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URBANA, IL – Many Illinois corn and soybean growers are busy planting their crops, with 42 percent of the corn and two percent of the soybean crop planted as of April 24. However, those producers who are just getting started or are still waiting for dry fields may not see a large yield penalty, according to University of Illinois crop scientist Emerson Nafziger.

“We’ve run 35 corn planting-date trials in central and northern Illinois over the past nine years, with four planting dates at each site beginning in early April and going through late May or early June,” Nafziger says.

Nafziger’s data indicated the planting date that gave the highest corn yield was April 17, but that date was not substantially different compared to April as a whole. For example, yields were within 1 percentage point (about 2 bushels per acre) of the maximum between April 5 and April 30.

“Beyond April, we predict yield losses of about 4 percent (8 bushels per acre) by May 10, 8 percent (17 bushels) by May 20, and 14 percent (29 bushels) if planting is delayed until May 30,” Nafziger states. “We don’t have a lot of data for June planting, but the yield loss going into June is at about 2 bushels per day of delay, and it’s accelerating.”

Nafziger’s research team has also gathered 23 site-years of planting-date data for soybeans in the same sites as their corn studies. The earliest planting date for soybeans was in the second week in April, with the latest dates in mid-June.

The data for soybeans showed that the maximum yield was obtained in mid-April, and that yield loss by the end of April was about 4 percentage points, or about 2.5 bushels. After April, losses totaled 7 percent (4 bushels per acre) by May 10; 10 percent (7 bushels) by May 20; 16 percent (11 bushels) by May 30; 21 percent (14 bushels) by June 10; and 29 percent (19 bushels) if planting was delayed to June 20.

“On a percentage basis, these loss numbers are slightly greater than those from planting delays in corn, but some of this is due to planting soybeans a little later in April than we started planting corn. Both crops lost yield at about the same rate as planting was delayed into late May,” Nafziger notes. “That runs counter to the earlier findings that corn loses yield faster when planting is delayed, and therefore needs to be planted earlier.”

Given that neither crop suffers dramatically from planting through early May, farmers might assume that planting priorities for both crops are similar. But because corn seedlings tend to emerge better than soybeans under soil conditions typical of early spring, Nafziger still suggests starting with corn, at least until soils warm up, to allow faster soybean emergence.

“While getting both crops planted on time is beneficial, we shouldn’t lose sight of the fact that yield losses for delays into and even past mid-May are not so large that we need to give up hopes for a good crop if we aren’t done planting by the end of April,” Nafziger says.

More details and data from field studies are available at: http://bulletin.ipm.illinois.edu/?p=3564

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