00 a.m. for two consecutive days).1 Warm soil temperatures result in faster emergence and prevent the risk of chilling injury during imbibition. Residue management should have a minor impact on plant population and yield if soybeans are seeded during optimal calendar dates and into warm soil.2
24-hour forecast – Seeds imbibe water within the first 24 hours after seeding. If cold and wet conditions are expected during this period, wait to plant to avoid chilling injury. Extended periods of poor weather after seeding can also delay emergence and reduce seedling vigour.
Personal risk profile – Adjust planting dates according to your individual frost risk, based on where you are located in the province, and the amount of soybean acres you are planting compared to other crops that may need to be seeded earlier. Ensure more frost-tolerant crops are seeded before soybeans. If you want to experiment with earlier planting, try a few acres first and spread out your planting dates.
Target Plant Stand and Seeding Rate
Target 140,000–160,000 live plants/ac. Adjust your seeding rate (seeds/ac) based on on expected seed survival. For example, with 75% expected seed survival, a seeding rate of 190,000 seeds/ac is required to reach a target plant stand of 140,000 plants/ac. Use the Bean App Soybean Seeding Rate Calculator to calculate customized rates for your farm. Seed survival will depend on seeding conditions, equipment and seed handling, and seed lot.
Plant stand assessment from previous years, a germination test and a soak test will help you estimate a more accurate survival rate. To conduct a soak test, place 200 seeds in water and calculate the percentage of seed coats that slough off. Seeds that lose their seed coat will not produce a viable plant.
Plant soybeans at 0.75–1.75 inches deep.3 Ensure good seed contact with soil moisture contact and avoid planting soybeans deeper than 1.5 inches. In dry seedbeds it may be tempting to plant deeper than 1.5 inches. However, planting soybeans too deep will cause seeds to exhaust their resources and can result in poor, non-uniform or delayed emergence.
Row Spacing and Equipment
Soybeans can be planted in narrow (<15 inch) or wide (15–30 inch) rows. Narrow rows offer faster and more complete canopy closure for better weed competition and moisture conservation. Wide rows (planters) result in greater planting precision and potentially lower disease pressure due to greater wind movement through the canopy. Existing narrow-row equipment can be used with a higher seeding rate, due to lower expected seed survival from mechanical damage. Higher moisture seed (>12%) and lower fan speed (i.e., variable among seeding systems) can reduce seed damage.4 Narrow rows have resulted in the same or greater yields than wide rows in Manitoba.5
As soybeans pod low to the ground, land rolling is beneficial on stony land to facilitate harvest. Land rolling can be done immediately after seeding or post-emergence at the V1 to V2 stage during the warmest part of the day.6 Rolling after seeding can cause soil crusting, reduce water infiltration and increase soil erosion due to pulverized surface soil. Post-emergent rolling can help prevent soil erosion. Avoid post-emergent rolling if plants have already reached the V3 stage.
1 Tkachuk, C. 2017. Evaluation of soybean planting dates and plant densities in the northern growing regions of the Northern Great Plains. M.Sc. Thesis. University of Manitoba.
2 Mohr, R. 2018. Effect of soil temperature at different planting dates and residue management on soybean. Manitoba Pulse & Soybean Growers Research Report.
3 MacMillan, K.P. 2020. Soybean Seeding Depth Evaluation. Soybean and Pulse Agronomy Lab, Department of Plant Science, University of Manitoba.
4 Grieger, L. 2018. Air seeder distribution and seed damage for wheat, canola and soybeans. Prairie Agricultural Machinery Institute (PAMI) Research Report.
5 Mohr, R. 2018. Agronomic management of soybeans in Manitoba: row spacing and seeding rate. Manitoba Pulse & Soybean Growers Pulse Beat Science Edition. 2:11.
6 DeJong-Hughes, J., D. Holen and P. Glogoza. 2012. Management considerations for rolling soybean in the Upper Midwest. University of Minnesota Extension.
Mohr, R. 2018. Evaluating the effect of soil temperature and planting date on soybeans in Manitoba. Manitoba Pulse & Soybean Growers Pulse Beat Science Edition. 3:4.
Tkachuk, C. and Y. Lawley. 2016. When to plant soybeans in MB – Should soil temperature be your guide? Manitoba Pulse & Soybean Growers Pulse Beat. 79:39.
MacMillan, K.P. 2018. 2018 Annual Report. Soybean and Pulse Agronomy Lab, Department of Plant Science, University of Manitoba.