Corn has two distinct phases of growth: vegetative and reproductive.

Vegetative Growth Stages in Corn

Vegetative stages are identified by the number of collars present on the plant. The leaf collar method is generally used for staging vegetative (V) development of corn. When corn seedlings emerge from the soil and no leaf collars have formed, plants are in the VE stage. When the plant has one visible leaf collar, it is in the V1 stage. The leaf collar is a light-colored band located at the base of an exposed leaf blade, near the spot where the leaf blade meets the stem of the plant. Leaves within the whorl, not fully expanded and with no visible leaf collar, are not included in the staging. For example, a plant with three collars would be called a V3 plant; however, there may be five to six leaves showing on the plant (Figure 1). Corn plants generally develop up to the V18 stage before reaching maximum height at tassel emergence (VT) and transitioning into the reproductive (R) stages of growth.

Beginning at about V6, the lowest leaves may fall from the plant and dissection of the lower stalk may be necessary to accurately stage the plant. To stage older plants, dig up the plant and split the stalk down into the root ball. Find the triangular “woody” base of the stalk and locate the first internode above the base. The woody, horizontal node is the point of attachment for the fifth leaf or collar. For example, if you can count five visible leaf collars above this point, the corn plant is in the V10 growth stage.

VE	Can occur four to five days after planting under ideal conditions, but up to two weeks or longer under cool or dry conditions.
V1-V5	At V1, round-tipped leaf on first collar appears, nodal roots elongate. By V2, plant is 2 to 4 inches tall and relies on the energy in the seed. V3 begins two to four weeks after VE, and plant switches from kernel reserves to photosynthesis and nodal roots begin to take over. Around V4, broadleaf weeds should be controlled to avoid loss. By V5, the number of potential leaf and ear shoots are determined. Plant is 8 to 12 inches tall and growing point remains below soil surface.
V6-V8	Beginning four to six weeks after VE, the growing point grows above the soil surface, increasing susceptibility to hail, frost, or wind damage. The nodal root system is dominant. At V7, rapid growth phase and stem elongation begin. Number of kernel rows is determined and potential kernels per row begins and continues through V15/16. By V8, the plant reaches 60 cm (24 inches) tall.
V9-V11	Around six to eight weeks after VE, corn begins steady and rapid period of growth and dry matter accumulation. At V9, tassel is developing rapidly, but is not yet visible. New leaves appear every two to three days and ear shoots are developing.
V12-Vnth	By V12, the plant is about 4 feet tall or more. Nutrients and water are in high demand to meet growth needs. All leaves are full size and roughly half are exposed to sunlight. Brace roots are developing and the potential number of kernels per ear and size of the ear are still being determined. Insect and hail injury can reduce the number of kernels that develop. The plant is about two weeks away from silking at V15. The tassel is near full size, but not visible. Moisture and nutrient deficiencies at this time can reduce the number of potential kernels per row resulting in shorter ears and lower yield potential.
VT	Beginning around nine to ten weeks after emergence, corn enters a critical period where successful pollination is required to convert potential kernels into viable, developing kernels. The plant has reached full size. Tassels are fully visible, and silks will emerge in two to three days. Pollen shed begins and continues for one to two weeks. Hail can be very damaging at this stage.

Reproductive Growth Stages in Corn

Corn plants enter reproductive growth after completing tassel emergence. Reproductive growth stages are determined by kernel development and not plant collars.

Corn Growth Stages

R1 Silking	Silking is one of the most critical stages in determining yield potential. A plant has reached R1 when the silks are visible. For a field to be in the R1 stage, the average silking date is used. The average silking date is when 50% of the plants have started to silk. Pollination begins at the base and proceeds toward the tip. Physiological maturity can be estimated by adding 50 to 55 days to the silking date.
R2 Blister	About 12 days after silking, silks darken and dry out. Kernels are white and form a small blister containing clear fluid. Each kernel develops an embryo. Kernels contain 85% moisture. Stress (especially drought) at this stage can reduce yield potential by causing kernel abortion.
R3 Milk	About 20 days after silking, kernels are yellow and clear fluid turns milky white as starch accumulates. Kernels contain 80% moisture. The effects of stress are not as severe after this stage, but can still result in shallow kernels, stalk cannibalization, or lodging.
R4 Dough	About 26 days after silking, the starchy liquid inside the kernels has a dough-like consistency. Kernels contain about 70% moisture, begin to dent at the top, and have accumulated close to 50% of their maximum dry weight. Stress can produce unfilled or shallow kernels and “chaffy” ears.
R5 Dent	About 38 days after silking, nearly all kernels are dented and contain about 55% moisture. Cob has distinct color: white, pink or red. Silage harvest begins sometime during this stage, depending on desired whole plant moisture.
R6 Black Layer	About 60 days after silking, physiological maturity is reached, and kernels have attained maximum dry weight at 30 to 35% moisture. Total yield is determined, and frost has no impact on yield.

Crop Heat Units (CHU)

In Canada, Crop Heat Units (CHU) are used to describe how heat accumulation is calculated each season. This is described below.

Always refer to each seed company on how hybrids are rated as relating to maturity and CHU accumulation. Some seed companies do not include the 150 CHU needed from germination to emergence in their maturity rating.

Our seed guide refers to the CHU accumulation needed to hit Black Layer (physiological maturity) for a corn hybrid in grain corn production.

For silage or grazing purposes, you can generally select a hybrid that is 150-200 CHU higher maturity rating than your normal accumulation in your growing area. This is due to the fact that silage corn does not have to reach full physiological maturity.

CHU is based on air temperature. CHU is calculated for each day and is accumulated when air temperature is above 4.4oC and below 30oC. Air temperature must reach 12.8oC for three consecutive days before CHUs begin accumulating. When -2oC is reached, CHU accumulation ends.¹

Heat unit maps are generated by Agriculture and Agri-Food Canada and can be found here.²