Dealing with the delayed start to corn planting
The fact that less than one percent of Illinois corn was planted by April 15 means that the start of corn planting is considerably later than it's been in recent years.
With so much on the line with high corn prices this year, there is growing concern that we are starting to lose some yield potential for each day that passes with little planting progress. Corn that we planted here at Urbana on April 2 was just starting to germinate on April 17, with roots and shoots less than a quarter of an inch long. The seed was still very sound, reflecting its past two weeks in the refrigerator that the soil has been. There has thus been little agronomic advantage to planting in early April this year.
Illinois is a large state, so characterizing effects of planting date on yield is not easy or very precise. If we use data for the whole state, we see that corn yield is not very well correlated with an early start to planting, as indicated by the percentage of corn planted by April 10. But, the percentage of corn planted by the end of April does show some correlation with yield.
Though it's not a perfect predictor of yield, this indicates that each percentage point increase in corn planted by April 30 means another half bushel in statewide yield. The three highest percentages (all above 72%) planted by the end of April during the past 14 years were in 2004, 2005 and 2006, long enough for some people to develop this as an expectation.
With typically rapid planting after April 30 regardless of how the year turns out, this correlation tends to decrease at later dates. But the message is that we get most of the crop planted during April in the better years and that failure to do so tends to result in low yields. The adage that "it matters when you finish planting, not when you start" appears to hold true. While there are plenty of anecdotes about individual fields that were planted late and still yielded well, years that allow timely planting tend to be better corn years.
We are not yet to the end of April, so there's little reason to start serious worrying about when we'll finish planting and how much the "delay" will cost. With today's equipment, we can plant faster than ever before. We have planted at more than five percent per day over some 10-day periods in several recent years, and if every field in Illinois were fit to plant at the same time, we likely could plant at least 75% of the corn crop in a week. We estimate that the median number of days Illinois producers need to plant their entire corn crop is about five. This might be higher by half a day or so this year because of increased corn acreage.
Even with the ability to plant fast, wet fields and some wet weather ahead will probably delay corn planting past the ideal time. Research shows planting in late April has produced the highest yields, and that yield declined by three percent from May 1 to May 10, by six percent from May 10 to May 20, and by 10% from May 20 to May 30. This is about half a bushel per day of delay for the first 10 days of May, one bushel per day during the second third of May, and one and one-half bushels per day for the last third.