Solar radiation over West Lafayette, Indiana, ran roughly 5% below the previous five-year average during the 2026 post-pollination window, Purdue University research trials show — and several individual days came in 55% to 75% below historical norms. For Indiana cornfields that pollinated in mid-to-late July, those low-light stretches likely overlapped the critical first two to three weeks of kernel development, offering a plausible explanation for the tip back growers are now spotting at harvest.

The excess rainfall that soaked Indiana in 2026 raised the question of whether the accompanying cloud cover cut into kernel set. The Purdue data suggests it did. During the post-pollination period, average and cumulative solar radiation came in 5% below the five-year average, 5% below 2025 and 7% below 2024. Beyond the averages, 2026 produced unusually large day-to-day swings, with several pronounced low-light events during early kernel development.

Pollination failure or kernel abortion?

Agronomists separate tip back into two distinct processes, and diagnosing which one occurred matters because they happen at different times and respond to different stresses.

Poor pollination leaves blank cob tissue with little or no evidence a kernel ever started developing. Causes include poor synchrony between pollen shed and silk emergence, drought, excessive heat, silk clipping and prolonged wet weather during pollen shed. Because tip silks emerge last, tip ovules are the most vulnerable to fertilization failure.

Kernel abortion looks different. Fertilization succeeded and the kernel began developing, then stopped. Aborted kernels appear during early grain fill as small, shrunken, whitish or yellowish translucent kernels; by maturity they may look shriveled or chaffy. Tip kernels, fertilized last and farthest from the photosynthate supply, are usually the first sacrificed when stress hits during early grain fill.

How sensitive are young kernels to light?

Previous research quantified the vulnerability. Corn plants subjected to a 50% reduction in intercepted light from pollination through 15 days after pollination lost on average 16% of kernels per row to abortion, with tip kernels most susceptible. Abortion generally began and finished within eight to 12 days after pollination.

The physiology is more complex than kernels simply running out of sugar. Studies found sugar, sucrose and starch concentrations initially stayed similar between aborting and non-aborting kernels. But reducing sugars accumulating in the cob were associated with aborting kernels, suggesting sugar transport or uptake from the cob into developing kernels became impaired early. Once abortion was complete, sugar supply to those kernels shut off and growth stopped permanently.

What should growers do at harvest?

Scouting is not finished even though harvest is underway. Pulling ears now shows how the crop responded to this season's conditions. Key diagnostic steps:

• Check whether missing kernels left blank cob tissue — pointing to pollination failure — or shriveled, partially developed kernels concentrated at the tip, pointing to abortion.
• Match the field's pollination date, especially mid-to-late July, against the timing of cloudy stretches, rainfall, heat and disease in the following two to three weeks.
• Count harvestable kernels rather than inches of blank cob. Dramatic tip back can still sit on an ear with enough kernels to support decent yields.

Growers should also avoid over-diagnosing. Large ears with high initial kernel counts can naturally abort some tip kernels as the plant adjusts final kernel number to available resources, so some tip back does not automatically signal a serious yield problem. Kernel abortion typically stems from multiple stresses — drought, heat, nutrient deficiency, hail, foliar disease, insect injury or consecutive cloudy days — rather than one isolated factor, and reduced solar radiation becomes more consequential in combination with other pressures.

Where substantial tip kernel abortion shows up, particularly in fields whose early grain fill coincided with the lowest-radiation days, Purdue's 2026 radiation record makes cloudy weather a credible contributing factor. A full diagnosis, however, still requires weighing pollination timing, plant health, disease pressure, moisture, temperature and overall kernel number field by field.