Plant soybeans before corn? Not so fast.

University of Missouri research still favors planting corn first.

In recent years, the agriculture community began discussing whether it makes sense to plant soybeans first and delay corn planting.

The University of Missouri (MU) Food and Agricultural Policy Research Institute predicts a 7% increase in soybean acres planted in 2021. That leaves farmers wondering whether they could, or should, plant soybeans before corn.

Corn is Still First

Multiyear research at MU’s Bradford Research Center near Columbia still favors corn over soybeans for first planting in Miidwestern corn-soybean rotations, says Bill Wiebold, MU Extension soybean specialist. 

The research shows the importance of considering the entire crop operation over individual crops or any one component, he says.

“If early soybean planting delays corn planting, the productivity of the enterprise could be harmed,” says Wiebold. “Under most circumstances, it makes sense to plant corn before soybean.”

Prior research conducted elsewhere favored early soybean planting but did not examine yields from both crops or look at the effects of delayed corn planting. MU data for both soybean and corn come from the same location, often in the same year. “This allowed us to observe the effects of planting dates on performance of the entire rotation, not just a single component,” Wiebold says.

Researchers looked at data from 12 MU soybean experiments using three and often five planting dates, with the first in late March. Data also give a picture of how weather influenced grain crop yields.

Missouri corn vs. soybean comparison
Left: Response of corn and soybean yields to planting dates relative to dates with highest yield for each crop. Data are expressed as relative yield. Right: Effect of planting date on corn and soybean yields. Data are expressed as relative yield.

Wiebold planted up to five varieties of high-yield, commercially available soybean with maturity ratings of mid-MG3 to mid-MG4 at a rate of 160,000 seeds per acre.
The corn yield data covers six years with five planting dates beginning in the last week of March. The number of hybrids varied from four to six, with relative corn maturity (RCM) varying from 106 to 114 using a seeding rate of 30,000 kernels per acre.

Soybeans and corn were planted into crop residue in a typical rotation with 30-inch rows. Corn yields more bushels per acre than soybean, so researchers compared relative yields to yield responses. Researchers averaged yields for each crop across all experiments.

Yields of both crops decline when planting is delayed. However, corn starts losing yield at least a month earlier than soybeans, and corn yields drop quicker than soybeans’ with delayed planting.

For example, 30 days after yields began to decrease, corn went down 13% and soybeans dropped about 8%. The rate of yield loss increased so that 10 days later yields dropped 20% for corn and 14% for soybeans.

Soybean yield data from the experiments came from March plantings and give a good comparison of the date when yields start to decrease. Corn yields start to decrease when planting begins after April 20. Soybean yields remain unaffected by planting dates in April and only decreased 4% by May 20.

Different Responses 

The two crops also respond differently to planting dates in other ways.
Soybean traits make it make it more stable than corn in its response to weather. Soybeans also have more opportunity to produce. Wiebold explains that soybean plants flower for 30 or more days while corn plants complete pollen shed in less than 10 days.

Soybean produces yield at nearly every node on the plant, while corn yield is on a single ear. Soybean also produces three to four times as many flowers than are accounted for as pods at harvest, so there is abundant yield backup.

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