Corn Growth and Development

LATE-SEASON CORN DEVELOPMENT


Did you visit Wyffels' web pages on Early Corn Development and Mid-Season Corn Development where we looked at corn plant development from planting through tasseling?

Here, we continue the three-part series with a look at the critical reproductive stages that must occur to put grain in the bin. We'll begin with a review of the growth stages.

Corn Growth Stages
There are a number of ways by which a particular stage of corn plant development may be labeled. One of the most helpful identification systems is the one Iowa State University has described in its publication, “How a Corn Plant Develops, Special Report No. 48.”

Vegetative Stages*   Reproductive Stages
VE  Emergence  R1  Silking
V1  First Leaf  R2  Blister
V2  Second Leaf  R3  Milk
V3  Third Leaf  R4  Dough
V(n)  (n)th Leaf  R5  Dent
VT  Tasseling  R6  Black Layer

* Each vegetative leaf stage in this system is defined according to the uppermost leaf whose leaf collar is visible.

The leaf collar is the yellow flared band appearing at the point where the leaf is attached to the sheath. The drawing above illustrates a corn plant in the third-leaf, or V-3 stage. Notice that while six leaves are visible, only the three lower mature leaves that have a definite leaf collar are counted when identifying the stage of development.

NOTE: Keep in mind that not all plants in a field will reach the same stage at the same time. A field of corn is defined to be at a specific stage when 50 percent or more of the plants in the field are in or beyond that stage.

R1 - Silking
The production of silks is one of the most critical and sensitive steps in corn development. One receptive, viable silk must be produced for each kernel you will put in the bin. Corn is said to be in the R-1 (silking) stage as soon as one silk appears from the tip of the husk.

The silk of the kernel is the pathway for the male component (pollen) to travel to the female component (ovule) and achieve fertilization. Every single kernel produced must be successfully fertilized. Silking actually begins during the VT (tasseling) stage. Under normal conditions, it takes 6 to 7 days for silks to extend from the ovule to the tip of the husk.

The silk has the highest water content of all the parts of a corn plant. This is one reason why the period from V-15 to VT is so sensitive to moisture stress (see growth stage chart at top of page). The plant must have adequate water to produce silks. When plants are under drought stress, silk formation can be delayed.

Unfortunately, these stresses rarely delay tassel emergence and pollen shed. Severe heat and/or drought stress can cause silks to appear after the pollen shed. Pollen will fall for 6-14 days, and in severe droughts the silks can miss this window.

The Pollination Process
A normal tassel will produce from 2,000 to 5,000 grains of pollen per each silk to be fertilized. The tassel itself is a very effective organism for fertilizing silks. A healthy tassel will only shed pollen during opportune periods. Most pollen shed occurs in the mid to late morning. In early morning, the tassel can be covered with dew which can cause the pollen to stick and not fall to the silks. Tassels will not shed pollen in the heat of mid-afternoon either, because heat can reduce the viability of the pollen. Once released by the tassel, pollen can be moved to the silks by either wind, insects, or gravity.

Because the tassel is very effective at timing its shedding periods for maximum effectiveness, and it has 2,000 to 5,000 chances to fertilize each silk, the tassel rarely fails to do its job. Almost all pollination failures observed in the field come from a silking problem. The old belief that hot weather "cooks" the pollen and causes a fertilization failure seldom happens.

Silks must emerge and remain receptive during the pollen shedding period for fertilization to occur. Hot dry winds during the silking period can dry and harden the silks so they cannot accept pollen. If these conditions don't moderate and allow viable silk to extend during pollen shed, fertilization may not occur.

Pollen can attach at any point along the length of a viable silk and achieve fertilization. The concept that pollen can only attach at the tip of the silk is incorrect. After a pollen grain attaches to a receptive silk, fertilization of the kernel occurs within 24 hours. The silks will detach 24 to 72 hours from fertilization. The silks have now completed their only function and will begin to turn brown and wither.

Rootworm beetles at pollination. Once the silks have detached, rootworm beetle feeding on the silks is not a threat to your yields. When scouting fields for rootworm beetle feeding, husk several ears carefully and shake them to see how many silks are detached. If 80 percent or more are detached, it is not necessary to treat for rootworm beetles.

The entire period from V-15 to R-1 is very sensitive to stress. Severe stress during this window can reduce final yields from 3-8% per day. With a yield goal of 180 bushels, this can cost you 5-14 bushels per day of stress.
R2 - Blister Stage
Under normal conditions, corn will be in the blister stage for 10-14 days after silking. At this point the kernel and the fluid inside are almost clear. The embryo inside the kernel is developing. The radical, coleoptile, and first embryonic leaf have already formed, so the young corn plant that would appear if this kernel were planted is present.

The kernel is now visible and resembles a small bubble or blister. At the end of R-2, the cob will be at or very near its full diameter. All the kernels that will be pollinated have done so, but from R-2 through R-4 (dough stage), the plant will retain the ability to abort pollinated kernels if stress occurs.

During the R-2 stage, starch will begin to accumulate in the endosperm. This is the beginning of kernel fill. Nitrogen and phosphorus are being used in large quantities by the plant. Kernel moisture is around 85% and will decrease from now through R-6 (black layer).
R3 - Milk Stage
Corn will usually reach the milk stage about 20 days after silking. The almost clear kernel of the R-2 stage is now turning yellow. The rapid starch build-up has changed the fluid inside the kernel to a milky white. The total number of cells contained in the kernel has been determined, and kernel growth will now come from enlargement of existing cells.

Stress in the R-3 stage may still result in kernel abortion. The kernels at the tip of the ear will abort first. If the stress continues, the abortion will proceed down from the tip to the butt of the ear.

R4 - Dough Stage
Once corn has reached the dough stage, it becomes less sensitive to stresses. The starch in the kernel is now thick and resembles biscuit dough. This will generally occur around 26 days after silking.

The plant can still abort kernels if stress occurs, but severe stress should reduce final yields by only 1-2 percent per day. Common stresses at this time of year are nutrient deficiencies, reduced sunlight, plant diseases, and drought. Long periods of gray, cloudy weather from R-2 to R-4 can shorten ears. Remember, the plant must have sunlight to produce the carbohydrates that power it; and without sun it will take the needed energy from its own tip kernels.

R5 - Dent
At about 38 days after silking, all or most of the kernels have dented. This is the R-5 stage of growth. At the beginning of R-5, the kernel will have about 55% moisture.

Once into R-5, the plant has lost the ability to abort kernels. Stresses now will reduce kernel weight but not the kernel count.

Anything that can cause early plant death will result in reduced yields and slow drydown. An early killing frost will cause light kernel weights and very high harvest moistures. Stalk rots can have the same effect by killing the plant prior to maturity.

Cool weather during the R-5 stage can cause a unique problem. A week of 50º average temperatures can trigger premature black layer formation. This can cause a serious reduction in yields, but the plant does not die like you would see from frost or disease. This condition is rarely noticed until harvest when yields are down and there does not appear to be another cause. Many times the hybrid is blamed when, actually, the cool weather was the culprit.
R6 - Black Layer
At maturity, the kernel forms a black layer where it attaches to the cob. At this point all the dry matter that each kernel will accumulate is in place. The plant should remain alive for some time after black layer formation. The black layer acts as a gate that stops movement of dry matter into the kernel, but allows water to leave.

As long as the plant remains alive, the moisture content of the kernel can continue to drop. If the plant dies at black layer, final yield will not be affected, but drydown can be very slow. A dead plant will not draw water from the kernel like a living plant. Once the plant dies, the only way for water to leave the kernel is through the hull. This is a very slow process and can increase drying expenses. With modern hybrids, staygreen— the ability of the plant to stay alive after black layer— is a very desirable characteristic. Many times, you will find yourself harvesting your driest grain from the greenest plants, because the plant has remained alive long enough to draw moisture out of the kernel.

Once black layer and drydown have been achieved, the plant has done its work. Now it is time to harvest and collect your pay-off.
Summary
To raise corn profitably, it is important to understand how the corn plant functions and gear your management to assist it along the way. Always remember to look at the work from the plant's point of view when making management decisions, and you will have more successful harvests.

* The Iowa State University Leaf Collar Staging Method is used in this series of articles to identify the stages of corn plant development.