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Extension > Agriculture > Crops > Corn Production >Growth and development > Maturity, frost, and harvest moisture considerations for corn

Maturity, frost, and harvest moisture considerations for corn

Jeff Coulter, Extension Corn Agronomist

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 (lb/bu) 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
1Derived from Behnken and Breitenbach, 2004
2Derived from Hicks (2004), and Afuakwa and Crookston (1984)
3Derived from Lauer (1997), and Schmidt and Hallauer, 1966
4Derived 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 40° 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 0.75-1.00
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
Source: Hicks (2004)

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

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

Drying, handling, and storing wet and frost-damaged corn:

Yield and harvest considerations for frost damaged corn:


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. (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.

Schmidt, J.L., and A.R. Hallauer. 1966. Estimating harvest date of corn in the field. Crop Sci. 6:227-231.

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