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September 2008 Archives

Energy costs for corn drying and cooling

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Bill Wilcke
University of Minnesota Extension Engineer, BBE Dept,
1390 Eckles Ave, St Paul MN 55108; email:; phone: 612-625-8205

Reviewers: Vance Morey Professor, UofM Bioproducts and Biosystems Engineering Dept, St Paul MN
Lizabeth Stahl UofM Extension Educator in Crops, Worthington MN

The purpose of this brief article is to provide enough information so that readers can estimate costs for drying and cooling corn. Grain needs to be dry to be stored through warm weather and it takes some energy to remove moisture from grain, but there are things that can be done to manage energy use. More information about managing dryers and storage can be found on the University of Minnesota Bioproducts and Biosystems Engineering Extension post-harvest web page (

Jeff Coulter
Extension Corn Agronomist

Corn Maturity and Frost
Due in part to late planting and cool temperatures this year, much of Minnesota's corn crop will reach maturity (black layer) a little later than normal. In southern Minnesota, a lot of the corn is expected to reach maturity around September 20. In and around the Red River Valley, much of the corn will not reach maturity until the last week of September. As a result, there is a decent chance some corn will receive a frost before reaching maturity, especially in northern Minnesota.

Corn grain and silage characteristics at various stages of development and the effects of an early frost are summarized in Table 1. It should be noted that days to maturity are relatively consistent among hybrids. This is because hybrid maturity mainly influences the amount of time spend in vegetative development, with late maturing hybrids requiring more time from planting to tasseling than early maturing hybrids.

Table 1. Reductions in corn grain yield due to frost, along with corn grain and silage characteristics at various stages of development.

Corn stage Days to maturity1 Grain yield loss due to frost damage2 (%) Test weight2

Grain moisture3

Silage moisture4

Leaves and stalk Leaves only
Early dent 25 32-40 25 47 59 73
Dent 19-22 23 12 50 51 72
Late dent 10-15 15 8 53 47 70
Half-milk line 5 8-12 5 55 40 66
Maturity 0 0 0 58 28-35 60

1 Derived from Behnken and Breitenbach, 2004

2 Derived from Hicks (2004), and Afuakwa and Crookston (1984)

3 Derived from and Lauer (1997), and Schmidt and Hallauer, 1966

4 Derived from Lauer, 1996

For those interested in assessing the developmental stage of corn and determining whether black layer has been reached, an excellent set of pictures have been compiled by Bob Nielsen at Purdue University, and are available at

The extended forecast for both northern Minnesota (Crookston) and southern Minnesota (Redwood Falls) predicts daily low temperatures at or above 40⁰ F through September 18, with a low in the upper 30s predicted for September 19. Assuming that corn maturity would be reached around September 27 in northern Minnesota, a first frost on September 19 (8 days before maturity) would reduce grain yield by about 8-15% if it is a killing frost (leaves and stalk killed), or by 5-8% if it is a light frost (Table 1). A frost-free window through September 18 would extend the growing season almost long enough for corn maturity to be reached in much of southern Minnesota.

Harvest Moisture
Another concern with delayed maturity is harvest moisture. This is because the rate of field drying slows as we move further into the fall (Table 2). Assuming that the crop reaches maturity before the frost, and that the average grain moisture at maturity is 31.5% (Table 1), one can come up with a rough estimate of grain moisture for various maturity and harvest dates (Tables 3 and 4). Harvest moisture values in Table 3 were estimated using the maximum dry-down rates from Table 2, while harvest moisture values in Table 4 were estimated using the average dry-down rates from Table 2.

Table 2. Field drying rates for corn in Minnesota.

Date Grain moisture loss (% per day)

September 15-25


September 26-October 5 0.50-0.75
October 6-15 0.25-0.50
October 16-31 0.00-0.33
After October 31 >very little

Table 3. Predicted grain moisture at harvest for various maturity and harvest dates, using the maximum dry-down rates in Table 2 and assuming 31.5% grain moisture at maturity.

  Date of maturity
Harvest date September 20 September 24 September 28
  Predicted grain moisture at harvest(%)
October 5 19.0 23.0 26.3
October 10 16.5 20.5 23.8
October 15 14.0 18.0 21.3
October 20 --- 16.4 19.6
October 25 --- 14.7 18.0

Table 4. Predicted grain moisture at harvest for various maturity and harvest dates, using the average dry-down rates in Table 2 and assuming 31.5% grain moisture at maturity.

Date of maturity
Harvest date September 20 September 24 September 28
  Predicted grain moisture at harvest (%)
October 5 20.9 24.4 27.1
October 10 19.0 22.5 25.3
October 15 17.1 20.6 23.4
October 20 16.3 19.8 22.6
October 25 15.5 19.0 21.7

According to the estimates in Tables 3 and 4, corn that is expected mature on September 20 would be 19% moisture on October 5 if there is a rapid rate of drying, and 19% moisture on October 10 if there is an average rate of dry down. For corn that is expected to mature on September 28, these estimates indicate that harvest moisture will be 19.6% on October 20 if dry down is rapid, and 21.7% on October 25 if there is an average rate of drying.

Overall, it appears that corn harvest in Minnesota may be delayed a little later than normal to allow drying in the field. It also appears that much of the corn in Minnesota will need to be dried before storing. For corn in northern Minnesota that is not expected to mature until late September, these estimates indicate that the grain will be quite wet at harvest, and that significant drying will be needed.

Useful Links
Managing corn dryers and storage:
Drying, handling, and storing wet and frost-damaged corn:
Harvesting frost-damaged corn for silage:

Afuakwa, J.J., and R.K. Crookston. 1984. Using the kernel milk line to visually monitor grain maturity in maize. Crop Sci. 24:687-691.
>Behnken, L., and F. Breitenbach. 2004. Minnesota SE region Ag newsletter. Available at (verified 10 Sep. 2008). Univ. of Minnesota, St. Paul.
Hicks, D.R. 2004. The corn crop - frost and maturity. Available at (verified 10 Sep. 2008). Univ. of Minnesota, St. Paul.
Lauer, J. 1997. Corn replant/late-plant decisions in Wisconsin. Available at (verified 10 Sep. 2008). Univ. of Wisconsin, Madison.
Lauer, J. 1996. Corn harvest in Wisconsin during cool growing conditions. Available at (verified 10 Sep. 2008). Univ. of Wisconsin, Madison.
Schmidt, J.L., and A.R. Hallauer. 1966. Estimating harvest date of corn in the field. Crop Sci. 6:227-231.

Jeff Coulter
Extension Corn Agronomist

Corn residue as a commodity?
In most fields, corn residue remaining after grain harvest is incorporated into the soil with tillage or is left on the soil surface. Currently, corn residue is being harvested by some livestock producers, and there is interest in producing ethanol from corn residue in the near future (Perlack et al., 2005). However, soil productivity (synonymous with soil carbon) will be reduced if all corn residue in a field is harvested regularly and there is not another source of carbon being returned to the soil to replace the carbon removed with the residue. Good sources of carbon include: i) manure; ii) bi-products from industrial processes such as ash; and iii) winter cover crops. Increased fertilization in fields where residue is harvested will help replace some of the nutrients removed in the residue, but it will not compensate for the lost carbon. In addition, nitrogen fertilizer rates in continuous corn should actually be reduced following corn residue harvest.

Why is soil carbon important?
Carbon is important because it is the backbone of soil organic matter. Soil organic matter represents decaying plant and animal residues, microscopic soil organisms that decompose plant and animal residues, and substances released by these organisms into the soil (Brady and Weil, 2002). Since plants are at the top of the soil food chain, they are the initial source of all soil organic matter. For producers, soil organic matter is important because: i) it is bank of nutrients that will be slowly released over time; ii) it improves the water-holding capacity of the soil; and iii) it promotes the aggregation of soil particles. Aggregation is important because it promotes water infiltration, increases the rooting ability of plants, and allows the soil to be tilled with less horsepower. As a result, soil organic matter (or soil carbon) is synonymous with soil productivity. The light-colored forest-derived soils in the eastern Corn Belt contain about half the organic matter that our dark prairie-derived soils contain in Minnesota. As a result, crop water stress is often common on these soils, even though rainfall in the eastern Corn Belt is generally greater than that in Minnesota.

How much corn residue can be harvested?
The amount of corn residue that can be sustainably harvested in the absence of supplemental carbon (manure, industrial bi-products, or cover crops) depends on the crop rotation and tillage system. On average, the amount of corn residue that needs to be retained to preserve soil organic matter and protect against wind and water erosion in the Corn Belt is shown in Figure 1 (Wilhelm et al., 2007). It should be noted, however, that the amount of corn residue needed to protect against soil erosion is less than the amount needed to maintain soil organic matter levels. For reference, Figures 2 and 3 show surface residue coverage with removal of none and approximately all of the corn residue in a chisel plow tillage system.

Figure 1. Corn residue to retain to preserve soil organic matter and protect against erosion, depending upon crop rotation and tillage system. Source: Wilhelm et al., 2007.

Corn Residue.gif

Figure 1 shows that a 200 bushel per acre corn crop produces 4.22 tons of dry matter per acre as corn residue (assuming a harvest index of 0.53). In a corn-soybean rotation where corn residue is moldboard plowed, the amount of corn residue that needs to be retained is greater than the amount produced with a 200 bushel corn crop. Thus, it is not sustainable to harvest corn residue in this system, and this system is actually reducing soil productivity over time. On the other hand, if continuous corn is grown with moldboard plow tillage, the amount of corn residue that needs to be retained is about 0.84 tons per acre less than that produced with a 200 bushel corn crop. This leaves 0.84 tons of corn residue per acre (20% of the total residue production) that could be harvested annually, but this would require a 200 bushel yield level every year. The potential for residue harvest is much greater when a conservation tillage system such as no-till, strip-till, or chisel plow tillage is used. With conservation tillage in continuous corn, up to 45% of the corn residue could be harvested annually if grain yields are consistently 200 bushel per acre.

Figure 2. Surface residue coverage after stalk chopping and chisel plowing in a field where no corn residue was harvested. Photo by Jeff Coulter
Surface Residue Coverage.jpg

Figure 3. Surface residue coverage after chisel plowing in a field where corn residue was chopped, raked, and baled. Photo by Jeff Coulter.
Surface Residue Coverage 2.jpg

Considerations for sustainable harvest of corn residue
Residue harvest is best suited to continuous corn systems that consistently have high yields and utilize little or no tillage. If corn residue is harvested, do not remove more than 45% of the residue. Harvesting only 45% of the corn residue is tricky, but it can be done if stalks are cut high during grain harvest and if stalks are not chopped prior to baling. If a rake is used prior to baling, make sure that the rake is set as high as possible to avoid collecting too much residue.

Another useful idea when harvesting residue is to rotate residue harvest among fields. This ensures that residue is not harvested from the same field every year. In addition, think seriously about reducing tillage following residue harvest. Also target manure applications rather than fertilizer for these fields if soil test levels indicate that phosphorus is needed. Winter cover crops should also be considered for fields where residue is removed. In addition to serving as a carbon source, the roots from winter cover crops are extremely effective at scavenging residual soil nitrate. This is especially important following dry years where uptake of nitrogen by the corn crop is lower than normal.

When residue is removed in continuous corn systems, reduce the nitrogen fertilizer rate for the next year's corn crop. Research that I conducted for my Ph.D. at three locations in northern and central Illinois on dark prairie-derived soils in 2006 and 2007 showed that the economically optimum nitrogen fertilizer rate in continuous corn is reduced by 13% when half or all of the corn residue is harvested. This was consistent for both chisel plow and no-tillage systems. In continuous corn, less nitrogen is needed for the following year's crop when residue is harvested because corn residue promotes tie up (immobilization) of nitrogen by soil microorganisms. While it is critical to maximize profitability from the land, we need to balance short-term economics with long-term sustainability. When removing residue this fall, use common sense to preserve soil organic matter and protect against erosion.


  • Brady, N.C., and R.R. Weil. 2002. The nature and properties of soils. 13th ed. Pearson/Prentice Hall, Upper Saddle River, NJ.
  • Perlack, R.D., L.L. Wright, A.F. Turhollow, R.L. Graham, B.J. Stokes, and D.C. Erbach. 2005. Biomass as feedstock for a bioenergy and bioproducts industry: The technical feasibility of a billion-ton annual supply DOE/GO-102005-2135 and ORNL/TM-2005/66 [Online]. Available at (posted Apr. 2005; verified 16 Sep. 2008). NTIS, Springfield, VA.
  • Wilhelm, W.W., J.M.F. Johnson, D.L. Karlen, and D.T. Lightle. 2007. Corn stover to sustain soil organic carbon further constrains biomass supply. Agron. J. 99:1665-1667.

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