Published 1996 (Rev. June 2007)
URL: http://www.kingcorn.org/news/timeless/EarSize.html

Ear Size Determination in Corn

R.L. (Bob) Nielsen
Agronomy Dept., Purdue Univ.
West Lafayette, IN 47907-2054
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• Ear shoots are initiated at multiple stalk nodes very early in a corn plant’s development.
• Ear size determination of the uppermost (harvestable) ear begins by the time a corn plant has reached knee-high and finishes 10 to 14 days prior to silk emergence.

he number of harvestable kernels per ear is an important contributor to the grain yield potential of a corn plant. Severe plant stress during ear formation may limit the potential ear size, and thus grain yield potential, before pollination has even occurred. Optimum growing conditions set the stage for maximum ear size potential and exceptional grain yields at harvest time. The size of what will become the harvestable ear begins by the time a corn plant has reached knee-high and finishes 10 to 14 days prior to silk emergence.

Ear Shoot Development

An axillary meristem forms at each stalk node (behind the leaf sheath) beginning at the base of the stalk and continuing toward the top (acropetally for you wordsmith fans) except for the upper six to eight nodes of the plant. Each axillary meristem initiates husk leaves at the nodes of the ear shank and eventually an ear itself at the distal tip of the ear shank.

By about the V5 or V6 stages of development (five to six visible leaf collars), the growing point (apical meristem) of the corn plant finishes the task of initiating leaf primordia and completes its developmental responsibilities by initiating the tassel primordium of the plant. At about the same time that the tassel is initiated, the uppermost harvestable (and final) ear is also initiated (Lejeune and Bernier, 1996). This uppermost ear is normally located at the 12^th to 14^th stalk node, corresponding to the 12^th to 14^th leaf of the plant.

Careful removal of the leaves from a stalk, including leaf sheaths, at about growth stage V10 (ten visible leaf collars) will usually reveal 8 to 10 identifiable ear shoots. Each ear shoot originates at a stalk node, behind its respective leaf sheath. At growth stage V10, these tiny ear shoots primarily consist of husk leaf tissue. The developing ears themselves are only a fraction of an inch in length.

Initially, the ear shoots found at the lower stalk nodes are longer than the ones at the upper stalk nodes simply because the lower ones were created earlier. As time marches on, the upper one or two ear shoots assume priority over all the lower ones and ultimately become the harvestable ears. Development of the upper ears is favored over the lower ones partly because of hormonal “checks and balances” and partly because of the proximity of the upper ear to the actively photosynthesizing leaves of the upper canopy.

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Ear shoot at Node #4 on a V6 corn plant.	Ear shoot at Node #5 on a V6 corn plant.
Ear shoot at Node #6 on a V6 corn plant.	Ear shoots at Node #3 and #5 on a V9 corn plant.
Upper ear shoots and tassel dissected from a V9 corn plant.

Ear Size Determination

Row number and kernel number per row are two of several yield components in corn. Typically, from 750 to 1000 ovules (potential kernels) develop on each ear shoot. The number of kernel rows multiplied by the number of kernels per row determines total kernel number per ear. Actual (harvestable) kernel number per ear averages between 400 and 600. For a 16-row ear, one kernel per row is equal to about five bushels per acre (for average populations).

Like so many other processes in the corn plant, kernel row number determination on an ear proceeds in an acropetal fashion (from base to tip). Kernel row number determination of the uppermost ear begins shortly after the ear shoot is initiated (V5 to V6) and is thought to be complete as early as V8 (Strachan, 2004).

Kernel rows first initiate as “ridges” of cells that eventually differentiate into pairs of rows. Thus, row number on ears of corn is always even unless some sort of stress disrupts the developmental process. True row number is often difficult to visualize in tiny ears dissected from plants younger than about the 12-leaf stage.

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Tip of uppermost ear shoot dissected from a V10 corn plant.	Tip of uppermost ear shoot dissected from a V14 corn plant.
Tip of uppermost ear shoot dissected from an early R1 corn plant.

Row number is determined strongly by plant genetics and less so by environment. This means that row number for any given hybrid will be quite similar from year to year, regardless of growing conditions. Some exceptions to this include the effects of injury from the post-emergence application of certain sulfonylurea herbicides or nearly complete defoliation by hail damage prior to growth stage V8.

The potential number of kernels per row is complete by at least V15 and maybe as early as V12 (Strachan, 2004). Kernel number (ear length) is strongly affected by environmental stresses. This means that potential ear length will vary dramatically from year to year as growing conditions vary. Severe stress can greatly reduce potential kernel number per row. Conversely, excellent growing conditions can encourage unusually high potential kernel number.

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Ear with 14 rows of kernels.

Ear with 18 rows of kernels.

Final Comments

Because ear initiation does not occur until about V5, stress prior to this leaf stage has no direct bearing on ear size determination UNLESS that stress eventually results in a severely stunted or weakened plant. This is particularly true for stress events that damage only the above ground portion of young seedlings without damage to the plants’ growing point regions. Such damaged plants usually can recover well with little evidence of the damage some weeks down the road.

Severe stress from about V5 to V12 that severely limits photosynthesis can directly interfere with ear size determination and result in fewer kernel rows (less likely) or fewer kernels per row (more likely).  While such early stress can be important, recognize that severe stress that occurs shortly before to shortly after pollination has a far greater potential to reduce yield per day of stress.

Related References

Bonnett, O.T. 1966. Inflorescences of Maize, Wheat, Rye, Barley, and Oats: Their Initiation and Development. Bulletin 721. Univ. of Illinois, College of Ag., Agricultural Expt. Sta.

Lejeune, P. and G. Bernier. 1996. Effect of environment on the early steps of ear initiation in maze (Zea mays L.) Plant, Cell and Environment, 19:217-224.

Ritchie, S.W., J.J. Hanway, and G.O. Benson. 1992. How a Corn Plant Develops (SP-48). Iowa State Univ. [On-Line]. Available at http://www.extension.iastate.edu/pages/hancock/agriculture/corn/corn_develop/CornPlantStages.html. (URL verified 6/16/07).

Strachan, Stephen D. 2004. Corn Grain Yield in Relation to Stress During Ear Development. Pioneer, a DuPont Company. [On-Line]. Available at http://www.pioneer.com/growingpoint/agronomy/library_corn/ear_development.jsp [URL verified 6/16/07. Note that access to this article requires registration (free) to Pioneer’s Growing Point Web site.