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Black layer occurs at physiological maturity and at roughly 30% moisture content. Grain drydown is primarily influenced by weather, especially temperature and humidity/rainfall. Late planting coupled with a cool growing season can push maturation into potential frost timelines.
Corn kernels are around 30% moisture content when physiological maturity or black layer occurs. Several factors influence field drydown after maturity. Kernel moisture content decreases faster with warm, dry weather and may decrease slowly in a wet and cool environment. Fuller season corn products, that require more growing degree days (GDDs) to mature, will likely be slower drying as the fall progresses within an area. Crop maturity can be hastened by dry weather conditions, which usually results in a loss of potential yield as plant death occurs before the kernels gain their full weight and size.
Typical drying rates after black layer range from 0.4% to 0.8% kernel moisture content loss per day.1 It is not uncommon for grain moisture to decline more than 1.0% per day when conditions are warm, sunny, windy, and dry. In contrast, minimal daily drydown may occur on cool, cloudy, rainy days. About 30 GDDs per point of moisture are required to dry corn from black layer to 25% moisture content.2 Conversely, about 45 GDDs per point of moisture are needed for grain to drydown from 25% to 20% moisture.2 Purdue University studies showed that a loss of 0.5% moisture content occurs when average daily accumulation of GDDs is 12 and 0.75% moisture content is lost when 22 GDDs are accumulated in a day, respectively.
Corn products differ from one another in drydown rates. Plant characteristics that can influence the drydown rate include:3
Field drying of corn at maturity (30%) to an acceptable harvest moisture can take two to four weeks or more. Grain bins and dryers may be necessary in the event of a wet harvest and potential variable grain moisture.
Late Planting and Cool Weather Effects. Late-planted corn can result in taller plants, smaller diameter stalks, pollination when temperatures are hotter, and delayed maturation. Delayed maturation can result in a less than desirable grain moisture content well into the harvest season. Cooler fall temperatures decrease the rate that kernels lose moisture content. This coupled with late planting dates, can push maturation into potential frost timeframes.
Frost Potential. A killing frost occurs when temperatures near 32° F persist for a few hours or temperatures near 28° F persist for a few minutes. Frost damage at higher temperatures is possible, but highly variable to topography and atmospheric conditions.
1 Elmore R. and Abendroth, L. 2007. How fast can corn drydown? Iowa State University Extension. http://www.agronext.iastate.edu/corn/showitem.php?id=59 2 Thomison, P. 2014. Corn drydown: What to expect? C.O.R.N. Newsletter 2014-31. http://agcrops.osu.edu/corn/newsletters/2014/2014-31/corn-drydown-what-to-expect 3 Nielsen, R.L. 2013. Field drydown of mature corn grain. Corny News Network. Purdue University. https://www.agry.purdue.edu/ext/corn/news/timeless/graindrying.html 4 Lauer, J. 2014. Frost. University of Wisconsin. http://corn.agronomy.wisc.edu Web sources verified 09/05/15. 141006083211