If the main concern is to identify a "safe" hybrid maturity that will reach physiological maturity before a typical fall freeze date, then the steps described in this article are appropriate for you to follow. Some growers may opt to select an "end of season" date earlier than the historical first fall freeze date to ensure that physiological maturity will occur earlier during a time period that may allow for some grain drydown in the field and thus minimize their expenses of drying the grain artificially. doi:10.2134/agronj20 High Plains Regional Climate Center. The question is based, of course, on the perceived risk of the crop not reaching physiological maturity before a killing fall freeze and the yield losses that could result.
The accumulation of heat on a daily basis can be quantified on the basis of calculated Growing Degree Days or GDDs (Nielsen, 2017) and the relative maturity of a hybrid can be more reliably characterized by how many GDDs it requires from planting to physiological maturity (kernel black layer).
NOTE: Most seed companies publish "GDDs to Black Layer" ratings for the hybrids in their lineup, but sometimes do not clearly state whether the GDD values are "from planting" or "from emergence".
The traditional "days to maturity" rating system for hybrids (Nielsen, 2012) does not literally refer to calendar time and so is not helpful in making decisions about switching to early-maturity hybrids with delayed planting.
How fast a corn plant develops (i.e., moves through growth stages) is very dependent on temperature (warm = fast, cool = slow).
However, by about the last week of May, some growers in the central and, especially, northern parts of Indiana need to consider switching to earlier-maturity hybrids to minimize the risk of not maturing safely prior to a killing fall freeze.
The steps outlined in this article will help growers and consultants determine "safe" hybrid maturities for late planting. The Corn GDD Tool estimates county-level GDD accumulations and corn development dates based on current and historical GDD data plus user-selected start dates, relative hybrid maturity ratings, GDDs to blacklayer, and freeze temperature threshold values. Hybrid Maturity Switches Based on Long-Term Research. The estimates are displayed graphically and in tabular form, plus the results can be downloaded in a Comma Separated Value (.csv) formatted file for you to work with in your own spreadsheet program. Pioneer (Corteva agriscience, an Agriculture Division of Dow Du Pont). https://noaa.gov/data-access/land-based-station-data/land-based-datasets/climate-normals/1981-2010-normals-data [URL accessed May 2019] Nielsen, R. Consequently, you can be led astray by the Tool if you do not modify the "Black Layer GDDs" value in the Tool's input area. 1 for a 112-day hybrid with a default GDD rating of 2691 planted in Tippecanoe Co. Screen capture of U2U GDD Tool graphical display of historical and estimated future GDD accumulations and predicted corn development stages for a 112-day hybrid planted May 31 in Tippecanoe County, IN. on May 31 indicates the hybrid would mature on or about October 27, almost a week after the average first killing 28F freeze. The discussion and guidelines provided in this article assume "GDDs from planting".