The formation of kernel black layer signals full corn kernel maturity. Corn products differ from one another in drydown rates. Plant characteristics and environmental conditions contribute to drydown rate after black layer. Kernel moisture content is during the drying period is lost faster with warm, dry weather and slower with wet, cool weather.
Corn kernels achieve physiological maturity or kernel black layer when a black film develops at the tip of a kernel (Figure 1). The black film or layer seals the kernel from further development or increase in test weight. The kernel moisture content at black layer formation usually ranges from 25 to 40 percent, but averages around 30 percent.
Physiological maturity is greatly influenced by an individual product’s bred-in relative maturity (RM) - shorter season products mature earlier than fuller season products. This genetic characteristic allows for the selection and planting of different corn products to spread out (over time) the maturation of a farming operation’s corn crop.
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 (Tbase) temperature 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 and 115 RM products. The kernel milk line can be used as a measure of kernel moisture content as the kernel advances toward black layer (Figure 2). 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; the environment has a great influence on the speed of moisture content loss from the kernel. Kernel moisture content is lost faster with warm, dry weather and slower in a wet and cool environments. 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).
Aside from product RM and environment, individual product characteristics can influence the speed of kernel moisture content loss. Characteristics that influence the rate of kernel drydown include:
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/ext/corn/news/timeless/GrainFill.html.
2 Nielsen, R.L. 2013. Field drydown of mature corn grain. Corny News Network Articles. Purdue University. https://www.agry.purdue.edu/ext/corn/news/timeless/GrainDying.html.
3 2014. Corn development. Corn Agronomy. University of Wisconsin. www.corn.agronomy.wisc.edu/management/L011.aspx.
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.