Why Corn Ears Don't Fill: Pollination, Tip-back, and Kernel Loss

Corn yield potential develops in stages: first the plant establishes the ear’s potential size before pollination even occurs, then pollination determines which ovules (the potentially future kernels) actually become kernels, and early grain fill determines whether those newly formed kernels survive.

Ear size determination

First, we must understand when ear size is determined. The ear begins to develop starting at growth stages V5-V7 when the plant begins initiating ear shoots along the stalk. The number of kernels rows (row number or ear girth) is set shortly after ear initiation at V5-V7 and is strongly controlled by hybrid genetics, less by the environment, and will not change much except under sudden, severe stress during the very short V5-V7 period. (Image 1.)

The number of kernels per row (ear length) is determined over a longer period, beginning around V5-V7 (depending on the hybrid) and ending around V12-V15, generally 10-14 days before silk emergence. (Image 2.)

At this time period, the ovules (potential kernels) have finished developing, and the plant cannot create additional kernels later in the season. Different than row number, the number of kernels per row is not as strongly determined by plant genetics and is more affected by the environment. Severe stress during this period can shorten the ear by reducing the number of ovules per row, which explains why the ear length can vary a lot from year to year, depending on growing conditions. Purdue's Ear Size Determination in Corn goes more in-depth on this process.

While severe stress from V5-V12 can directly reduce ear size by limiting photosynthesis and allocation of carbohydrates to the potential kernels, issues with pollination have a much greater negative effect on grain yield.

Pollination and early grain fill

At the silking stage (R1), the plant determines which of those potential kernels are successfully pollinated (in technical terms, fertilized). In ideal conditions, pollen shed occurs during silking, and each ovule that is successfully pollinated becomes a kernel, while ovules that are never pollinated will remain empty and not develop any further. After pollination, kernel number can still decline through abortion, which can happen throughout the early grain fill stages, up to around milk stage (R3).

Kernel abortion is when a kernel is successfully pollinated, starts to form but later stops developing and shrivels because the plant cannot supply enough photosynthate during early grain fill. Therefore, the number of kernels per row is determined between VT and R3 encompasses pollen shed, silk emergence, fertilization (kernel set), and early kernel survival.

For successful pollination to occur, viable pollen must be shed from a tassel and land on a receptive silk. Therefore, synchrony between silk emergence and pollen drop is critical to ensure successful pollination. Pollen shed usually lasts about 10 days. Complete silk emergence from an ear generally occurs within four to eight days after the first silks appear and each silk remains receptive for about 10 days. In ideal conditions, the wide windows of silking and pollination can ensure synchrony between these developmental phases and thus good pollination. 

However, many plant stressors such as extreme heat, severe drought, or clipping by insects that feed on the silks, can delay or even completely prevent silk emergence, resulting in poor pollination. In particular, drought stress tends to slow silk elongation but accelerate pollen shed, leading to pollen shed occurring before silk emergence.

Each silk connects to one ovule, or potential kernel, on the ear. Silks from the base of the ear the first to emerge, followed by the middle ear silks, and lastly the tip silks. Because tip silks emerge last, the ovules near the tip are naturally the most vulnerable to pollination failure due to delayed silking caused by stress. Poor pollination will leave absent kernels, while abortion (caused after successful pollination during early grain fill stages) will leave shriveled kernels (see Table 1.)

Tip dieback/Tip-back

Tip dieback, or tip-back, is corn ears that fail to fill all the way from the base of the ear to the tip, resulting in an ear that exhibits little or no kernel development towards the tip of the ear. This is a symptom of either poor pollination or post-pollination kernel abortion which occurred due to the plant experiencing stressful conditions during early kernel development.

Stresses may include high temperatures, severe drought, nutrient deficiencies, high plant populations, cloudy weather, or leaf area losses (foliar diseases, hail), which delay silk emergence resulting in poor pollination and impact the plant’s ability to photosynthesize, leading to kernel abortion.

It's important to note that tip dieback is not always a sign of lost yield potential. It's important to look at the whole ear's size and evaluate the number of rows and kernels per row as it may be larger than normal and still yield well.

Zipper ears

Zipper ears, or banana ears, are ears that exhibit missing or incomplete kernel rows. Ears may be misshapen and bent (banana ears) due to the missing kernels. Zipper ears are typically associated with severe drought stress during early grain fill. Hybrids differ in their susceptibility to zippering. See R.L. Nielsen's The "Zipper" Pattern of Poor Kernel Set in Corn for greater detail on this phenomenon.

Drought stress and pollination failure

Drought may:

• Slow silk elongation, causing delays or prevention of silk emergence
• Emerging silks can dry out and become unreceptive
• Accelerate pollen shed, worsening the timing between pollen drop and silk emergence
• Cause persistent wilting in the two weeks before silking up to after silk emergence, impacting the plant’s determination of ear size, pollination, and early kernel development.
• Severe drought combined with heat, especially with low humidity, may desiccate silks or damage pollen if temperatures reach 95°F or above

In-field investigations

Inspect multiple ears across representative areas of the field. Determine where the kernel loss occurs. Shrunken, shriveled kernels indicate post-pollination abortion while empty spaces indicate failed pollination. Compare ear symptoms with the field’s weather and crop conditions during three windows:

• V5 – V15: determination of ear size and potential number of kernels per ear 
• VT/R1: pollen shed, silk emergence, and fertilization
• R1 to R3: early grain fill and kernel retention

Note observations of drought, high temperatures, leaf rolling, leaf loss, nutrient shortage, prolonged cloudiness, and disease pressure.  MSU's "Recognizing water stress and critical growth stages in corn and soybeans" can help inform you of stress signs to catch them early. Knowing the cause of the damage may help inform changes in management for future years. For example, if the same problem persists year after year, one could consider switching to a different hybrid that is more drought tolerant, or that has a different maturity so that pollination does not coincide with periods of drought stress.

If you need help understanding growth stages of corn, Corn Growth Stages can help or you may reach out to your local Extension educator.

To summarize, grain yield potential is impacted by several factors directly affecting the number of kernels per ear. Ear size establishes the number of potential kernel sites (ovules), pollination determines which sites set kernels, and early grain fill determines how many of those kernels survive. 

References:

Gradiz, Angie, and Younsuk Dong. "Recognizing Water Stress and Critical Growth Stages in Corn and Soybeans." Michigan State University College of Agriculture & Natural Resources, 28 July 2025.

Nielsen , R.L. (Bob). "Drought & Heat Stress Effects on Corn Pollination", Purdue University, 14 July 1995. 

Nielsen, R.L. "The Zipper Pattern of Poor Kernel Set in Corn", Purdue University, October. 2019.

Ortez, Osler. "Understanding Corn Ear Abnormalities: Tip Dieback.", Agronomic Crops Network, Ohio State University, 28 August 2026.

Quinn, Dan. "Tip Back' in Corn – What Is It and What Does It Mean?" Pest&Crop Newsletter, Purdue, 4 October 2022.