You need only do two things to reassure yourself that you better have your grain dryer ready to go this year. First, look out the window. Unless you're one of the lucky ones that planted early, or unlucky ones where it's not raining, odds are you see corn that's either just finished pollinating, or maybe even still tasseling. Hopefully that's just a few spots, not the whole field.
Bob Nielsen has done a good job of calculating cornfield math for what needs to happen after pollination fro corn to reach black layer. It's presented in the Purdue University Corn & Soybean Field Guide, 2009 edition. The bottom line is that just like fine wine, so they say, you can't rush progress. Corn running behind may speed up the process somewhat, but it takes a certain amount of time and heat units to get from pollination to black layer. And when you're at black layer, you're still only at 32 to 35% moisture in most cases. But the significance is that a frost will no longer hurt yield if the black layer has formed and the kernels are no longer putting more nutrients into themselves from sugars produced by the plant during photosynthesis.
Here's how the math shakes out. About 10 to 14 days after silking, corn enters the blister stage. Hopefully, your fields are past that stage by now. Ear silks are brown and drying rapidly at this stage, and only clear fluid is inside the kernels. Kernel moisture content is 85%, and kernels are still subject to abortion due to stress at this point.
Milk stage is defined as the point where kernels are yellow on the outside, and the inside is a milky substance. Now we're at 18 to 22 days after silking under normal conditions. It's the infamous roasting ear stage, and moisture content is still off the charts, up there at about 80%.
Dough stage covers a range, depending upon hybrids and conditions. It can happen anywhere from 24 to 48 days after silking. The inside of the kerns take on a pasty or doughy appearance. It's technically considered the R4 stage. You're making progress- let's just hope your crop is at least at this point now, not two weeks from now.
Crowns dent 35 to 42 days after silking. Nearly every kernel should dent at this stage. The milk line, the infamous line between the original milky material and the solid starch of mature kernels, begins moving through the kernels. Dry matter content can still be limited by stress at this point. Moisture content is about 60%. By half milk-line stage, moisture content is down to about 55%.
Finally, about 55 to 65 days after silking, dry weight accumulation in the kernel peaks. The black layer forms. The kernel is physiologically mature, but moisture contest is still 32 to 35%. If corn silked August 1, we're talking the last week of September into the first week of October for black layer. Let's hoped yours silked earlier.