“Tip Dieback” and “Zipper Ears” in Corn

During the past week, I’ve heard several comments about corn with unfilled tips. In some cases, no kernels are 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 certain conditions only a limited amount of pollen may be available to germinate late emerging silks. Pollen shed may be complete before the silks associated with the tip ovules emerge. As a result, no kernels form at the ear tip. Severe drought stress may result in slow growth of the silks that prevents them from emerging in time to receive pollen. Uneven 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 tips and ears.

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. Stress conditions, such as heat and moisture stress, 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 very high plant populations can also contribute to kernel abortion. Some agronomists and farmers characterize the kernel abortion that occurs at the end of the ear as “tip dieback” or “tip-back”, although poor pollination is also usually a factor affecting poor kernel set at the tip. 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.

Image Caption: Tip Dieback

Is the presence of barren 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 getting attention involves ears with missing kernel rows (usually on the side of the cob away from the stalk that give sort of a zippering look on the ears”).  The zippering often extends most of the cob’s length. The zippering is due to kernels that are poorly developed and/or ovules that have aborted and/or not pollinated. 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 usually present. Differences in the degree of zippering among hybrids are evident. What’s difficult to explain is why this very distinct "missing row" anomaly occurs on the outside or underside of the ears fairly consistently.

Image caption: Zipper Ears

Some of the explanations for zipper ears that I’ve heard include the following: 1) silks attached to the kernels (associated with the missing row) were covered up by other silks and simply did not get pollinated or, more likely, were pollinated late and as a result were more prone to abortion; 2) differential corn rootworm beetle silk clipping and feeding, i.e. beetles are below the ear during daytime hours, preferentially clipping silks of kernels facing downward? 3) differential kernel growth rate on the ear. Under drought stress, silk emergence can be slower than pollen shed. Perhaps silks on the outside or underside of the ear emerge more slowly than those facing the stalk? As a result, they may be pollinated later or emerge after pollen shed is complete. The later pollinated  kernels may be outcompeted for limited photosynthates by other kernels which are larger and further along in development, and thus more effective in competing for the limited supply of photosynthates (similar to the problem that occurs with kernel abortion that occurs at the tip of the ear - "tip dieback"). 4) Small, short ear shanks might play role in this problem - if the shanks collapse or pinch (due to drought) perhaps it might impair the vascular tissue conducting nutrients to kernel rows on the outside or underside of the ear.

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.

For more on ear development problems, including images, check the following:

Nielsen, R.L. 2009. Effects of Stress During Grain Filling in Corn

Available at http://www.kingcorn.org/news/timeless/GrainFillStress.html

(URL verified 8/23/10)

Thomison, P. and A. Geyer. 2007. Abnormal corn ears. Ohio State University Extension. ACE-1. available at  http://agcrops.osu.edu/specialists/corn/resources/of-interest/AbnormalCornEarsPoster_000.pdf/view  (URL verified 8/23/10)