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.