Illinois Planting Date Studies for Corn & Soybean

It looks like more rain is coming to the corn belt. That'll concern farmers hoping to plant this year's crop. However, they've got time says University of Illinois Extension Agronomist Emerson Nafziger.
 

There's not huge losses of yield as long as you can get corn planted by the second week of May - Emerson Nafziger, University of Illinois 

The fact is Nafziger would rather wait than put a crop in the ground under not so great soil conditions, "I think it is easy at this time of year to do more harm than good by planting it when you say, " well I don't think this soil is quite ready, but I think we'll have to get started and go." And our goal is to get it planted when it is fit, and as soon as we can when it is fit".

Corn planting date response over 35 Illinois site-years, 2007-2015. Yields are expressed as a percentage of the yield produced by the highest-yielding date at that site.

Nafziger's planting date studies across the state of Illinois over the last nine years put the optimum planting date for corn at April 17th. Planting dates from April 5 to April 25 maximize corn yield within a two bushel range. Corn planted April 30th loses two bushels off the top, and a delay to May 10th puts the expected loss at 8 bushels to the acre.
 

It's clear, by the University of Illinois planting date studies, that soybeans sown in April can do well. This is the case even in southern Illinois, although it's really hard to get a good early stand. Yields in the top two-thirds of the state respond the same way to earlier planting dates. The earliest dates, starting around the 10th of April, have the highest yields and things fall off as time passes, however, Nafziger is a bit cautious about planting so very early. He simply states to start when field conditions are good to go.

Our work is showing the best time to plant soybeans is the last week of April to the first two weeks of May. - Emerson Nafziger, University of Illinois 

The average maximum yield for soybeans over the 23 site years of the study, gathered from 2010-2015, is 67 bushels to the acre. There is a two-and-a-half bushel decline from April 10 to April 30th, four bushels by May 10th, seven bushels for a delay to May 20th, 11 bushels to the end of the month, 14 by the 10th of June, and 19 by the 20th.

Soybean planting date response over 23 trials in central and northern Illinois, 2010-2015. Yields are expressed as a percentage of the yield of the highest-yielding date within each trial.

Interestingly, comments Nafziger, the usual halfway point for soybean planting in Illinois is about May 20th. That is, he says, only because of the wet conditions that keep farmers out of the field. Given all of this, the U of I agronomist says he wouldn't wait after planting corn to start planting soybeans, "We've seen some sizable yield losses with soybeans by planting too early, but by too early I mean the first half of April".

There are two ways to get lower yields from planting soybeans too early. First, there are drought years like 2012. When planting late in 2012 you picked up moisture later in the season to get better yields. In-other-words, too much dry weather during flowering can really do a number of the crop. The other is if it gets really cool early after soybeans have emerged. It can actually keep them physiologically below their maximum yield says Nafziger.

Pragmatically speaking, Emerson Nafziger says as long as soil conditions are good, he'd begin planting soybeans as soon as corn planting is completed and, after some momentary consideration, says he'd move to a soybean field if soil conditions in the next corn field weren't up to par.