The recent low temperature events damaged emerged soybeans and will probably lead to reduced stands in these fields. The extent of the damage will not be known until about one week after the low temperatures occurred because the plants in the photo may still produce new growth from the axillary node located at the base of the cotyledons.
If new growth is not detected from the main growing point or the axillary node at the base of the cotyledons after a week has passed, the plants are no longer viable.
When poor soybean emergence and thin stands occur, producers need to make timely and informed replant decisions. Accurately assessing your soybean stand and diagnosing the cause of the emergence problems are the first steps in the process.
Once the existing stand has been determined, the following information may help producers make an informed replant decision.
Forty-eight planting rate trials conducted in Michigan from 2015 to 2019 are summarized in Table 1. The average final plant stands and average yields for the lowest and the highest planting rates are shown. There were 10 locations in 2015, 10 sites in 2016, 11 sites in 2017, eight sites in 2018 and nine sites in 2019.
Detailed information from all the planting rate trials conducted from 2015 to 2019 is available in the 2017, 2018 and 2019 On-farm Research Reports available at the Michigan Soybean Promotion Committee website.
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The data shows that thin soybean stands can produce surprisingly high yields. However, there were exceptions as yields from the 80,000 seeds per acre planting rate were reduced by more than 4 bushels per acre at 12 of the 48 sites (25% of the time).
Four bushels per acre is the approximate breakeven yield loss for the 80,000 planting rate given current seed and crop prices making the 80,000 rate less profitable than the 160,000 rate at these sites. At three of these sites, the yield loss in the lowest planting rate was more than 7 bushels per acre. None of the varieties in the trials were thin or straight-line plant types.
|Table 1. Average final plant stands and yields by year for the lowest and highest planting rates compared in 48 on-farm planting rate trials conducted in Michigan from 2015 to 2019.|
|Year||Target planting rates (seeds per acre)|
|Stand (plants per acre)||Yield (bushels per acre)||Stand (plants per acre)||Yield (bushels per acre)|
Soybean agronomists have identified 100,000 plants per acre in narrow rows and 80,000 plants per acre in 28- and 30-inch rows as the minimum plant stands required to produce optimum yields. However, the information presented in Table 1 shows that stands of 60,000 to 70,000 plants per acre can produce high yields.
Also, realize that soybean yields decrease by 0.3 to 0.4 of a bushel per acre per day when planting after the first week of May. I urge producers to consider this information, the information presented in Table 1, and to refer to an excellent publication from the University of Wisconsin, “Think Twice Before Replanting Soybeans,” when making replant decisions.
The case for keeping thin stands becomes even stronger for fields having a history of white mold. The lowest planting rate increased soybean yields by 5 bushels per acre and income by $80 per acre over the highest planting rate at two sites infested with white mold.