Elmore also handed out six bags of corn ears he picked that morning from a nearby field where he has a date of planting study. The bags were labeled Group Number 1,2,3,4,5 and 6. People sitting in the front row were handed the "Group 1" bag of ears. I was sitting there next to Barb and Jim Halbur who farm at Carroll in western Iowa. Everyone opened their bag, took out an ear and noted the stage of corn growth. Ours was "early dent" stage. Dent is the R5 growth stage of a corn plant (see the table).
Next, Elmore had us break an ear in half, and instructed us to "Look and see how far the milk line has developed on the kernels. The milk line separates the milky white portion nearest the cob and the starchy solid portion at the top of the kernel. The milk line moves down from the top of the kernel toward the cob as the kernel matures."
Ears from our Group 1 bag on August 30 were still 24 days from maturity
The ears from our Group 1 bag had the milk line formed about halfway down the kernels. We looked at the chart and saw that at the "half milk" stage, our ear of corn still had 24 days left before it would reach physiological maturity. That's the R6 stage of growth, which precedes what is commonly known as "black layer" stage. The black layer is the first visible indication that physiological maturity has occurred and the corn is safe from frost.
Will that ear of corn reach maturity before first killing frost? The answer was "yes"—if the date of the first killing frost comes no earlier than normal. Corn in the Group 1 bag was planted April 29, said Elmore. The average date of the first killing frost in central Iowa is around October 10. So if frost occurs around that date this year, the field where this ear of corn came from will be safe. Adding 24 days to August 30 you figure the corn will reach maturity on September 22 or so.