Predicting black layer or physiological maturity in corn

• By Angela McClure•

milk line in corn

Recent showers in some counties will help maximize the yield potential of corn even at this late stage. Overall yield potential of the crop is good, however heat stress and limited rainfall will affect yield numbers in some portions of the state.

Achieving physiological maturity (R6 or black layer) is the first step towards corn dry down and harvest. The date at which corn black layer forms depends on planting date, relative maturity of hybrid and, to some extent, planting location in Tennessee.

For example, fields located in extreme Southwest Tennessee will accumulate growing degree-day units faster than other counties for a similar relative maturity hybrid planted the same day.

Because development stages of corn are related to heat unit accumulation, the date of black layer can be predicted fairly accurately for a field if long-term weather data is available. One such site that can generate black layer prediction charts by county is

approximate black layer dates
What is ‘black layer’?

Black layer (R6) is the culmination of starch conversion in the seed during a process requiring about three to 3.5 weeks. A “milk line” or white line displays across the kernel and indicates the amount of hard starch accumulation.

At ¾ milk line, the starch line has moved about ¾ distance down the kernel towards the cob. When the milk line is no longer visible, a dark brown or black layer of cells forms to separate the kernel from the cob, hence the term “black layer.”

Seeds are fully dented at black layer, but grain moisture typically averages above 30%. Mechanical harvest loss and kernel damage are minimal when grain drys to about 25% moisture.

Tennessee grain drydown may be affected by hybrid characteristics (husk density and coverage) and especially heat unit accumulation; therefore, producers should expect faster drydown rates (about 0.5 percentage point per day) in September compared to October or November.

Regardless of planting date or hybrid, drydown rates ultimately depend on how much warm, low-humidity, dry weather a field receives. Although late-planted beans may need moisture in September, corn does not during drydown.

Dr. Angela McClure is University of Tennessee Extension corn and soybean specialist. She may be reached at

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