Growers with corn planted in the late May to mid-June time frame are wondering if the crop will mature before frost injury.  Our summer temperatures were on the cool side, leading to fewer than expected growing degree days (GDD’s).  Peter Thomison, OSU Extension Corn Specialist, addressed this concern in a recent article and I will use some of that information in this column.

In Ohio, physiological maturity (when kernels achieve maximum dry weight and black layer forms) typically occurs about 65 days after silking. At physiological maturity (kernel moisture approximately 30-35%), frosts have little or no effect on the yield potential of the corn crop.

Dr. Bob Nielsen at Purdue University has summarized research findings from Indiana and Ohio that provide insight into both the calendar days and thermal time (growing degree days, GDDs) typically required for grain at various stages of development to achieve physiological maturity (kernel black layer, R6) and available on-line at http://www.agry.purdue.edu/ext/corn/news/timeless/RStagePrediction.html. The calendar days from various grain fill stages to black layer (R6) for a 111-day hybrid maturity are shown in the following table where R3 = milk stage, R4 = dough stage with no visible denting and R5 = Late dent where all kernels are visibly dented:

Using this chart, if corn is only at the R4 stage, do we have the 38-40 days indicated in the chart before a killing frost to allow the grain to mature to R6 physiological maturity?  For the Wooster area, the average date for a frost that would stop corn growth is October 10-15. Corn at the R5 stage of development has a much better chance of reaching physiological maturity before frost injury.