How A Corn Plant Grows

Corn plants increase in weight slowly early in the growing season. However, as more leaves develop and are exposed to sunlight, the rate of dry matter accumulation gradually increases. The leaves of the plant are produced first, followed by the leaf sheaths, stalk, husks, ear shank, silks, cob, and finally the grain.
Highest yields will be obtained where environmental conditions are favourable at all stages of growth. Unfavourable conditions in early growth stages may affect the leaves (the photosynthetic factory) or root development, leading to decreased yield potential. In later stages, unfavourable 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.

Identifying Stages of Development

There are several methods of counting leaves. One method counts all leaves including any leaf tip that has emerged from the whorl at the top of the plant (leaf tip method). The second method only counts those leaves that are fully emerged and are arched over with the next leaf visible in the whorl but standing straight up (leaf over method).
The staging system used in this manual was developed by Iowa State University and involves dividing plant development into vegetative (V) and reproductive (R) stages. Subdivisions of the V stage starts with VE (emergence) and then numerically proceeds with V1, V2, V3, etc. where the number represents the number of fully emerged leaves (leaf collar is visible). The number of leaves will be specific to the hybrid. The last vegetative stage is the VT stage which is the tasseling stage. From there, the reproductive stage starts. The first stage will be R1 which is silking. From there, it proceeds as R2 - blister, R3 - milk, R4 - dough, R5 - dent, and R6 - physiological maturity.
The following table compares growth stages using the three leaf-counting methods.
TABLE 1: Comparative growth stages using three different leaf-counting methods
Collars Visible Leaf Tips Leaf Over
1 3 2
3 5-6 4
4-5 7-8 6
5-6 9-10 8
8 12 10
10 14-15 12
Note: It is important when reading pesticide labels or other information to know which leaf counting method is being referred to.

Vegetative Development (VE-VT)

Germination and Seedling Development (VE)
Germination begins with the seed absorbing soil moisture equal to about 30% of the weight of the seed and at a soil temperature of 10ºC. The first external sign of germination is the appearance of the "radicle" near the tip of the kernel. Under favorable conditions (warm, moist soil), this may occur within 2 or 3 days after planting. The first roots of the seedling develop from the radicle. Shortly after the appearance of the radicle, the shoot emerges and begins to grow towards the soil surface. The tip of the shoot is protected by a needle-like leaf called the "coleoptile". Because it is pointed and stiff, the coleoptile is able to penetrate through the soil without damage to the leaves inside. Once the coleoptile is exposed to light, it stops growing and splits open allowing the leaves to emerge. At emergence, the growing point of the plant and all the nodes of the stem are below the soil surface.
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  • Depth of planting influences the length of time from planting to emergence. Seedlings from deep-planted seeds have a greater depth of soil to penetrate. In addition, temperatures are colder at greater depths and growth is slower. Depth of planting determines the depth at which the primary roots (radicle and seminal roots) develop but does not influence the depth at which the nodal (permanent) roots develop.
  • Nutrients and food reserves within the seed generally supply the young plant adequately before emergence. Fertilizer placed in a band to the side and slightly below the seed may be contacted by the primary roots before the plant emerges from the soil. However, placement of too much fertilizer too near the seed can result in salt injury to the young plant.
  • If the seed is planted too deep, or if growing conditions are unfavourable, the coleoptile may open underground. This could also be caused by damage from insects, diseases or some herbicides. Because the exposed leaves are soft, they are unable to push through the soil. The result is reduced emergence and/or badly distorted plants.
  • Corn seedlings are very susceptible to disease organisms and insects during this early stage of growth, especially in cold soil. Application of a seed treatment containing suitable fungicides and insecticides is very important to ensure uniform emergence of healthy seedlings.
Early Stage Development: 1-4 Leaf Stage (V1-V4)
Leaves are produced at the growing point which remains below the soil surface for the first 3 or 4 weeks following emergence. All leaves and ear shoots that the plant will produce are starting to be formed at this stage. Once the seedling is established, the first set of nodal roots begin elongation from the first node. The nodal root system is the major supplier of water and nutrients.
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  • Since the root system is relatively small and the soil is cool, higher concentration of fertilizer nutrients stimulate early plant growth. However, the amount of nutrients required is relatively small. Fertilizer placed in a band where the primary roots (especially the radicle) will contact it will be effectively taken up at this stage. Roots are not attracted to this fertilizer band, so the fertilizer must be placed where the roots will be.
  • When the root system is well distributed in the soil, the plant begins to absorb greater amounts of nutrients. The placement of fertilizer relative to the developing plant is not as critical. Fertilizer applications in amounts adequate to supply those nutrients that are deficient in the soil will be beneficial.
  • A frost (light freeze) or hail may destroy the exposed leaves but will not damage the growing point, which is still below the soil surface; thus it usually results in very little reduction in the final yield.
Mid Vegetative Development: 5-8 Leaf Stage (V5-V8)
All the leaves are formed by the time the seedling reaches the 5 to 6-leaf stage (V5 to V6). The number of leaves produced will depend on the hybrid. Longer-season hybrids generally have more leaves than short-season varieties. Also, above-normal soil and air temperatures 2 or 3 weeks after emergence will result in the production of more leaves. Consequently, late-planted corn is often taller and leafier than early-planted corn, but total dry matter production is reduced.
At approximately the 6-leaf stage (V6), the growing point and tassel are above the soil surface and the stalk is beginning to elongate. The nodal root system is also the major supplier of water and nutrients to the plant. Also at the 6-leaf stage, some ear shoots (potential cobs) and tillers can be visible. Ears can be initiated at several nodes along the growing point (a node is the point at which the leaf sheath is attached to the stalk), but only the upper one or two ear shoots develop into harvestable cobs. Tillers (suckers) may never develop fully since development will vary with hybrid, plant density, fertility and other environmental conditions.
At the 8-leaf stage (V8), a critical period in the determination of the potential size of the ear starts. The number of ovules (potential kernels) that will produce silks is determined during this period. Since the upper limit of grain yield is set by the number of kernels initiated, losses at this time cannot be completely compensated for by good conditions later on.
By the time the corn plant has reached the 8-leaf stage (V8), the roots have reached the middle of the corn rows and penetrate to a depth of about 18 inches. Most of the root growth to this point has been by elongation, downward at a diagonal. As the plants become larger, the soil surface becomes filled with a mass of roots.
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  • Cultivation too near the plant will destroy some of the permanent root system.
  • Nutrient deficiencies at this stage seriously restrict leaf growth. Nitrogen fertilizer may be effectively side-dressed before the crop reaches 6 inches in height if the fertilizer is placed in moist soil and if serious injury to the root system, through root pruning, is avoided.
  • Removal of all of the unfurled leaves at this stage (by frost or hail) may result in 10-20 % reduction in final grain yield.
  • Spraying with 2,4-D and/or Banvel (dicamba) may cause the developing stalk to become temporarily brittle, and the stalks can be easily broken at the soil surface.
  • Adverse growing conditions, such as drought, nutrient deficiency, flooding, herbicide damage, or very high temperatures can limit the number of ovules formed.
Late Vegetable Development: 9-12 Leaf Stage (V9-V12)
At the 9-leaf stage (V9), many ear shoots are now visible. Generally, these ears are located 6 to 8 nodes below the tassel. Hybrids that produce more than one harvestable ear are termed prolific. At low plant densities, plants will often develop more harvestable ears. Also at this stage, the tassel is developing rapidly and the stalk continues to elongate.
During later vegetative development, the brace roots begin to emerge from the nodes above the soil surface and enter the soil. Brace roots serve a supportive role as well as absorb phosphorus and other nutrients from the soil.
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  • Moisture and nutrient deficiencies from this stage on will markedly influence the growth and development of the ears. Since the root system is extensive and the soil is warm, banded, high concentrations of nutrients are not essential. But the nutrients must be in moist soil to be absorbed. Thus, banded fertilizer is preferred since surface applied nutrients (especially phosphorus and potassium, which do not move any appreciable distance in most soils) may be in dry soil and unavailable to the plants.
  • The stalk is almost at full height. Yield losses from hail or leaf damage are greater than at any previous stage.
  • Plant-available moisture is critical at this stage because stress conditions can reduce seed set.
Tasseling Stage (VT)
Tasseling indicates the last vegetative stage of growth and it is where the tassel is completely emerged. Tasseling starts just prior to silking and the number of days between tasseling and silking will vary by hybrid and environmental conditions.
Pollen shed normally begins 2 to 3 days after the tassel has fully emerged from the whorl and continues for several days. Pollen shed is not a continuous process. Pollen will not release if the tassel is too wet or too dry. Thus, there is no danger of the pollen being washed away by rain. Pollen shed will begin again when environmental conditions are favorable or when more pollen has matured.
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  • Complete leaf removal (by hail) at this stage will result in essentially complete loss of grain yield because the tassel and all the leaves are exposed.

Reproductive and Grain Filling Stages (R1-R6) 

Silking Stage (R1)
Silking begins when any silks are visible outside of the husk. The silks are pollinated within 4 to 10 days after tassel emergence. When a pollen grain lands on a silk, the pollen germinates to produce a "pollen tube" which must grow down the entire length of the silk before fertilization can occur.
The period surrounding flowering is the most critical in determining yield. Under hot, dry conditions, the silks dry out and may not be able to support growth of the pollen tube. Moisture stress can also delay silking. In extreme drought, the silks may not emerge until after pollen shed has been completed. In either case, the result is incomplete seed set and reduced yields.
This period represents a peak in demand for all growth factors. The corn plant is devoting nearly all of its energies into producing an ear. If the plant is under too much stress, it is unable to supply enough materials to support all the developing kernels. Even kernels which have been fertilized may abort during the first few days following fertilization. Stress caused by drought, disease, insects, nutrient deficiency (especially nitrogen), over-population, or even several days of cloudy weather can reduce yields.
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  • The number of kernels that develop silks is being determined. Any nutrient or moisture deficiency or injury (hail or insects) may seriously reduce the number of kernels that develop.
  • Moisture stress or nutrient deficiencies usually increase in intensity from the top to the bottom of the plant and will delay silking more than tassel emergence and pollen shedding.
  • Complete leaf removal (by hail) at this stage will result in essentially complete loss of grain yield. Removal of half of the leaves would result in 25-30% yield loss.
Blister Stage (R2)
At this stage, the kernels are white on the outside and resemble a blister in shape. Starch is starting to accumulate and the kernels are beginning a period of rapid dry matter accumulation, lasting approximately 30 to 40 days. Also at this time, relocation of nutrients from vegetative to reproductive plant parts has started.
Milk Stage (R3)
At this stage, the kernel are yellow in color and the inner fluid is a milky white due to starch accumulation. Grain filling is still occurring and dry matter accumulation is still occurring at a rapid rate. Stress at this point can still affect yield by reducing the number of kernels that fill out or the final size and weight of the kernels. Kernel moisture is approximately 80%.
Dough Stage (R4)
The starch accumulation in the endosperm has caused the milky inner fluid to thicken to a paste. At this stage, kernels begin to dent or dry on top. Kernel moisture is approximately 70% and kernels have accumulated almost half their mature dry weight.
Dent Stage (R5)
When the kernels have dented, the exposed face of the upper end of the ear will reveal the smooth surface of the kernels. The milk line should be visible near the top of the kernels. This line represents the boundary between the solid, starchy portion of the kernel and the milky lower part. As the grain matures, the line moves downward towards the tip of the kernel. When the milk line is halfway down the kernel, the grain moisture content will be about 40%. The line will disappear at the tip of the kernel shortly before black layer formation, giving a good indication of when the crop is about to reach physiological maturity.
Maximum whole-plant dry weight is usually reached when the grain moisture content is about 45% and the kernels are well glazed. Maximum corn silage yields can be obtained by harvesting at this stage. This is roughly one to two weeks prior to the achievement of maximum grain dry weight. At this stage of the grain filling period, the increase in kernel weight results from the transfer of carbohydrates from the stalk to the grain. Therefore, silage feeding value remains constant over this period, although the moisture content decreases as the plant matures.
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  • Stress at this stage will reduce yield by reducing kernel weight, not kernel number. An early frost at the dent stage can also severely reduce yield potential and delay harvest.
Physiological Maturity (R6)
All kernels at this stage have attained their maximum dry weight. The achievement of maximum grain weight can be detected by observing the formation of a distinct black layer at the base of the kernel. Usually this occurs when the moisture content of the grain is 31 to 35%. However, black layer formation can occur at much higher moisture contents if kernel development is stopped prematurely by frost or disease.
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  • The grain is not ready for safe storage as the moisture content must be reduced to 13-15% moisture level. In order to reduce drying costs, it is advisable to let the crop partially dry in the field before harvesting. The rate of dry-down will depend on the hybrid and environmental conditions.