It has been a tough spring. In South Central Minnesota, most of the corn and soybeans planted by Memorial Day have now emerged. Cool and wet soil conditions have held back development until recent warm weather put us back on track. For corn planted by May 20th we are probably only one to two weeks behind normal. Growing Degree Days show that we are only 8% behind the long term average for heat units. Everything depends on what happens weather-wise from here on out. Corn is an amazing plant that can make up ground in a hurry with good weather conditions.
The number of harvestable kernels per ear is an important contributor to the 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.
By the time the V5 or V6 stages of development (fie to six visible leaf collars), the growing point of the corn plant finishes the task of initiating new leaf growing points and completes its developmental responsibilities by initiating the tassel of the plant. At about the same time that the tassel is initiated, the uppermost harvestable (and final) ear is also initiated. This uppermost ear is normally located at the 12th to 14th stalk node, corresponding to the 12th to 14th leaf of the plant.
Ear Size Determination
Row number and kernel number per row are two of several yield components in corn. Typically, from 750 to 1000 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). Kernel row number determination of the uppermost ear begins shortly after the ear shoot is initiated (V5 toV6) and is thought to be complete as early as V8.
Kernel rows first initiate as "ridges" of cells that eventually differentiate into pairs of rows. Thus, row numbers 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 that about the 12-leaf stage. 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.
Potential Kernel Numbers
The potential number of kernels per row is complete by at least V15 and maybe as early as V12. Kernel number (ear length) is strongly affected by environmental stresses. This means that potential ear length will vary dramatically from year to yare as growing conditions vary. Severe stress can greatly reduce potential kernel number per row. Conversely, excellent growing conditions can encourage unusually high potential kernel numbers.
Severe stress from V5 to V12 (current growth stages) 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.
The critical growth stages of corn for water use will be at tassel until grain is fully formed. The highest water use by corn will be during July and August. For soybeans, critical growth stages for water availability are during bloom and fruit set. Shortage in moisture supply during these growth stages will cause yield reduction.