It’s getting to the time of the season when corn is begging to reach physilogical maturity. Here is an article I share every year on how to know when your corn has reached black layer.
The best way to describe the milk line method of determining the water needs of you crop is it takes approximately 20 days for the kernel to create the starch. The top half (the darker half) is the starch. It will slowly progress down the kernel to the cobb. At this point, the kernels are completing their starch production, so any water deficiency will directly affect your bottom line. A good rule of thumb is to maintain a good soil moisture profile until the starch/milk line on the kernel reaches the 50-75% mark (which is about half way down the kernel ~ see photos below). Pick about six ears from different locations in the field, break the ears in half and pull kernels from the middle of the ear. Slice these kernels lengthwise and evaluate how far the starch layer has progressed.
For pivot irrigation, we would like to see the layer progress from ½ to ¾ down the kernel and have good soil moisture at that time before irrigation is terminated.
Fig. 1 Starch line progression in grain corn. Fig. 2 Starch/milk line on the cob.
Once the corn has reached physiological maturity, precipitation in the form of rain or irrigation will not influence yield. It is the point at which the starch line disappears and just before the kernel black line forms at the tip of the kernels. Physiological maturity can be determined by examining the kernels in the same manner as monitoring the starch line. Once the kernels are sliced, the black line will be evident toward the bottom of the kernel (Fig. 3 & 4). Just keep in mind that different cultivars mature differently and at different speeds, but these maturity indicators should work universally.
Fig. 3 Black layer from kernel cross-section Fig. 4 Black layer at kernel bottoms.
For more information or for help in determining if you have reached black layer, please contact your local Extension Agent.