How to Make Soybean Replant Decisions

Flooding and hail damage have left some fields looking pretty bad.

Some soybean fields have been flooded by heavy rainfall. Others have been hit by hail.

Hail damage early in the growing season often looks worse than it really is and flood damage is often more detrimental than hail damage in the beginning of the growing season. "But that doesn't mean you should ignore hail damage," says Palle Pedersen, Iowa State University Extension soybean agronomist.

As soon as a bean plant emerges, the growing point, located in the cotyledons, is above ground, he explains. This makes soybeans particularly susceptible to damage from hail, frost, insects such as bean leaf beetle, or anything that cuts the plant off below the cotyledons early in its life.

Examine growing point carefully

"Consider a soybean plant dead if it is in the cotyledon stage and is cut off below the cotyledons, or if it is damaged by hail to such a degree that there is no remaining green leaf tissue or regrowth," says Pedersen. "The reason is that nutrients and food reserves in the cotyledons supply the needs of the young plant during emergence and for about seven to 10 days after emergence, or until about the V1 stage - one fully-developed trifoliate leaf."

Cotyledons are the first photosynthetic organs of the soybean seedling and also are major contributors for seedling growth. That's why stand reductions are likely to follow hailstorms. After V1, photosynthesis by the developing leaves is adequate for the plant to sustain itself, he adds.

Estimate the plant population

Accurately estimating soybean plant population is important before making replant decisions. "Plant population should be based on an accurate stand count, along with factors such as yield potential of the existing stand, date of replanting and the real cost of replanting," says Pedersen.

To evaluate the existing stand, you should look at uniformity of stand and overall health of plants. Only some areas of the field may require replanting if the majority of the field seems to have enough viable plants remaining.

Pedersen says it's important to wait several days (3 to 5) after a crop has been damaged or has emerged before replanting. Injury can look very serious the day after the hail damage event but recovery may be possible.

How low can your stand go?

Previous ISU studies show that a final stand as low as 73,000 plants per acre has consistently yielded more than 90% of the optimum plant population. That is a little more than two plants per foot of row in 30-inch row spacing. That may not sound like a lot but it is, says Pedersen.

The reason is that soybean plants can compensate for missing plants and reduced stands by branching out to make up for a thin stand. "Keep in mind the lower the stand count, the more weeds will become a problem due to less shading, especially later in the growing season. If a reduced stand is saved, weed control must be a top priority," he says.

Other problems associated

There are some secondary problems associated with flooding and hail damage. Disease may increase and consequently further reduce stands since plants that have been damaged or wounded are more susceptible to infection from plant pathogens like Phytophthora root rot and Pythium.

In addition to all this, seed quality was a serious issue this year. Flooding and pathogens have a greater impact when poor quality seed is used than when the seed isn't mechanically damaged and is free of seedborne disease.

Watch for wounds on plants

"Soybean plants that have torn stems should be watched closely in the coming weeks for evidence of pathogen infection," says Pedersen. "Lesions around the base of the stem and plant wilting are often good indicators. If this is the case, it will be necessary to estimate the number of viable plants in the field again and make a decision concerning replanting."

However, it is difficult to assess this type of injury soon after flooding or a hail event. Thus, if the field has a history of pathogen problems and if it continues to rain, loss of wounded plants will probably increase, he says.

Make estimate of viable plants

"Remember, yields will not necessarily be reduced just because the plant stand has been reduced," says Pedersen. "When it is possible to get back into the fields, take plenty of time to visit each of your fields and take the time to make a good estimate of the number of viable plants in the stand where flooding or hail has occurred."

A replant decision based on a quick look at a field may therefore underestimate the existing plant population. "We recommend that you plant the original full season variety until June 20 in northern and central Iowa and until early July in southern Iowa," says Pedersen.

More information on soybean replant decisions can be found at www.soybeanmanagement.info.

TAGS: Extension
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