The process of estimating yield potential in corn fields prior to grain harvest includes an assessment of the success of "kernel set" on the ears. Poor tip fill on ears, resulting from a combination of pollination failure and kernel abortion, is not uncommon in fields where severe crop stress has occurred during pollination or in the early weeks following pollination.

The absence of kernels on the tips of ears as a result of stress "makes sense" from the standpoint that most agronomists will tell you that the tip silks are the last to emerge from the husk during pollination and, thus, are usually the last to receive pollen if pollen is still available. If pollen is no longer available, then the tip ovules are never fertilized (in a sexual context) and no kernels develop.

If pollen is available and the tip ovules are fertilized, then the resulting tip kernels are younger relative to the others on the cob and so are more vulnerable to abortion if severe photosynthetic stress occurs early in the grain fill process that greatly limits the availability of photosynthate to the developing kernels. The causes of severe photosynthetic stress are varied and include drought stress, heat stress, severe defoliation (e.g., hail damage), and nutrient deficiencies (e.g., nitrogen).

The "zipper" pattern of poor kernel setA less common pattern of poor kernel set is one that is often described as the "zipper" pattern wherein 1 or more entire rows of kernels along one side of a cob are absent due to some combination of pollination failure and kernel abortion. A subsequent symptom that often develops on such "zipper ears" is a noticeable curvature of the cob, sometimes to the extent that folks describe it as a "banana ear". These curved ears are a consequence of the absence of kernels on one side of the cob coupled with the continued development of kernels on the other side that "force" the cob to bend or curve.

While most recognize that the absence of kernels down one side of the ear is the result of severe photosynthetic stress, it is less obvious why the pollination failure or kernel abortion occurred along that side of the ear rather than being localized at the tip of the ear. Silk development typically begins with the basal ovules at the butt of the ear and progresses up the ear which means that the first silks to emerge and be fertilized are primarily from the basal half of the ear. This acropetal progression of silk elongation is thought to occur uniformly from base to tip such that silk emergence occurs uniformly around the circumference of the ear at any particular position on the ear. If this is true, then what is the cause of the "zipper" pattern of poor kernel set?

I can only offer an opinion based on observations. Most of the time when I have discovered "zipper" ears, the side of the ear with the kernel set problem is the same side over which the silks draped during the pollen shed period. This leads me to speculate that perhaps the draping of the silks resulted in the underlying silks being "shaded" from initial contact with pollen with the result being those silks never coming into contact with pollen (ovules not fertilized) or those silks being pollinated later than the rest (delayed kernel development, more vulnerable to abortion under stress).