By Rebecca Vittetoe

The 2017 growing season seems to be in the terrible twos — either too wet or too dry depending upon where you live in Iowa. The “too dry” situation has left 71% of the state abnormally dry, 40% in a moderate drought and 12% in a severe drought, according to the U.S. Drought Monitor that was released Aug. 8.

July was hot and dry, which is less-than-ideal conditions for corn pollination. These stressful conditions have a direct impact on corn ear development. Ideally, we like to see warmer-than-normal temperatures in May and June and cooler-than-normal temperatures in July and August. These ideal conditions help to speed up crop development until pollination, but then slow maturity after pollination, giving us a long grain fill period. The conditions we’ve experienced this year have not only sped up crop development until pollination, but after pollination as well.

What are you seeing in fields?
Drought stress during pollination and grain fill can affect ear and kernel formation. What are we seeing in fields this year?

The limited precipitation and high temperatures, ranging in the mid-to-upper 90s during pollination resulted in some plants to be barren or missing ears entirely. This seems to be more common in compacted areas of a field, or areas with really sandy or clayey soils.  In addition to the barren plants, nubbin ears or ears that are severely reduced in size with few scattered kernels can also be found.
One of the more common ear issues this year is the tipping back on the ears. Usually these ears are normal or near normal in size around, but the number of kernels is markedly reduced. It’s not uncommon to see no kernels on the last 2 or more inches of the ear tip.

Causes of kernels tipping back on ears
Different factors can attribute to tipping back. The kernels at the tip of the ear are the last to be pollinated, and there may have been only a limited amount of pollen available to pollinate those later-emerging silks. Pollen shed may also have been complete before those silks had a chance to emerge. Also, pollen feeding and silk clipping by insects, such as Japanese beetles and corn rootworms, can also result in pollination problems. 

Another factor contributing to the tipping back is abortion of kernels that did pollinate, but the plant decided it could not support any more due to limited plant nutrients (sugars and proteins). Since the kernels at the tips of the ear are the last to be pollinated, they are the first to be aborted.

You can distinguish between poor pollination and tipping back by the color of the tip kernels. While both aborted kernels and unfertilized kernels appear dried and shrunken, aborted kernels often have a yellowish color to them. Aborting of kernels happens for only about two weeks post-pollination. After that, if conditions are still stressful, kernel fill will be less than ideal.

Zippering, odd-shaped ears, ear molds
Zipper ears are also evident in fields this year due to the drought. Zipper ears exhibit missing kernel rows. The missing kernel rows are usually found on the side of the ear away from the stalk. The zippering is due to kernels that are poorly developed, from kernels that have been aborted, or from kernels that never pollinated. Often this is from silk balling inside the husk, preventing silks from emerging.

ZIPPER EARS: Zippering is another common problem of poor kernel set observed in cornfields this year. It’s the result of an ear missing one or more entire rows of kernels along one side of the cob due to a combination of pollination failure and kernel abortion.

In most cases, the zippering extends the full length of the ear. Sometimes the zippering can cause ears to have a curvature to them, which is also called a banana ear.

In addition to some of the odd-shaped ears we may be seeing in fields this year, one should keep an eye open for ear molds, particularly Aspergillus. This ear mold favors hot and dry conditions and insect feeding on kernels can also contribute to the disease development. Aspergillus is very distinguishable by its olive-green color. Typically, this ear mold is found near the tip of the ear, but it may be found all the way to the base if the disease is really severe.

Should you test corn grain for mycotoxin?
The concern around Aspergillus is that the fungus produces a mycotoxin known as aflatoxin inside diseased corn kernels, which greatly affects grain quality and marketability and health concerns if consumed by livestock. If you find Aspergillus that does not mean the aflatoxin is present, but you should test the grain to see if it is.

ASPERGILLUS: Check ears for presence of Aspergillus mold. Here, the green mold is starting to form on a kernel near the ear tip.

To determine if your field is at risk, scout for the ear rot. Scout from dent through harvest. Stop at several (five to 10 spots) throughout the field to assess ears for the presence of Aspergillus by pulling back the husks of 10 ears and inspect them for the characteristic olive-green powdery mold of Aspergillus. If greater than 10% of the ears show signs of Aspergillus ear rot, it is recommended to schedule the field for an early harvest.

Notify your crop insurance adjuster immediately if you have fields with Aspergillus ear rot. Infested fields need to be inspected before harvest or strips need to be left in the field for a claim to be made.

More information on managing Aspergillus and aflatoxins can be found in the ISU Extension publication Aflatoxin in Corn.

If you are covered either by Federal Crop Insurance or by private insurance, check with your insurance agent before harvesting a drought-damaged crop for either grain or silage. Inspections should be made to determine the difference between actual anticipated yield and amount covered by the policy. Insurance settlements are based on this difference. If you are expecting severe yield reductions due to drought, report this to your county FSA office before the crop is harvested. If drought is widespread, there may be the possibility of emergency or low-interest loans.

Vittetoe is an ISU Extension field agronomist covering south-central and southeast Iowa. Contact her at [email protected]u.