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To help achieve optimum corn yields, moisture should be available to corn plants until physiological maturity. Water requirements decrease as the plant gets closer to maturity, but maintaining proper soil moisture levels until maturity can minimize plant stress. For corn, physiological maturity occurs when the abscission layer, commonly known as the black layer, forms at the base of the corn kernel.1 Kernels will continue to accumulate seed weight until the black layer forms.
Figure 1. Pictured is the milk line, the border between the dark yellow hard starch layer and the light milky dough layer.
The maturing process begins as the hard starch layer starts to develop at the top of the kernel. The borderline separating the hard starch layer and the dough layer is referred to as the milk line (Figure 1).2 The progression of the milk line can be monitored by breaking a corn cob in half and observing the flat side of the corn kernel. The milk line separates the darker yellow hard starch layer from the dull light yellow soft dough layer.
At complete physiological maturity (R6) the hard starch layer has reached the kernel base, the abscission layer has formed, and the plant no longer needs water for growth. At this point, kernels have attained their maximum dry weight and should have a moisture content of 28 to 35 percent. To confirm the presence of the black layer, the seed coat at the tip of the kernel can be rubbed off. The black layer should be visible at the kernel base on the opposite side of the embryo.
Black layer formation occurs progressively from the tip of the ear (furthest from the stalk) to the base (closest to the stalk).1 To verify total black layer formation always check the youngest kernels on the ear.
1 Corn: When should irrigation of corn be terminated? 2009. Mississippi State University Cooperative and Research Extension Service. http://msucares.com 2 Corn: What does the milk-line look like? 2009. Mississippi State University Cooperative and Research Extension Service. www.msucares.com 3 Corn growth stage development. University of Nebraska Lincoln. WeedSOFT. Crop Growth Stage Learning Module. http://weedsoft.unl.edu Web sources verified 7/12/2014. 140711080201