- Kernel black layer seals the kernel from further test weight gain.
- Product relative maturity and environmental issues can influence the timing of black layer formation.
- Kernel milk line can be used as a measure of kernel moisture content as the kernel advances toward black layer.
- Kernel moisture content is lost faster with warm, dry weather and slower with wet, cool weather.
What is Black Layer or Physiological Maturity
Corn kernels achieve kernel black layer or physiological maturitywhen 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) - short 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. This helps with harvest scheduling as all corn acres are not maturing at the same time.
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 delay the normal maturation for an individual product because the necessary growing degree units (GDUs) 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.
Reproductive Corn Growth Stages
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 harvest timing 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 on product RM) to achieve black layer.3,4
Kernels begin their drying process after black layer formation; environmental conditions have 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 GDUs per point of moisture are required to dry corn from black layer to 25% kernel moisture content.2 Purdue University studies showed that a loss of 0.5% moisture content occurs when the mean accumulation of GDUs is 12, and 0.75% moisture content is lost when the mean GDU accumulation is 22 per day (Table 2).
Product Characteristics Impacting Drydown
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:
- 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 kernel 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. Online sites include:
- The University of Wisconsin offers a calculator for Android mobile devices: http://ipcm.wisc.edu/blog/2015/02/5-free-apps-for-wisconsin-crop-production/
- South Dakota State University offers a web-based calculator: http://climate.sdstate.edu/awdn/archive/de greedays.asp
- Michigan State University offers weather-based pest, natural resource and production management tools online at: http://enviroweather.msu.edu.
- Illinois State Water Survey offers a web-based calculator: www.isws.illinois.edu/warm/cropdata/cro pddcalc.asp
- University of Missouri Corn Maturity Calculator: http://plantsci.missouri.edu/grains /corn/calculator/index.cfm
Once corn planting is delayed, the yield potential of the crop varies with the rest of the growing season. Insect pressure, especially from late generations of European corn borer, corn ear worm, and fall armyworm, can significantly damage a late-planted corn crop. Planting a B.t. corn product can greatly reduce this risk.3 Plant population should reflect the yield expectation and it is important to be very timely with fertilizer applications and weed control as a late-planted corn crop will accumulate heat units faster than an earlier-planted corn crop.
Delayed corn planting may also increase the chance of heat and drought stress during the critical water use periods (two weeks prior to silking and during pollination). Planting corn products that range in GDU requirements for flowering and physiological maturity can help reduce the chance that the whole corn crop flowers during a period of high heat or is damaged by frost later in the season (Figure 2).
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 Corn growth stages with estimated calendar days and growing-degree units. South Dakota State University. https://www.sdstate.edu.
5 Nielsen, B. 2001. Post-maturity grain drydown in the field. Agronomy Tips. Pest & Crop. No. 24. Purdue University. http://extension.entm.purdue.edu.
6 Lauer, J. 1997. Healthy corn growth and development in Wisconsin. Agronomy Advice. University of Wisconsin.
7 Nafziger. E. 2009. Corn. Chapter 2. Illinois Agronomy Handbook. University of Illinois.
8 Hoeft, R.G., Nafziger, E.D., Johnson, R.R., and Aldrich, S.R. 2000. Modern Corn and Soybean Production. Additional source: Geyer, A. and Thomison, P. Corn drydown. C.O.R.N. Newsletter 2006-28. The Ohio State University.
Web sources verified 8/1/2016. 130905070122