Deciding when to plant is a balance of the right calendar date, soil conditions, weather forecast and personal tolerance to risk. According to MASC records1 and local small plot research2, soybeans planted in early May yield higher than those planted in late May and June. Soybeans are highly susceptible to frost damage, so planting date needs to be considered in tandem with emergence rate and last expected spring frost. A general rule of thumb is to plant soybeans within two weeks of the last spring frost. See map (Figure 1) to find the average date for your region. Emergence rates vary with soil temperature, so beware that soybeans planted into warm soil may emerge in a little as four days (Figure 2).2
Planting into soils >10°C is recommended to avoid chilling injury, which occurs in the first 12-24 hours while the seed is rapidly imbibing water. Chilling injury can inhibit or delay germination and emergence, so be mindful of the forecast immediately after planting. Check Manitoba Agriculture’s provincial weather stations for an estimate of soil temperature and moisture conditions. Residue management will impact soil temperature so measure soil at planting depth (5 cm) in each field for a few days before planting. Soil temperature fluctuates diurnally so take an average of soil temperature in the morning and evening.
Impact of Residue
To minimize chilling injury, soybeans are often planted into blackened soil. Seedbed preparation doesn’t come without cost: discing, vertical tillage or strip tillage costs $19-32/ac, including fuel, machinery and labour3 and leaves soils vulnerable to erosion and drying.
Research conducted at Brandon, Carberry, Portage la Prairie and Roblin in 2015-2017 compared tilled wheat stubble to zero-tilled wheat, oat and canola stubble. Despite tillage reducing soil moisture and increasing soil temperatures compared to no-till at about half of the locations, there was no impact on soybean plant population4. Effect on soybean yield was also limited and differences observed were not associated with soil conditions4. Since most sites were planted on May 17 to 26 and all sites were seeded into soil that was at least 10°C, we can conclude that residue management may have little impact on soybean emergence or yield if planting occurs within the optimum window.
1 MASC. 2005-2013: Seeding Date vs. Average Yield Response. Available https://www.masc.mb.ca/masc.nsf/mmpp_seeding_dates.html
2 Tkachuk, C.F. 2017. Evaluation of soybean (Glycine max) planting dates and plant densities in northern growing regions of the northern great plains. M.Sc. Thesis. University of Manitoba. Winnipeg, MB.
3 Walther, P.A. 2017. Corn (Zea mays l.) Residue management for soybean (Glycine max l.) production: On-farm experiment. M.Sc. Thesis. University of Manitoba. Winnipeg, MB
4 Mohr, R. 2018. Effect of soil temperature at different planting dates, and of residue management, on soybean. Available https://www.manitobapulse.ca/research-project/effect-of-soil-temperature-at-different-planting-dates-and-residue-management-on-soybean/