Understanding how corn ears respond to stress can help determine what stress was present, when the stress occurred, and how to mitigate the stress in the future.
Environmental stresses during any of four ear development stages significantly affect the number and weight of harvestable kernels. The four critical stages are: (1) when the corn ear is setting the maximum number of kernel rows around the ear (approximately V7), (2) when the ear is establishing the maximum number of ovules along the length of the ear (just before pollination), (3) when the maximum number of ovules are pollinated to form kernels (at pollination), and (4) when the ear sets maximum kernel size during the latter portion of grain fill (approximately R3 to R5).
The first yield component determined by a corn plant is the development of ovule rows on the ear, which begins at about the V7 growth stage. Each ovule row will eventually divide to produce a pair of rows. This division explains why a corn ear always has an even number of kernel rows around. If a particular parent line normally has 16 kernel rows around the ear, and the ear in question has less than the normal number, then some sort of stress was present at or just before this critical stage.
The number of kernels per row is established between ear initiation (V5 to V6) and pollination. Ovules near the base of the ear develop first and newer ovules will continue to form as development progresses toward the tip of the ear. If resources become limited, ovules will be sacrificed to allow the corn plant to support the remaining viable ovules. The amount, type, and duration of the stress can affect which ovules are aborted. If the stress is a long-term general stress, ovules near the tip of the ear are sacrificed, resulting in viable ovules only at the base of the developing ear. Ovules near the base of the ear are more likely to remain viable because they are further developed and closer to the source of nutrient supply. If the environmental stress is very short but intense, the ovules that are lost may be anywhere along the corn ear.
The first three weeks of corn reproduction (silking through the milk stage) are devoted to kernel set. Kernel formation begins with successful fertilization of mature ovules. Synchronization of pollen shed and silk emergence is essential. The most critical time for corn yield determination is the two week period before and after silking. Yield reduction during silking and pollination after four consecutive days of severe drought stress can be as high as 40 to 50%. Drought stress delays silk emergence and accelerates pollen shed, thereby narrowing the window of opportunity for successful pollination.
Because silks are made up of more than 90% water, the amount of moisture available for silk growth substantially influences when silks emerge. More drought tolerant hybrids are often able to “push silks” under conditions of lower available water.
Even after successful pollination, kernel set is not certain. Post-pollination stress can cause the ear to sacrifice the tip kernels in favor of kernels at the butt of the ear. Depending upon the severity of the stress, tip kernel die-back will continue until the point at which the corn plant has the ability to supply adequate water and nutrients to support growth of the remaining kernels.
Corn plants use the latter five weeks of reproduction to deposit starch layers into the kernels. Final seed weight is reached at kernel black layer. If the tip portion of the cob shows either very small kernels or kernel die-back, stress occurred sometime early in the grain-filling process. If tip kernels did not abort but overall kernel depth or test weight is decreased, the stress occurred only during the very latter part of grain fill.
Kernel formation or the lack thereof is an indicator of the time of stress occurrence — whether it occurred before or during pollination or during grain fill. Additionally, the size, placement, and amount of corn kernel set documents when the ear was subjected to environmental stresses and the severity of these stresses. Understanding stresses that limited yield potential can provide a starting point for developing management practices to mitigate these stresses in the future. This could lead to more complete pollination/grain fill and subsequently higher grain yields.