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Angie Peltier

Angie Peltier
Former Extension Educator, Commercial Agriculture

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Hill and Furrow

Current topics about crop production in Western Illinois, including field crops research at the NWIARDC in Monmouth.
Figure. Growing degree days accumulated each day between April 10 and August 15, 2014 (blue bars) and total growing degree day accumulation during this period (red line). Data obtained using cli-MATE tools from the Midwest Regional Climate Center),

Cool temperatures and crop development, corn planting dates and yield estimates

July temperatures. Temperatures below the 30-year "normal" prevailed over much of July and are continuing into August (Table). Comparing the temperature data collected at the Northwestern Illinois Agricultural Research and Demonstration Center (NWIARDC) to the "normals" at a weather station 4 miles away in Monmouth (with a larger data set) gives us a decent indication of just how much temperatures may have strayed from normal. The monthly average high and low temperatures were 5 and 7 degrees below the 30–year normal respectively.


Soil Temperature

Air Temp

4" (Bare)

4" (Sod)


Growing Degree Days


Monthly average high


Monthly total: 575



Monthly average low




Observed high (date)

87 (12)

26 (8)

86 (31)

86 (8)

Observed low (date)

48 (24)

9 (3)

64 (4)

67 (4)

How might the cool July weather affect corn development? The enzymes that drive the chemical reactions required for growth and development have optimal temperatures under which they work. This is the case for the enzymes that are needed to change carbon dioxide into complex sugars using sunlight and water: photosynthesis. Corn plants require heat (between 50 and 86 °F) in order to grow, develop and reach vegetative and reproductive milestones on their way to physiological maturity.

Growing degree days are a measure of this accumulated heat and associated corn growth and development during the growing season. Chapter 2 in the Illinois Agronomy Handbook has a good description of daily GDD calculations and the number of GDD that typically must accumulate in order for corn to reach specific growth stages.

The corn grown in a planting date trial at the NWIARDC has been slowly plugging along during this unseasonably cool summer (Table). Although growing degree days have been accumulating, they have been doing so very slowly, particularly in August (Figure).

When contemplating how this unseasonably cool weather may delay corn maturity, we can compare the average daily GDD accumulation during the second half of June (16 through 30) to the first half of August (1 through 15). The average daily GDD accumulation during the second half of June (25.3) accumulated 6 more units on average per day than the average daily GDD accumulation during the first half of August (19.3)!

Let's use one of the 109 day corn hybrids planted at the NWIARDC as an example. This hybrid is estimated to require approximately 2,630 GDDs to reach physiological maturity. If the season continues with cooler temperatures similar to the period between August 1 and 15, corn planted April 9, May 5, May 19, and May 30 would require an additional 31, 37, 44, and 56 days to mature, respectively. However, if the season gets warmer, similar to the period between June 16 and 30, corn planted on April 9, May 5, May 19, and May 30 would require only an additional 24, 28, 34 and 43 days to mature, respectively.

Yield estimates.

Table. Planting date, days since planting, total growing degree day (GDD) accumulation on August 15 and yield estimates in a corn planting day trial at the NWIARDC in 2014

Planting date

Days since planting

Date at silking (R1)

Total GDD accumulated

Yield Estimate (bu/A)

April 9


July 7



May 5


July 11



May 19


July 21



May 30


July 31



NWIARDC Research Agronomist Brian Mansfield recently estimated yield in this planting date trial. Cool weather and adequate soil moisture did allow for very favorable weather during pollination for each of the planting dates in the corn planting date trial.

Differences among the number of plants and kernels per row influenced yield estimates. The corn planted April 9 was sown into dry soils which were soon followed by a long period of heavy rainfalls and cooler weather. This lowered both corn stands and yield potential in those plots. While corn planted on May 5 tends to have fuller ears with more kernels per row, and estimated higher yields than in corn planted on May 19 or 30. Only time will tell how the season will progress and how close these estimates will come to the actual grain yields.

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