By David Nicolai, University of Minnesota Extension

The corn grain fill period begins with successful pollination and initiation of kernel development, and ends approximately 60 days later when the kernels are physiologically mature. During grain fill, the developing kernels will be the primary sink for photosynthate produced by the corn plant. What this means is that the photosynthate demands of the developing kernels will take precedence over that of much of the rest of the plant. In essence, the plant will do all it can to "pump" dry matter into the kernels, sometimes at the expense of the health and maintenance of other plant parts including the roots and lower stalk.

A stress-free grain fill period can maximize the yield potential of a crop, while severe stress during grain fill can cause kernel abortion or lightweight grain and encourage the development of stalk rot. The health of the upper leaf canopy is particularly important for achieving maximum grain filling capacity.

Following is a review of major corn stages left for the balance of the growing season:
Kernel Milk Stage (R3)
About 18 to 22 days after silking, the kernels are mostly yellow and contain "milky" white fluid. Starch continues to accumulate in the endosperm.

Kernel Dough Stage (R4)
About 24 to 28 days after silking, the kernel's milky inner fluid is changing to a "doughy" consistency as starch accumulation continues in the endosperm.

Kernel Dent Stage (R5)
About 35 to 42 days after silking, all or nearly all of the kernels are denting near their crowns. A distinct horizontal line appears near the dent end of the kernel and slowly progresses to the tip end of the kernel over the next 3 weeks or so. This line is called the "milk line" and marks the boundary between the liquid (milky) and solid (starchy) areas of the maturing kernels.

Physiological Maturity (R6)
About 55 to 65 days after silking, kernel dry weight usually reaches its maximum and kernels are said to be physiologically mature and safe from frost. Physiological maturity occurs shortly after the kernel milk line disappears and just before the kernel black layer forms at the tip of the kernels. If your corn pollinated around Aug. 1 than you need to hope that a killing frost does not occur until after Oct. 1!

(David Nicolai is a crops educator with University of Minnesota Extension in Hutchinson.)