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Is this stand worth keeping?

Sometimes bad things happen to soybean fields resulting in stand losses, so decisions need to be made on whether it will pay to replant.

Keep in mind it is still common to plant soybeans thicker than is needed for optimum yields. There is no yield advantage with soybean harvest stands of greater than 100,000 established plants per acre. With stands of less than 100,000, some yield loss may occur, but it seldom pays to replant soybeans with uniform stands of around 70,000 to 75,000 plants, because of the cost to replant and the likelihood of reduced yields with the later planting date.

Unfortunately, I don’t recall seeing a stand loss that is uniform, so some judgment needs to be made. University of Minnesota studies showed about a 1 bushel-per-acre yield loss with stands reduced from 150,000 to a uniform 75,000 plants, but about a 4-bushel-per-acre loss when the 75,000 remaining stand had numerous 2-foot-long gaps in the 30-inch rows.

It is tempting to “thicken up” a marginal soybean stand rather than replanting. This can be successful, especially if done soon after soybean emergence, but generally it’s better to make a decision one way or the other. That is, either keep the existing stand, or tear it up and replant.

As the season approaches mid-to-late June, even stands of less than 75,000 may be a keeper because of the additional yield penalty for the delayed planting.

When the stand is lost due to hail or a late frost, the decision becomes more difficult because you also need to determine which plants are going to survive.

Is this stand a keeper?

It’s important to remember there are two auxiliary buds at each node, including at the cotyledons (seed leaves). Each of these buds can grow when the growing point above these buds is killed. This often results in both buds growing at a node, resulting in a more branched plant.

I’m always amazed at how much of a beating soybeans can take early in the season and come back with a vengeance and yield as if nothing had happened. It’s best to not be in too big of a hurry to tear up a stand; rather you should wait to see if the plants regrow. Even when there is a freeze that kills the soybean leaves, buds often survive and regrow because the sugars are more concentrated in the buds, making them more tolerant to the cold. If the plant is cut or frozen below the cotyledons, it’s history.

What I do when evaluating soybean stands after hail has hit them is to make three piles of plants: 1) plants that are OK, 2) plants that are dead and 3) plants that are questionable; and I count each questionable plant as a half a plant. Even when there are buds on the stem, if there isn’t some leaf tissue or at least half a cotyledon, it is not likely that the plant will survive, so I would count it as dead.

How many plants per foot?

It takes about five plants per foot of row in 36- to 38-inch rows, four plants per foot in 30-inch rows, three plants per foot in 20-inch rows and two plants per foot in 15-inch rows to equal about 70,000 to 75,000 plants per acre. For drilled beans it’s easier to use a hoop to take stand counts. It takes 10 plants per hoop with a 33-inch-diameter hoop, and 12 plants per hoop with a 36-inch-diameter hoop to equal about 70,000 to 75,000 plants per acre.

If the stand loss is due to damping off and plants are still dying, you may want to consider replanting stands greater than 75,000 plants per acre because it will be more difficult to determine what stand you are going to end up with after the disease has run its course.

If the replant is being done because of early-season disease problems, be sure to use fungicide-treated seed when you replant. An ISU publication that can help in making decisions on replanting is “Soybean Replant Decisions” (PM 1851), which is available at www.extension.iastate.edu/Publications/PM1851.pdf.

Fawcett is an ISU Extension field agronomist in eastern Iowa at Iowa City.

This article published in the May, 2011 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.