Physiological Maturity or Black Layer
Physiological maturity for corn usually occurs when the plant has obtained the number of growing degree units (GDUs) required for full maturity and the kernel becomes sealed off from the flow of nutrients and water. The sealing is a result of the formation of a black layer or a thin film of tissue on the tip of the kernel (Figure 1). Black layer formation can be influenced by environmental conditions. Drought can cause the black layer to form prematurely and cool conditions can delay its formation. The kernel begins its drying process after the black layer forms.
Simple Versus Killing Frost
A light frost can develop on leaves with temperatures well above 32°F with very clear and calm conditions.1 Simple or light frosts do not kill the entire corn plant but can damage or kill some leaf tissue. In the days following a simple frost, undamaged remaining leaves can continue to photosynthesize and carbohydrates from stalk tissue are remobilized to immature grain. The stem itself is a temporary storage organ and if not killed by frost, can contribute a 7 to 20 percent grain yield increase after light frosts even if leaves are killed.2
Killing frosts occur when temperatures remain at or below 32°F for several hours. Plant metabolism is stopped, and no further dry matter accumulates in the grain. Kernel black layer develops after a killing frost and is triggered by the reduction or lack of photosynthates.
Late Planting and Timing of a Killing Frost
A late-planted corn crop could be at risk of being killed by frost. However, research demonstrated corn products require about 6.8 fewer GDUs per day of delayed planting as compared to “normal” planting dates – after May 1 and through mid June - to reach black layer compared to an earlier planting.3 Below-normal temperatures during the growing season, followed by warm, sunny fall conditions keeps green plants photosynthesizing. These conditions could result in delayed black layer formation based on calendar date and expose the plant to frost damage. However, this situation also provides an opportunity for late season grain fill and may, in the end, be beneficial.
Black Layer Development
Under normal Midwest planting dates and growing conditions, the calendar time from grain fill to physiological maturity is similar across a wide geographical area. Kernel black layer for adapted corn products typically occurs about 65 days after silking in the central Corn Belt and 55 to 60 days after silking in the northern corn growing region.4 Physiological black layer may not be achieved before a late- September or early-October killing frost if silking does not occur until early August or later. This is especially true in the northern corn-growing area where black layer can pre-maturely be induced by frost.
Estimating Corn Plant Maturation Before Frost
There are two ways to estimate the potential for a corn plant maturing before a killing frost: adding calendar days or calculating GDUs.
Calendar Day Method. Depending on geography, add the 55 to 65 calendar days required for black layer development after the pollination date and check historical information for the average first killing frost date in the area. This rule of thumb can be influenced by temperature during grain fill period; warmer than normal temperatures can shorten the grain fill period and hasten maturity, while cooler than normal temperatures can lengthen grain fill and delay maturity.
Growing Degree Unit. Estimating maturity date based on the anticipated GDU requirement (which could be in a range around 1500 GDUs) from silking to black layer. It typically takes early-maturing corn products fewer GDUs to reach black layer after silking. There is also potential for early season soil temperatures and growth-limiting stresses to influence timing of maturation. Models have been developed by several Land Grant Universities to predict physiological black layer and may be helpful in forecasting the date that it is reached. Please see https://hprcc.unl.edu/gdd.php for more information.
Yield potential and grain quality can be affected if a killing frost occurs before physiological maturity (black layer) occurs. The effect on corn yield depends on the stage of corn development, the low temperatures reached, the duration of the low temperature, and other factors. The closer the plant is to physiological black layer, the less effect there is on grain yield (Table 1). Even if a frost damages 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.
1Nielsen, R.L. 2019. Frost or freeze damage to immature corn. Corny News Network. Purdue University.
2Lauer, J. 1997. Killing frost in corn. Wisconsin Crop Manager. Corn Agronomy. University of Wisconsin. http://corn.agronomy.wisc.edu/WCM/W048.aspx.
3Nielsen, R.L. 2019. Hybrid maturity decisions for delayed planting. Corny News Network. Purdue University.
4Nielsen, R.L. 2019. Predicting corn grain maturity dates for delayed plantings. Corny News Network Articles. Purdue University.
Web sources verified 5/27/20. 6012_S1