Corn Growth and Development

MID-SEASON CORN DEVELOPMENT


Did you visit Wyffels' web page on Early Corn Development where we looked at corn plant development from planting through emergence? Here, we continue the three-part series with a look at the growth and development that occurs in the plant from the third-leaf stage (V-3) to tasseling. We've included helpful information on timing your post-planting operations to minimize plant stress.

Stages of Development
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."

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.

At the V-6 Stage and Beyond
At the V-6 stage and beyond, it becomes a little more difficult to correctly identify the stage of development using the leaf-collar method because some of the first leaves and their collars may fall off. After lower leaf loss, you can accurately determine the growth stage by splitting the lower stalk or crown lengthwise to determine where these leaves were attached. Inside this crown section, you will see a series of root nodes as bands or lines across the width of the stalk. These root nodes indicate where the early leaves were attached to the stalk. At the very bottom tip of the stalk, four of these nodes are stacked one on top of another. Then you’ll notice a gap of one-fourth to one-half inch before the fifth node. This gap is your key identifier in locating the fifth node, the attachment point for the intact or missing V-5 leaf. From here you can count upward (V-6, V-7, V-8 and so on) to directly determine 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 V stage when 50 percent or more of the plants in the field are in or beyond that stage.

The Management Window, V-3 to V-5
The growth stage period from V-3 (plant has three collared leaves) to V-5 (plant has five collared leaves) is also known as the “management window.” During this time frame, your corn is the most tolerant to the stresses that can accompany post operations and has the least risk of yield reduction. Because the growing point of the plant is still below ground level, this is the optimal window of time to spray for weeds, cultivate, and sidedress nitrogen.

It is important to recognize that this optimal, minimal-risk window is brief. Under good growing conditions the plant will pass from one stage to the next in about three days. Your optimum management window (V-3 to V-5) will usually be only 6-7 days long. It is important to have your equipment ready to go so you can complete your post operations in this brief, six to seven day period.

Stress Impact, V-5 to V-12
From the V-6 stage onward the plant begins a series of critical yield determining decisions. Even the slightest stress at V-6 or later can reduce your final yields.

From growth stages V-7 to V-12 the final ear girth, i.e., the number of kernel rows, will be determined. Stress to the plant during this determination will result in fewer kernel rows—even though there may be no outward symptoms of plant stress. Hybrids that usually produce 18 to 20 row ears could end up producing 16 to 18 row ears. In some cases, you can actually see a reduction in the rows on an ear. An ear may have 20 rows around the butt, and only 18 halfway up.

At harvest, a reduction in girth will indicate some type of stress occurred in the V-7 to V-12 stage. A two-row reduction in ear girth as a result of natural or management-related stresses can reduce yields by as much as 23 bushels per acre, so it pays to be careful and perform all of your post-plant operations prior to V-6.

Herbicide stress. A common cause of reduced ear girth is late herbicide application. While most of the modern post-applied corn herbicides are relatively safe for corn, they are much safer if applied during the V-3 to V-5 stages of growth. It also pays to note that the weeds are smaller and easier to kill during these early stages.

Sulfonylurea herbicides like Accent, Accent Gold, Basis Gold, Exceed, and Hornet attack the growing points of the plant, and this can include the developing ear. Applying these products in the V-3 to V-5 stage helps to minimize the possibility of injury and enhances weed control. Many times these products are used late in the season to clean up "cosmetic" weed problems. Because of the potential for reduced ear girth, late applications of these compounds should be reserved for serious weed problems that are threatening your yields.

Applications of growth regulating herbicides like 2,4-D, Banvel, Clarity, and Marksman should be completed even earlier—before the V-3 stage. The potential for injury from these products grows substantially after V-3, and using 2,4-D on today’s hybrids presents serious risks at any stage after emergence.

Cultivation stress. Cultivation from V-7 to V-12 is another common cause of reduced ear girth. While cultivating small corn is tedious, later cultivation can prune roots and create enough stress to reduce girth. If the weather forces you to cultivate later, cultivate shallow to minimize root damage.

Ammonia application stress. Late sidedressing of ammonia will burn roots and can produce the same results. If the wet spring prevented your ammonia application, remember to apply it as soon after emergence as possible.

Weather-related stress. Drought stress in this stage also has the potential to reduce your yields. If you are irrigating, be sure your crop doesn't run low on moisture during this period. Hail during this time frame can also reduce girth.

Checking the effects of stress. Once the plant reaches the V-12 stage, the girth of the ear will be fixed. It is possible to determine the girth of the ear at this stage. Carefully extract the uppermost ear shoot from the plant and examine it. Using a magnifying glass you can count the rows of ovules. Generally, you will find from six to eleven rows, but don’t be disappointed. Each row of ovules will form two rows of kernels. (This is why you never see an odd number of rows around the ear.) Eleven rows at V-12, as an example, will be 22 rows of kernels at maturity.

Stress Impact, V-12 to V-T
Stress at this stage and later will no longer affect ear girth. During the growth period from V-12 to tasseling (V-T), the plant is determining the number of kernels per row, i.e., the length of the ear. The plant is growing rapidly and forming an ear at the same time. Large amounts of water and nutrients are required to fulfill these elevated requirements. If your field is producing ears with normal girth for the variety, but disappointing length, ask yourself what was going on from V-12 to V-T. During this period it is critical to maintain nitrogen availability to the plant, since deficiencies of nitrogen will have serious effects on final yields. Any type of heat or moisture stress in this period can shorten ears substantially.

From V-15 to tasseling is a very moisture-sensitive time in the life of the corn plant, because it encompasses the silking period. Silk has the highest water content of the entire plant, and a great deal of water is required to complete silk formation. There is also increased sensitivity to nutrient, heat, or hail stress at this time. If silk formation is hindered, pollination will be incomplete, and dramatic yield reductions can occur.
Summary

Stress occurring during specific stages of corn plant development impacts ear girth, ear length, and kernel development, even though there may be no outward plant symptoms. Remember that timing your post-planting management operations for completion prior to V-5 will minimize impact on the crop and maximize final yield potential.

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