We often start April with some corn already planted and we normally end April with about 50 percent of the corn planted, based upon state averages published by the National Agricultural Statistics Service. The latest planting start in the last 20 years was in 2013 when only 1 percent of the crop had been planted by May 1 but by mid-May we had recovered significantly with 75 percent planted. The slowest year was probably 2009, that year at mid-May we were only at 20 percent planted. Even with the slow starts back in in 2009 and 20013 we ended up with average trendline yields.
The Soybean Seeding Calculator that Emerson Nafziger and I put together has moved to a new server. The old links went through a Dropbox folder. Unfortunately, Dropbox changed their sharing protocol and the links would no longer work and the links could not be changed. The University of Illinois Crop Science Department gave me a spot on their server to host some of my apps. I am now trying to get the word out. Unfortunately old documents on the Internet have a long lifespan and are out of my control.
Here are links to Nitrogen application resources for Fall 2018:
Stealing a phrase from the "Hitchhiker's Guide to the Galaxy", there is still time to plant and get above average yields. The attached chart shows the Illinois corn planting progress chart since 1999 and the corresponding corn and soybean yields. I included the soybean yields because I have heard the question, "If corn planting is late, then soybeans will be even later and we have been pushing for early planting beans for higher yields." The USDA NASS data in the chart does not really show a penalty for years when things start really slow.