Research in Manitoba has proven that yield potential is reduced when soybeans are planted at or beyond the end of May (Figure 1). We can also expect a yield reduction of 0.23 bu/ac for each one-day delay in planting (Figure 2). However, this yield reduction does not equate to a crop failure, suggesting there is still potential for soybeans if spring conditions delay planting in Manitoba.
How late can soybeans be planted in Manitoba?
One particular study has evaluated the potential of late-seeded soybeans in Manitoba and the feasibility of extending crop insurance deadlines. Current soybean seeding deadlines for full insurance coverage are June 6 for Area 1 and May 30 for Areas 2, 3 and 4.
From 2015 to 2017, three soybean varieties (very early, early and mid-season) were planted in three 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.
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.
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.
1 MacMillan, K. 2018. 2018 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.