Recently I’ve received reports of corn ears exhibiting “nosing back” symptoms. This condition, also referred to as “tip-back”, or “tipping back”, is not unusual and we encounter it every year although the magnitude of the problem varies greatly. Tip dieback is associated with unfertilized ovules and aborted kernels at the ear tip. Even in fields that receive timely rains, corn ears with unfilled tips may be common. No kernels may be evident on the last two or more inches of the ear tip. Several factors may cause this problem. The ovules at the tip of the ear are the last to be pollinated, and under stress conditions only a limited amount of pollen was available to germinate late emerging silks. Pollen shed was complete or nearly complete before the silks associated with the tip ovules emerge. As a result, no kernels form at the ear tip. Severe drought stress resulted in slow growth of the silks that prevented them from emerging in time to receive pollen. This was evident in many drought stressed fields in 2012. Uneven soil conditions and plant development within fields may have magnified this problem. Pollen feeding and silk clipping by corn rootworm beetles and Japanese beetles also contribute to pollination problems resulting in poorly filled ear tips.
Incomplete ear fill may also be related to kernel abortion. If plant nutrients (sugars and proteins) are limited during the early stages of kernel development, then kernels at the tip of the ear may abort. Kernels at the tip of the ear are the last to be pollinated and cannot compete as effectively for nutrients as kernels formed earlier. Heat and drought (as well as other stress conditions, such as nitrogen deficiency, hail, and foliar disease damage) may cause a shortage of nutrients that lead to kernel abortion. Periods of cloudy weather following pollination, or the mutual shading from high plant populations can also contribute to kernel abortion. Kernel abortion may be distinguished from poor pollination of tip kernels by color. Aborted kernels and ovules not fertilized will both appear dried up and shrunken; however aborted kernels often have a slight yellowish color.
Are unfilled ear tips a major cause for concern? Not always. In many cornfields this year, favorable growing conditions may have resulted in a larger number of potential kernels per row than normal. So even if corn ear tips are not filled completely, due to poor pollination or kernel abortion, yield potential may not be affected significantly, if at all, because the numbers of kernels per row may still be above normal. The presence of ears consistently filled to the tip may actually indicate that a higher plant population is needed to optimize yields.
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.