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Make soybean replant decisions early

Tom J Bechman Field with soybeans at the VE to VC stage
CAN YOU SEE THEM? These soybeans are at the VE to VC stage. The field still looks more brown than green. Final population varied from 105,000 to 125,000 plants per acre.
Making the decision early gives soybeans a better shot if you replant.

You plant soybeans and the weather doesn’t cooperate. You don’t get the stand you want. Should you replant?

Either way, make the decision while soybeans are still small. Otherwise, you’re giving up yield potential if you decide to replant or overseed. That’s what Shaun Casteel, Purdue University Extension soybean specialist, told Indiana Certified Crop Advisers.

“Timing is critical on when you do stand counts,” Casteel says. “You can take stand counts from VE to V2, but if you wait until V2, it’s pretty late. If you decide to replant then, you’ve shortened the season and likely forfeited yield potential.”

According to the Purdue University Extension Corn and Soybean Field Guide, VE occurs when cotyledons and growing point are above ground. Soybeans reach VC when larger unifoliate leaves are fully expanded. V2 occurs when the margins of leaflets of the second trifoliate no longer touch.

Studies indicate that if you only have 50,000 plants at VC, you can add more plants or replant. However, if you act on those options at V2, you’re not gaining much. And if you’re trying to add more plants at V4, you’re wasting your time. Larger plants shade out late-planted seedlings.

“We need to make the call at the VC stage,” Casteel emphasizes. “That seems early because, traditionally, people have waited longer.

“Our data indicates the best time to make a decision to replant a thin stand or add more beans by planting into an existing stand is at VC. If you wait until V2, you’ve essentially already made the call. At that point you’re going to live with the stand you have.”

New scouting techniques

One problem with making a replant decision is getting accurate stand count information across the field. Guidelines suggest using a hula-hoop and sampling several areas. Even then, Casteel says, it’s difficult to get an accurate picture of what the stand is like across the entire field.

He and a graduate student, Richard Smith, are in the process of refining how to determine if a soybean stand is adequate using a drone. The advantage for the drone is that it can quickly “see” the entire field.

Various tech companies are working on programs to produce accurate stand counts in corn using images from drones. “We’re not trying to count plants in soybeans — populations are too high,” Casteel explains. “We’re trying to use an indication of how much of the soil surface is covered by vegetation and relate it to population.”

They assessed stands in a field-scale trial in 2020 with a DJI Matrice drone with a RGB camera at six locations with seed rates ranging from 50,000 to 250,000, with three to five replications per site. The challenge, he says, is that the percentage of soil covered when soybeans are at VC is relatively small.  

They used an algorithm to count green pixels representing vegetation and to calculate canopy coverage. That is the percent of ground covered with plant pixels. They found that at VC to V1, a typical field may be only 4% green, 10% to 15% at V2 to V3 and 30% at V4 to V5.

“We’re hoping we can refine the process by using vegetative indices like the Green Leaf Index so that we can use it to make replant decisions early enough in the season,” Casteel says. “It’s still a work in progress.”

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