Bean Counting

June 17, 2008 07:00 PM

Pamela Henderson Smith
, Farm Journal Crops & Issues Editor
Your good soybean days are numbered. Iowa State University research shows soybean yield loss increases rapidly every day planting is delayed after May 15.
"It's a disaster here,” says Palle Pedersen, Iowa State University agronomist. Not only has the season been delayed by wet weather, but now flooding has many farmers considering replant.
Why the yield hit? Pedersen says the benefit from early planting is a result of increased seasonal canopy photosynthesis, greater number of main-stem nodes, potential for earlier flowering, increased crop growth rate during pod set and greater seed filling rate.
When soybean planting is delayed, vegetative growth is reduced since flowering can start as soon as the plants have one or two nodes. Soybeans planted later in the season don't canopy the row like their full-season counterparts. Plants are often shorter and so is the podding height because sunlight reaches the nodes in the bottom of the canopy.
 "High yielding varieties respond the same if they are planted early or late,” says Pedersen. The time from flowering to harvest maturity is usually the same when a variety is planted because it is controlled by maturity group.
"However since we are running so late, I will soon begin to suggest that farmers change maturity groups soon so the soybeans can mature,” he adds. That cutoff date is generally June 20 in northern and central Iowa and early July in southern Iowa.
Jim Trybom, research scientist with Pioneer at Monticello, Ill., says replant decisions are never easy.
At this late date, he encourages growers to carefully measure the surviving stand. "It's very easy to underestimate how many plants are out there. A stand of 80,000 plants per acre doesn't look like much when they are little—that's only five plants per foot of row. Just remember, soybeans can compensate for low stands and still produce good yields,” Trybom says.
He cautions growers against overseeding into established plants to thicken stands—that results in uneven stands and emerging soybeans that can act like weeds in an established crop.
For more information on how to make replant decisions, see Pedersen's full report at:

You can email Pamela Henderson Smith at

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