Will The Corn Get Ripe?

Tim Jergenson
Agriculture/Horticulture/Natural Resources


Corn growers in Northwestern Wisconsin are concerned about slow maturing corn fields this season due all of the cool wet weather that the area has seen during the month of August.   The accumulated Growing  Degree Units (GDUs) for this area are running behind normal.  With  less than 20% of the corn in Barron County in the early dent stage at the time this article is being written (late August),  it may take another 25-30 days of near-normal temperatures and frost-free nights  for corn plants to reach maturity.

Most concern exists when corn does not reach the silk stage until early August or later. Killing frosts can easily occur by late September, so corn silking in early August would not be safe from major yield reductions due to frost until early October.  At the dent stage, corn has accumulated 75-85% of silage yield and 60-75% of grain yield and needs about 27-32 days to avoid significant yield reductions due to frost .  See Table 1 below.


Table 1.—Relationship between kernel growth stage and development.
Percent of Maximum Yield Moisture Content
Stage Calendar days to
maturity (average)
Growing degree units
(GDUs to maturity)
Grain Whole plant Grain Whole Plant
Silk  (R1) 55-60 1100-1200 0 50-55 80-85
Blister (R2) 45-50 875-975 0-10 55-60 85-95 80-85
Late milk-dough (R4) 35-40 650-750 30-50 65-75 60-80 75-80
Early Dent (R5) 25-30 425-525 60-75 75-85 50-55 70-75
Fully Dented (5.50-5.75) 13-17 200-300 90-95 100 35-40 65-70
Physiological maturity (R6) * 0 0 100 95-100 25-35 55-65
* Black layer formation and/or milk disappearance from kernels under development. Premature frost or extended cold temperatures may cause black layer formation at earlier stages and wetter moistures.

All hybrids require a similar amount of GDUs to complete grain-filling (~1000 to 1200 GDUs). The main difference between hybrids with different maturity ratings is the time required to achieve silking (Table 1). Long-season hybrids (110-115 d RM) in Wisconsin require about 1500-1700 GDUs, while shorter season hybrids (80-85 d RM) require about 800-900 GDUs. Plants respond to these GDU requirement differences by producing fewer leaves which can be further influenced by photoperiod (latitude).

Historically, the number of days from planting to harvest has been used to classify the maturity of corn hybrid; but in recent years, commercial corn hybrid maturity is often determined by growing  degree units (GDU) or heat units (Hu). Growing degree units are systems used to classify the maturity of corn hybrids. GDU result from summation of mean daily temperature.

The calculation procedure of the cut-off method assumes that the maximum sustainable corn growth rate is 86° F . This assumes that higher temperatures do not sustain faster growth rates. So, all daily maximum temperatures above 86° F  are set to 86° F for GDU calculation purposes; and if the daily mean temperature is greater than the “base temperature” for corn of 50° F and the daily minimum is below 50° F, then the daily minimum is set at 50° F to calculate the GDU value on such a day.

As noted in Table 2, approximately 384 GDUs accumulate each September.  If corn has reached the mid to late dent stage by  September 1, it should reach its full potential yield by the end of that month barring an early frost.   Scout corn fields now to evaluate each hybrids potential for reaching maturity and then make plans accordingly for harvest options.

Table 2.—September Growing Degree Units—Spooner Agricultural Research Station 2012– 2016
Year 50° F. Base Corn 50/86° F. Base
2012 271 355
2013 367 410
2014 266 308
2015 457 484
2016 341 365
Average 340 384