The corn crop in Wisconsin is entering one of the most critical stages in its development. Grain yield in corn is comprised of the components: ears per unit area, kernel number per ear consisting of kernel rows and kernels per row, and kernel weight. Each of these yield components is determined at different stages in the lifecycle of the plant. Yield components develop by initial cell division near the growing point and formation of numerous primordial tissues that eventually become ears or kernels. Often the number of these early structures is greater than what the plant is later capable of supporting. The plant "adjusts" yield components according to environmental and management stresses that take place during the growing season.

The plant has the "potential" to produce more ears and kernels than what is "actually" harvested. For example, the corn plant typically produces 6 to 10 ear shoots, but only one ear (at most two) actually develops. In some years, hybrids may produce 20 rows of kernels on an ear, but most of the time only 12 to 16 rows of kernels develop on the hybrids used in Wisconsin. If you were to examine the ear shoot at the V18 stage (just prior to tasseling) using a microscope, you could count 50 to 60 kernel ovules in a row. Multiplying the number of kernel ovules by the number of kernel rows indicates that 600 to 1200 kernels could potentially grow on an ear. Usually only 300 to 600 kernels develop on the ears of Wisconsin hybrids. Likewise, test weight (an indirect measure of kernel weight) is affected by environmental stresses. In the Wisconsin Corn Hybrid Performance Trials, test weight has ranged between 45 and 62 pounds per bushel. Usually we consider 56 pounds per bushel an average for corn.

The tasseling, silking, and pollination stages of corn development are extremely critical because the yield components of ear and kernel number can no longer be increased by the plant and the potential size of the kernel is being determined. Table 1 describes when yield components are at their greatest potential and when under normal conditions are actually determined and are not further affected under typical conditions. For example, the potential number of ears per unit area is largely determined by number of seeds planted, how many germinate, and eventually emerge. Attrition of plants through disease, unfurling underground, insects, mammal and bird damage, chemical damage, mechanical damage, and lodging all will decrease the actual number of ears that can be produced. The plant often can compensate for early losses by producing a second or third ear, but the capacity to compensate ear number is largely lost by R1 and from then on no new ears can be formed.

Likewise, kernel number is at its greatest potential slightly before R1, the actual number of kernels formed is determined by pollination of the kernel ovule. The yield component of kernel number is actually set by pollination and fertilization of the kernel ovule. If the ovule is not pollinated, the kernel cannot continue development and eventually dies. No new kernels form after the pollination phase is past.

The only yield component remaining with some flexibility is kernel weight. For the first 7 to 10 days after pollination of an individual kernel, cell division occurs in the endosperm. The potential number of cells that can accumulate starch is determined. At black layer formation (R6) no more material can be transported into the kernel and yield is determined.

Table 1. Corn growth and development stages when yield components are at maximum potential and actually determined (105 day hybrid).
	GDU required to reach growth stage	Yield components
Stage	GDU required to reach growth stage	Potential	Actual
VE (Emergence)	125	Ears/area	-----
V6 (six leaf collars)	470	Kernel rows/ear	"Factory"
V12	815	-----	Kernel rows/ear
V18	1160	Kernels/row	-----
R1 (Silking)	1250	Kernel weight	Kernel number Ears/area
R6 (Black layer)	2350	-----	Kernel weight