Will corn mature before the killing frost?

September 5, 1996 3(24):141-142

Joe Lauer, Corn Agronomist

It looks like we will be going "right down to the wire" this year to produce corn grain, and in many parts of the state it may even be difficult to produce good quality corn silage. Due to late planting and a cool growing season, corn development is behind and there is great potential for frost to occur before maturity, especially in the eastern half of Wisconsin.

Typically in a normal year, corn should be "silking at the end of July and denting on Labor Day." After corn silks, it normally takes about 55 to 60 days for it to mature. Right now heading into Labor Day, we are seeing many fields which are between the silking and milk stages of development. These fields will require 700-1200 growing degree units in order to mature and another 150 units to be at a harvestable moisture (Table 1).

Table 1. Required growing degree units between corn development stages and maturity (black layer).
Corn Relative maturity zone (days)
development 85-90 95-105 110-120
stage Growing degree units
R1 (silking) 1000 1100 1200
R2 (blister) 800 880 960
R3.5 (late milk / early dough) 600 660 720
R4.5 (late dough / early dent) 400 440 480
R5 (dent) 200 220 240
R6 Maturity (black layer) 0 0 0
Harvest (kernel moisture at 25%) 150 150 150
derived from Carter, 1991

Normally during September, growing degree units in Wisconsin accumulate at the rate of 12 to 19 units per day for a total accumulation of 400 to 450 units (Table 2). Likelihood of a 32 ° F freeze by September 20 is 3 years out of 5 in northern, and 1 year out of 5 in southern Wisconsin.

Table 2. Corn growing degree unit accumulation in Wisconsin.
  North   South
Month Daily Monthly Total   Daily Monthly Total
  Growing degree units
May 8-11 300 300   10-13 350 350
June 11-17 400 700   13-20 500 850
July 17-20 575 1275   20-23 650 1500
August 20-17 575 1850   23-19 650 2150
September 17-12 400 2250   19-13 450 2600
October 12-8 300 2550   13-10 350 2950
derived from Mitchell and Larsen, 1981

Use tables 1 and 2 to determine the likelihood that a field will mature. For example, if on September 1, your field is at R3.5 (late milk / early dough) and you are in a 95-105 relative maturity zone, it will take about 660 growing degree units to mature the crop before it is killed by a frost. Since corn is usually killed in 3 out of 5 years by September 20 the field in all likelihood will accumulate about 300 to 380 growing degree units and be at the early dent to dent stage of development when it is killed by a frost.

Carter, P.R. 1991. Corn development and growing degree days. Agronomy Advice Mimeo Series 28.10.

Mitchell, V.L., and R.W. Larsen. 1981. Growing degree days for corn in Wisconsin. UWEX. 22 pp.

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