How do you determine if you have enough soybean plants to leave the stand? Shaun Casteel, Purdue University Extension soybean specialist, is working on a way to make that decision based on a drone flight. Until he does, your best option is field scouting, using a hula-hoop rolled at random in several places within a field to determine average stand count.
Casteel says the key is to take those stand counts early, when soybeans are no bigger than the VC stage, with a pair of unifoliate leaves extended over the cotyledons.
Here are two examples where a grower examined a field at the VC stage. Some beans were still at VE, or just showing cotyledons. It was about 12 days after planting. He used a hula-hoop with a 30-inch inside diameter and the Purdue University Extension Corn & Soybean Field Guide to evaluate his stand.
Example 1. The stand seemed thicker in one side of the field. After rolling several times, the grower determined there were an average of 14 plants inside the hoop on the east side.
The Purdue Guide contains a table that helps determine the total population based on number of plants within the hoop. All you need to know is the diameter of the hoop.
Finding an average of 14 plants inside the hoop equates to 124,000 plants per acre. That’s a thick enough stand in almost anyone’s book. The Purdue Guide indicates you can expect 100% of original yield potential.
Tom J. BechmanCOUNT PLANTS: There are 14 plants in either the VE or VC stage inside the hoop. This number can be used to determine plants per acre.
Example 2. On the west side of the field, he found an average of 11 plants per hoop. Is that enough to leave the stand?
According to the table, that’s 98,000 plants per acre. The guide indicates yield potential is still near 100% in 15-inch rows.
Tom J. BechmanENOUGH PLANTS? There are 11 plants inside the hoop here. Since it’s the VE to VC stage, replanting is still an option.
“I usually figure we’re okay if we have 80,000 plants per acre, especially if they’re evenly spaced and weed control isn’t an issue,” says Steve Gauck, a regional agronomy manager for Beck’s.
Possible replant date if it was a close decision is also a factor. The Purdue Guide indicates that if you replant a midseason variety on May 30 and achieve a good stand, yield potential is only 96%, compared to planting May 20 or earlier. By a June 10 planting date, it’s 92% and by June 20 it’s 82%.
That’s why replant decisions need to be made early, Casteel points out.