Optimal vs. Late Soybean Seeding
We now have extensive research results on soybean planting dates in Manitoba, including information on the impact of late soybean seeding. Different studies at the University of Manitoba led by Kristen P. MacMillan (Soybean and Pulse Agronomy Lab) and Dr. Yvonne Lawley (Farming Systems Lab) have determined that planting at or beyond the end of May can result in yield loss. However, if spring conditions push soybean planting to the late end of this window, you can still produce a good soybean crop that matures before your average fall frost. If you are not dealing with these planting constraints, it’s important to know the limitations of later planting.
In the first of two Pulse and Soybean Agronomy Lab studies, four different planting windows were tested to determine the optimal time to plant soybeans in Manitoba: 1) very early (April 28-May 4), 2) early (May 8-14), 3) normal (May 16-24) and 4) late (May 31-June 4). Yields were statistically similar from April 28 to May 24. However, yield was reduced by 15%, on average, when soybeans were planted from May 31 to June 4.
Research conducted by Dr. Lawley examined the overall yield response of soybeans to a range of planting dates from 2014 to 2015 at Carman, Manitoba and Carrington, North Dakota. This study showed that soybean yield was reduced by 0.23 bu/ac per day for each one-day delay in planting from April 27 to June 16 (Figure 2).
Overall, the optimal time to plant soybeans is…
- Flexible throughout the month of May.
- During the second to third weeks of May. From the same Soybean and Pulse Agronomy Lab study, relative yield was maximized when soybeans were seeded between May 8 to 14. This timeframe also coincides with less risk from spring frost and less risk from cold soil temperature.
- Within two weeks of the last expected spring frost at your location to ensure they emerge into a frost-free environment. Frost is the biggest early-season threat to soybeans.
Late Seeding and Crop Insurance Deadlines
In another Soybean and Pulse Agronomy lab study, soybean planting was pushed much later to test the feasibility of extending crop insurance deadlines in Manitoba. Current soybean seeding deadlines for full insurance coverage are June 8 for Area 1, June 4 for Areas 2, 3 and May 30 for Area 4.
From 2015 to 2017, three soybean varieties (very early, early and mid-season) were planted during three different seeding windows at Arborg, Portage and Morden. These locations represent three distinct crop insurance areas (Arborg – Area 3, Portage – Area 2 and Morden – Area 1). Seeding dates ranged from May 24 to June 24 overall.
Maturity
Arborg had the highest risk associated with late seeding. Soybeans seeded on May 31 or later matured 5+ days after the normal frost date (Sep 22). At Portage, soybeans matured within at least one day of the normal frost date (Sep 25), regardless of seeding date. At Morden in 2017, all soybeans matured before September 25, but late- and very late-seeded soybeans matured after this date in 2016.
Yield
An acceptable benchmark for late seeding deadlines is 80% yield potential. One goal of this study was to determine if late-seeded soybeans can maintain 80% yield potential compared to the normal planting date. Yields were similar across seeding dates within most site-years, except at Arborg where the very late planting date reduced yields to 65-67% of the normal planting date (Figure 3). Very late seeding at Portage in 2015 also reduced yield, but 84% yield potential was maintained (Figure 3).
Based on maturity and yield data from this study, Portage and Morden demonstrated good yield potential and little risk for soybeans seeded as late as June 12. At Arborg, seeding soybeans beyond June 6 typically resulted in loss of yield potential and a greater risk of not reaching maturity.
The results of this project are being reviewed in consultation with Manitoba Agriculture and MASC to support a review of soybean seeding deadlines for Areas 1-3.
References
1 MacMillan, K. 2020. 2019-2020 Annual Report. Soybean and Pulse Agronomy Lab, University of Manitoba.
2 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.
3 MacMillan, K. 2018. Yield and maturity of late-seeded soybeans. Manitoba Pulse & Soybean Growers Pulse Beat Science Edition. 3:3.