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.

In southern Wisconsin, each corn plant typically develops about 20-21 total leaves, silks about 65 days after emergence, and matures about 120 days after emergence. All normal corn plants follow this same general pattern of development, but the specific time interval between stages and total leaf numbers developed may vary between different hybrids, seasons, planting dates and locations. An early maturing hybrid may develop fewer leaves or progress through different stages at a faster rate than indicated here. A late maturing hybrid may develop more leaves or progress more slowly than indicated here.

Numerous systems are used to stage growth and development of corn. These systems use morphological indicators including plant height, total leaf collars, total leaves that have appeared, or the uppermost leaf tip pointing below a horizontal line. Three of the most common systems are:

Iowa State University System
National Crop Insurance Association (NCIA) System
Pesticide Labels

This article will concentrate on the Iowa State System for staging corn development (Table 1).

Each leaf stage is defined according to the uppermost leaf whose leaf collar is visible. The first part of the leaf collar that is visible is the back, which appears as a discolored line between the leaf blade and leaf sheath.

The characteristic oval-shaped first leaf is a reference point for counting upward to the top visible leaf collar. Beginning at about V6, increasing stalk and nodal root growth combine to tear the small lowest leaves from the plant.

To determine the leaf stage after lower leaf loss, split the lower stalk lengthwise and inspect for internode elongation. The first node above the first elongated stalk internode generally is the fifth leaf node. The internode usually is about one centimeter in length. This fifth leaf node may be used as a replacement reference point for counting to the top leaf collar.

All plants in a given field will not be in the same stage at the same time. Each specific V or R stage is defined only when 50% or more of the plants in the field are in or beyond that stage.

Table 1. Iowa State University staging system for corn growth and development.
Vegetative Stages	Reproductive Stages
VE emergence	R1 silking
V1 first leaf collar	R2 blister
V2 second leaf collar	R3 milk
V3 third leaf collar	R4 dough
V(n) nth leaf collar	R5 dent
VT tasseling	R6 maturity

The rate of plant development for any hybrid is directly related to temperature, so the length of time between the different stages will vary as the temperature varies, both between and within growing seasons. Environmental stress such as nutrient or moisture deficiencies may lengthen the time between vegetative stages but shorten the time between reproductive stages. The number of kernels that develop, final kernel size, rate of increase in kernel weight, and length of reproductive growth period will vary between different hybrids and environmental conditions.

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.

GDU accumulation varies during the growing season (Table 2). Peak daily rates occur during late July and early August. Typically about 2600 GDU accumulate between May 1 and September 30.

SSignificant year effects are also observed (Table 3). Some years are "cooler" or "warmer" than others which greatly affects corn development.

Corn plants increase in weight slowly early in the growing season. But as more leaves are exposed to sunlight, the rate of dry matter accumulation gradually increases. Cell division in the leaves occurs at the growing point of the stem. Leaves enlarge, become green, and increase in dry weight as they emerge from the whorl and are exposed to light. The leaves of the plant are produced first, followed by the leaf sheaths, stalk, husks, ear shank, silks, cob and finally grain.

Table 2. Average growing degree unit accumulation for corn planted on May 1 in the southern production zone of Wisconsin.
Date	Rate	Accumulative
	GDU per day	total GDU from May 1
May 1	--	0
May 11	10	100
June 15	14	600
June 30	20	900
July 15	20	1200
July 31	22	1550
August 15	23	1900
August 31	19	2200
September 15	13	2400
September 30	13	2600
derived from Mitchell and Larsen, 1981

Table 3. Range in growing degree unit accumulation for corn planted on May 1 in southern Wisconsin.
Date	Average	Range
June 30	900	800-1000
July 31	1550	1450-1650
August 31	2200	2100-2300
September 30	2600	2500-2700
derived from Mitchell and Larsen, 1981 Range = expectation that any particular year is greater than or less than normal once in five years.

If we know how many GDU are required for various crop developmental stages, we can predict and time management operations. Table 4 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, WI. Using 30 years of weather data, I calculated the calendar date for several developmental benchmarks of a 100-day hybrid. 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.

Many factors affect corn growth and development, especially early in the growing season.

Conservation tillage: more than 75% residue, add 30-60 GDU (Swan et al., 1977; Imholte & Carter, 1987)
Soil texture: fine = add 30-60 GDU; coarse = subtract 30-60 GDU
Planting date: < April 25 = add 10-25 GDU; > May 15 subtract 50-70 GDU
Seed-zone soil moisture: below optimum, add 30 GDU (Schneider and Gupta, 1985)
Seed-bed condition: soil crusting or massive clods add 30 GDU (Schneider and Gupta, 1985)
Seeding depth: add 15 GDU for each inch below 2 inches (Hunter and Kannenberg, 1972)
Severe drought or heat stress
Hybrid differences for development

Table 4. Growth and development of a 100-day (MN RM) corn hybrid planted on May 1 at Arlington, WI.
Corn Growth Stage	GDU required to reach growth stage	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
sources: Neild and Seeley, 1977; Swan et al., 1987; Schneider and Gupta, 1985; Imholte and Carter, 1987; Bauer and Carter, 1984, Crookston et al., 1982; Mitchell and Larsen, 1981; Bland, 1997

By V10, enough leaves are exposed to sunlight so the rate of dry matter accumulation is rapid. Under favorable conditions, this rapid rate of dry matter accumulation in above-ground plant parts will continue at a nearly constant daily rate until near maturity. Highest yields will be obtained only where environmental conditions are favorable at all stages of growth.

Unfavorable conditions in early growth stages may limit the size of the leaves (the photosynthetic factory). In later stages, unfavorable conditions may reduce the number of silks produced, result in poor pollination of the ovules and restrict the number of kernels that develop; or growth may stop prematurely and restrict the size of the kernels produced.

The yield components of corn consist of the number of ears per unit area, the kernel number per ear (row number and kernels per row), and kernel weight. Yield components are determined at different times during the life cycle of the corn plant (Table 5). Each component has a maximum genetic potential followed by attrition to an actual level. The attrition of yield components is caused by environmental and management factors. The actual level can still be lowered mechanical harvest losses or through poor management.

The impact of environmental temperature effects and stresses on yield varies with the development of the corn plant (Table 6). For example, flooding while the growing point is below ground (prior to V6) can be devastating on yield, but frost or hail during this time will have little or no effect. Other management practices and environmental factors such as fertility, insects, diseases, weeds, pesticides can affect corn growth and development.

Table 5. Corn growth and development stages when yield components are at maximum potential and actually determined.
Stage	GDU required to reach growth stage	Yield components
Stage	GDU required to reach growth stage	Potential	Actual
VE	125	Ears/area	-----
V6	470	Kernel rows/ear	"Factory"
V12	815	-----	Kernel rows/ear
V18	1160	Kernels/row	-----
R1	1250	Kernel weight	Kernel number Ears/area
R6	2350	-----	Kernel weight

Table 6. Impact on grain yield of various factors occurring during corn development.
Factor	Corn development stage
Factor	VE	V6	V12	V18	R1	R6
	Percent yield impact
Frost (< 28 F)	0	100	100	100	100	0
Hail (max)	0	53	81	100	100	0
Drought/Heat (%/day)	--	--	3	4	7	0
Flooding (<48 h)	severe	0	0	0	0	0