One of the neat things about agriculture is the changing of the seasons. The fact that plants grow uniformly within a field during a cropping season, ripen synchronously, and are harvested at about the same time is truly amazing. The synchrony of development between and within plants is largely taken for granted by farmers. Yet synchrony allows a farmer to spray herbicides, cultivate, irrigate, and most importantly harvest in a single operation.

Synchrony is a remarkable feat of coordination by the plants. Days or weeks may elapse between leaf, tiller, spikelet or floret initiation, and yet each plant flowers at about the same time and grain ripens simultaneously within the span of a few days. The observed synchrony of crop development cannot be achieved without the coordinated responses of a number of developmental processes to the environment. How this coordination between plant and environment is achieved is largely unknown.

Farmers often use benchmarks to track corn development and time various management operations. Commonly heard phrases include:

"Plant corn when oak leaves are as big as a squirrel's ear."
"Plant corn when you can drop your drawers and sit on the ground for two minutes under a full moon."
"Knee-high by the Fourth of July."
"Pollinated by August, dented by Labor Day."
"Corn will dry down right when frost doesn't occur until after the first full moon in September."
"Harvest when 8 of 10 ears float in a stock tank."

The accuracy of some of these statements may be debated, but the reality is often borne out of careful observations by farmers in their environment.

Under normal planting date situations, corn growth and development is largely temperature driven. To more accurately describe the environment, agronomists often use Growing Degree Units (GDU) to describe the amount of heat that drives the metabolic reactions for growth and development in the corn plant. The formula for calculating GDU is:

Growing Degree Units = ((Tmax + Tmin)/2) - Tbase

where Tmax = maximum daily temperature (upper limit = 86 F), Tmin = minimum daily temperature (lower limit = 50 F), Tbase = base or threshold temperature for corn growth = 50 F. The daily range of GDU that can accumulate is between 0 and 36 GDU.

If we know how many GDU are required for various crop developmental stages, we can predict and time management operations. Table 1 shows our current working model for "healthy" corn development in southern Wisconsin. It is for a 100-day hybrid planted on May 1 at Arlington Wisconsin. Using 30 years of weather data provided by Dr. Bill Bland (UW-Soils), I calculated the calendar date for several developmental benchmarks of a 100-day hybrid. I will use this model to report the development progress of actual fields at Arlington in 1997.

As of May 20, Arlington had received 109 GDU since May 1. With some warm weather forecasted, we should see fields planted around May 1, begin to emerge sometime later this week. So far, we are running slightly behind the 30-year average, but GDU accumulation can be made up relatively quickly during this time of year.

At Arlington, we made several planting dates in April. Corn planted on April 18, emerged on May 10. Corn planted April 24 emerged on May 17. The corn plants on these planting dates were able to develop during the relatively cool growing conditions in April.

Use the following table to chart and compare corn progress in your fields this season. It can also be used to schedule management operations such as herbicide application "windows," cultivation, and other field operations.

Table 1. Growth and development of a 100-day (MN RM) corn hybrid planted on May 1 at Arlington, WI.
		Predicted date of growth stage using 30-year Madison average (1961-90)
Corn Growth Stage	GDU required to reach growth stage	Average	Range (4 of 5 years)
VE: (Emergence)	125	May 14	May 12-16
V2: (2 Leaf Collars)	240	May 24	May 22-26
V4	355	June 2	May 31-June 4
V6	470	June 9	June 7-11
V8	585	June 16	June 14-18
V10	700	June 23	June 21-25
V12	815	June 29	June 27-July 1
R1: Silking	1250	July 20	July 18-22
R5: Dent	2130	September 5	September 3-7
R6: Black layer	2350	September 23	September 20-26
Kernel moisture at 25%	2500	October 11	October 7-15