Why did the Kernels near the Ear Tip not Fill?

Incomplete kernel set is something hard to ignore when you peel back husks to evaluate your corn crop. Accordingly, we often wonder why those kernels near the ear tip didn’t fill out. Invariably, the next thought that comes to mind is “this crop didn’t have any stress.” However, the fact is if everything was favorable, then plants wouldn’t abort those kernels near the ear tip. Therefore, if we want to improve our yields, we need to identify potential reasons why kernel set is less than desired, so we can potentially improve our management in the future.

Kernels near the ear tip are more prone to failure than lower on the ear for several potential reasons. Silks responsible for receiving pollen emerge last from kernels near the ear tip. Thus, fertilization failure might occur, because these silks may not emerge in time to receive viable pollen. Severe drought stress is also known to delay silk development, often promoting synchrony issues and resultant pollination failure. However, we commonly see plenty of unfilled ear tips in our irrigated and healthy dryland crop. This occurs because kernels near the ear tip abort when plants do not have enough energy to support them. Although often referred to as a “pollination” issue, kernel abortion is unrelated to pollination, and I believe it is by far the most common reason why kernel tips do not fill.

Corn generally prioritizes available energy to developing kernels at the base of the ear, which is why kernels abort near the ear tip. Corn is very sensitive to any issue at early reproductive stages because plants are swapping energy allocation from vegetative to reproductive organs, and small, immature kernels have little ability to draw energy reserves from vegetation at this time. This makes developing kernels very dependent on current photosynthetic rate to supply energy needed to develop and ultimately produce high yield. Therefore, relatively modest issues may limit yield potential at this time. It is during the 15-20 days after pollination (until milk stage or R3) that kernel number is determined.

There is a long list of issues capable of reducing the photosynthetic rate necessary to feed developing kernels, but these are some of the most common:

High night temperatures increase wasteful plant respiration rates and expend energy which could otherwise support kernel development in all corn fields. Therefore, Mississippi corn yields, especially for irrigated fields, are generally closely correlated to night temperatures during grain fill, when the average normal temperature is about 70 degrees for the 30 day period following pollination. Our later planted corn is more vulnerable to this issue, because night temperatures generally increase in the summer.

Photosynthesis requires light and its rate is closely correlated to light intensity. Thus, lots of cloudy, overcast days shortly after pollination, or shading from high plant population, particularly when grown in wide rows, can limit photosynthetic rate and kernel development.

Drought stress is a common factor known to reduce transpiration and photosynthesis.  Water deficit causes leaf stomates to close, directly restricting carbon dioxide uptake needed for photosynthesis.

Hail damage can remove leaf tissue which directly reduces photosynthetic capability and energy available to feed developing kernels and ears.

Excessive rainfall or irrigation will saturate soil, depriving oxygen, stunting or halting physiological processes, damaging root systems, promoting nutrient loss, and can even escalate heat stress when temperatures are warm.

Nutrient deficiencies reduce the vegetative “plant factory” and restrict resources needed for growth and development. Besides not supplying adequate nutrients to support high yield potential, we often see wet soils and soil compaction severely restrict root development, nutrient availability and uptake.  Rainy weather can also hinder fertilizer application and increase nutrient loss, so plants do not have sufficient fertility throughout the season. For example, soils saturated by abundant rainfall or irrigation over an extended time can promote nitrogen loss.

Hybrids may differ considerably in their kernel filling characteristics or tendencies. There are three different yield component variables in an ear: number of kernel rows around the cob, the number of kernels per row from the base to the tip of the ear, and the weight of kernels. One of our popular and high yielding hybrids typically produces fewer kernels per row than most, likely appearing to be a problem. However, this hybrid also produces very heavy kernels, and a high number of kernel rows, which helps overcome this shortcoming to produce exceptional yields. Thus, the take-home lesson is that all three of these characteristics are important components of corn grain yield.

When planting conditions are challenging, we often see plant growth disparity resulting from variable seedling emergence caused by cool, wet soils. This plant growth disparity puts late plants at a distinct competitive disadvantage for resources throughout the season. Late plants will have spindly stalks, and small, poorly-filled ears, because their development lags behind the neighboring plants.