• The formation of kernel black layer signals physiological maturity.
• Corn kernels are around 30% moisture content when physiological maturity occurs.
• Plant characteristics and environmental conditions contribute to drydown rate after black layer.
• Kernel moisture content during the drying period is faster with warm, dry weather and slower with wet, cool weather.
• Delayed maturation can result in a less than desirable grain moisture content at harvest time.
Corn kernels achieve physiological maturity or kernel black layer when a black film develops at the tip of the kernel (Figure 1). The kernel moisture content at black layer formation usually ranges from 25 to 40%, but averages around 30%.
Physiological maturity is greatly influenced by an individual product’s relative maturity (RM). Shorter season products mature earlier than fuller season products.
Environmental issues can affect the timing of maturation. Severe drought can cause products to die prematurely and in the process, kernels form a premature black layer. Cool weather during the growing season can seemingly delay the normal maturation for an individual product because the necessary growing degree units (GDU) required to achieve maturity accumulate slower.
Growing degree units are calculated by determining the mean daily temperature and subtracting that from the base temperature (Tbase) for favorable corn growth (50° F). The upper limit for favorable corn growth is established at 86° F. Therefore, the GDU formula is: GDU = (Tmax + Tmin)/2 - Tbase. If the high temperature (Tmax) for the day is above 86° F, 86° is used for the calculation; if the low temperature (Tmin) for the day is below 50° F, 50° is used for the calculation.
Based on GDU accumulations, general relationships for black layer attainment and kernel moisture content can be determined. This can help provide a guideline to help determine the timing of harvesting and fall grain marketing. Table 1 provides GDU information for approximately 100 RM (2350 GDU) and 115 RM (2700 GDU) products. The kernel milk line can be used as a measure of kernel moisture content as the kernel advances toward black layer. Fully dented kernels require about 13 to 20 calendar days or 200 to 375 GDU (depending of product RM) to achieve black layer.3,4
Kernels begin their drying process after black layer formation. Kernel moisture content is lost faster with warm, dry weather compared to a wet and cool environment. Regardless of the environment, it is normal to see later RM corn products reach harvestable moisture levels (around 25%) later than earlier RM products.
Typical drying rates after black layer range from 0.4% to 0.8% kernel moisture content loss per day.1 About 30 GDU per point of moisture are required to dry corn from black layer to 25% moisture content.2 Purdue University studies showed that a loss of 0.5% moisture content occurs when the mean accumulation of GDU is 12, and 0.75% moisture content is lost when the mean GDU accumulation is 22 per day (Table 2).
Individual corn product characteristics can also influence the speed of kernel moisture content loss. Characteristics that influence the rate of kernel drydown include:
• Thickness and Number of Husk Leaves. Thinner and fewer husk leaves can promote quicker moisture content loss.
• Husk Death. Quicker death of husk leaves promotes quicker moisture content loss.
• Ear Tip Exposure. Exposed ear tips may provide for quicker grain moisture content loss.
• Husk Tightness. Husks that are loose and open may help increase grain drying.
Ear Angle. Drooping ears tend to lose moisture content more quickly. Upright ears can capture moisture from rainfall.
• Kernel Pericarp Properties. Thinner pericarps (outer layer covering a corn kernel) have been associated with faster moisture content loss.
Corn maturity calculators are available online from universities and other sources. By entering a location, planting date, and the GDU to silk or black layer, a maturity date can be estimated.
The Corn Growing Degree Calculator from the High Plains Regional Climate Center compiles current conditions into a 30-year historical perspective and offers trend projections through the end of the calendar year at the county level for 12 states in the Corn-Growing Area.
1 Nielsen, R.L. 2013. Grain fill stages in corn. Corny News Network Articles. Purdue University. https://www.agry.purdue.edu.
2 Nielsen, R.L. 2013. Field drydown of mature corn grain. Corny News Network Articles. Purdue University. www.agry.purdue.edu.
3 2014. Corn development. Corn Agronomy. University of Wisconsin. www.corn.agronomy.wisc.edu.
4 Nielsen, B. 2001. Post-maturity grain drydown in the field. Agronomy Tips. Pest & Crop. No. 24. Purdue University. http://extension.entm.purdue.edu.
Web sources verified 8/30/2018.