Some corn is starting to reach black layer now. This is the point when the corn is physiologically mature and no more nutrients are being put into the kernels. This means irrigation can be cut off in these fields.
The photo above shows the kernels of this ear with no milk line present. The photo below shows a kernel that has been cut in half. You can see the black layer pretty easily here.
The kernels are at about 35% moisture once they reach black layer. Harvesting will start soon, though growers without the ability to dry their own grain should wait until they are closer to 15.5% moisture to avoid drying costs.