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Physiological maturity (black layer) for corn occurs when the kernel becomes sealed off from the flow of nutrients and water, typically when the plant has acquired the number of growing degree units (GDU) for its maturity. The black layer is a film of black tissue that forms at the kernel tip where it attaches to the cob (Figure 1). The formation of the black layer stops potential gains in test weight and initiates the drying process of the kernel.
Figure 1. The black layer prevents the flow of nutrients and water from the cob and initiates kernel drydown.
Environmental conditions can cause premature or delayed formation of the black layer. Drought stressed plants can shut down too early for the product’s maturity and force an early black layer. A cool growing season, which results in a slow accumulation of GDU, can delay development of the black layer.
Late-planted corn fields can become a risk of being damaged or killed by frost. Generally, the same corn product planted late requires fewer GDU to achieve black layer than an earlier planting. However, if temperatures are below normal during the growing season, GDU requirements for each stage of growth for the product may track closely with the normal GDU requirements for an earlier planting, increasing the chance of frost injury.
Under normal Midwest planting dates and growing conditions, the calendar time from grain fill to physiological maturity is similar across a wide geographical area. Maturation for adapted corn products typically occurs about 65 days after silking in the central corn growing region and 55 to 60 days after silking in the northern corn growing region.1 If silking does not occur until early August or later, black layer may not be achieved before the occurrence of a late September or early October killing frost, especially in the northern corn growing area.
Two ways to estimate the potential for a corn plant maturing before a killing frost are:
A killing or light frost prior to black layer can affect yield potential and grain quality. The impact on corn yield depends on several factors including, the stage of corn development, the actual low temperature, and the duration of the low temperature. The closer the plant is to black layer, the less effect there is on grain yield (Table 1). Even if a frost kills most of the leaf tissue on the plant, the translocation of sugars from stalks to ears can still increase kernel dry weight unless the freeze is severe enough to kill the husks, stalks, and kernels.
Sources: 1 Nielsen, R.L. 2011. Predicting corn grain maturity dates for delayed plantings. Corny News Network Articles, Purdue University. https://www.agry.purdue.edu/ext/corn/news/timeless/RStagePrediction.html. 2 Hall, R.G. Corn growth stages with estimated calendar days and growing-degree units. South Dakota State University. https://www.sdstate.edu. 3 Lauer, J. 1997. Killing frost in corn. Wisconsin Crop Manager. Corn Agronomy. University of Wisconsin. http://corn.agronomy.wisc.edu/WCM/W048.aspx. Web sources verified 8/31/15. 150706090825