"Nosing back" and "zipper ears" in corn
Another ear development problem involving poor kernel set that’s been getting more attention recently is “zippering” in which corn ears exhibit missing kernel rows (or parts of rows) often on the side of the cob away from the stalk that gives sort of a zippering look on the ears”. The zippering often extends most of the cob’s length. Zippering is often associated with a curvature of the cob, to such an extent that zipper ears are sometimes referred to as "banana ears". This ear deformation is caused by 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.
Zippering is due to kernels that are poorly developed and/or ovules that have aborted and/or not pollinated along some length of the ear. Affected ears are often associated with corn plants which have experienced drought stress during early grain fill; cobs associated with the zippering are usually smaller than normal and poor tip fill is often present. Recent OSU studies indicate that some hybrids are much more susceptible to zippering than others and that zippering among such hybrids is more pronounced at higher seeding rates. In studies in which corn plants have been subjected to severe defoliation during the late silk and early blister stages, we’ve observed the resulting ears to show zippering, which suggests that a sudden reduction in photosynthate supply may be a factor. The zippering did not occur when plants were subject to similar defoliation at the milk or dough kernel development stage.