Corn in Mississippi
What is the black layer and why is it important?
Corn kernels continue to accumulate seed weight until physiological maturity is reached. Physiological maturity is signified by a "black layer" located within the base of a kernel, which normally forms about 60 days after silking or 20 days after denting.
Hard starch begins forming at the kernel tip when denting occurs. This hard starch layer gradually progresses to the kernel base over the next 20 days as kernels near maturity. This layer within a kernel between hard starch and dough layers is referred to as the "milk-line." The milk-line can be identified by the borderline between the bright, clear yellow color of the seed coat overlying the hard starch layer, compared to the milky, dull yellow color of the soft seed coat overlying the dough layer. To observe the milk line, break a corn ear in half and observe the cross-section of the top half of the ear (the flat side of kernels opposite the embryo (photo).
The progression of the milk-line as it relates to the timing of black layer development is very important to irrigation scheduling and application of pre-harvest aid herbicides, since growers may reduce grain yield by terminating irrigation or applying a harvest-aid too early. It generally takes about 20 days for the milk-line to progress from the kernel tip, down to the base. Growers can use this guideline to estimate maturity. For instance, if the milk-line is half-way down the kernels, it will take about another 10 days to reach physiological maturity. Thus, the field needs supplemental irrigation water to supply moisture and should not have a harvest-aid applied for 10 more days.
An abscission layer forms when the hard starch layer reaches the kernel base. Physiological maturity is signified by this black or brown abscission layer which is referred to as the "black layer". This abscission layer cuts off water and dry matter transfer into the kernel. Kernels will have a moisture content of about 28-35 percent at this stage. The black layer may be found by shelling kernels from an ear and gently scraping away the seed coat to expose the abscission layer. The black layer is located at the kernel base on the opposite side of the embryo. The black layer formation occurs progressively from kernels at the tip of the ear to the base.