Corn physiological maturity and potential harvest dates

Early planted corn in the Thumb and Saginaw Valley has reached physiological maturity. What does this mean for potential harvest dates?

Early planted corn (planted around May 10 and earlier) in the Thumb and Saginaw Valley has reached physiological maturity – the stage when a black or brown layer has formed right above the kernel tip. This layer forms when the starch layer has advanced completely down to the cob. The black layer cuts off water and dry matter movement into the kernel. Thus, no more starch accumulation occurs at black layer.

You can determine when black layer has occurred by snapping a corn cob in half and removing a kernel. Cut the kernel in half lengthwise. If the black layer has developed, you will see a crescent-shaped black layer at the base of the kernel. Kernels reach black layer starting at the tip of the cob and progressing to the base.

At black layer formation, depending on hybrid and weather conditions, the average kernel moisture usually ranges from 30 to 35 percent. This raises the question “When will the corn be ready to harvest?” According to Bob Nielsen at Purdue University, corn reaching physiological maturity in mid- to late September will dry down in the field at an average rate of 0.4 percentage points per day. Of course, air temperature and relative humidity play a big part in that dry down rate. Corn may lose one percentage point of moisture on a warm, breezy day, and no or very little moisture on a cool, cloudy day. Assuming an average loss of 0.4 percent per day, it would take 25 days for corn to lose 10 percentage points of moisture after black layer development (10 percentage points / 0.4 percent moisture loss per day = 25 days).

Differences in hybrids also play a part in in-field dry down. Hybrid differences that influence dry down include the number and thickness of husk leaves, husk coverage of the ear, and how quickly the ear drops from the upright to the downward position. Think of it like this – the less husk material covering the ear, the more potential the kernels have to dry faster. And, when the ear drops down, the rain will not accumulate as much in the ear.

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