Updated August 2025
URL: http://www.kingcorn.org/news/timeless/KernelSet.html
Causes of Poor Kernel Set in Corn
R.L. (Bob) Nielsen
Professor Emeritus & Retired Extension Corn Specialist
Agronomy Dept., Purdue Univ.
West Lafayette, IN 47907-2054
Email address: rnielsen at purdue.edu
- Kernel set problems typically stem from herbicide injury, ineffective pollination, ineffective fertilization of the ovaries, or kernel abortion.
- Diagnosing the specific cause(s) of poor kernel set can be challenging.
Poor kernel set, meaning an unacceptably low kernel number per ear, is not surprising in fields that are obviously severely stressed by drought, but can also occur in fields that otherwise appear to be in good shape. The time period for determining good or poor kernel set begins at pollination and continues through the early stages of kernel development; typically 2 to 3 weeks after pollination is complete.
Problems with kernel set primary stem from herbicide injury, ineffective pollination, ineffective fertilization of the ovaries, or kernel abortion. Distinguishing the symptoms is easy. Determining the exact cause of the problem is sometimes difficult.
Potential Yield Loss
The potential loss in grain yield caused by lower kernel numbers per ear can be estimated using the formula of the so-called Yield Component Method (Nielsen, 2021a). For example, the loss of only 1 kernel per row for a hybrid with 16-row ears and a stand count of 30,000 ears per acre would equal a potential yield loss of approximately 5 to 6 bushels per acre (1 [kernel] x 16 [rows] x 30 [thousand ears per acre] divided by 85 [thousand kernels per bushel]). Actual yield loss due to poor kernel set is a bit more complicated because of the potential for compensating changes in kernel weight by the remaining kernels on a poorly filled ear (Jeschke et al., 2025).
Herbicide Injury
Injury to ovules on developing ear shoots from late applications of certain herbicides, most notably glyphosate, can render some proportion of the ovules inviable to pollen (DuPont Pioneer, 2014). Glyphosate-tolerant hybrids are either homozygous (both inbred seed parents tolerant) or heterozygous (one parent tolerant, one parent susceptible) for the trait. The ovules (and pollen) of heterozygous tolerant hybrids segregate genetically for glyphosate tolerance and susceptibility in a ratio that is typically 3:1. In other words, about 25% of the ovules on developing ear shoots of a heterozygous glyphosate-tolerant hybrid are susceptible to glyphosate and so late, off-label, applications of glyphosate will consequently result in about 25% of the kernels not developing normally. Initially, the affected kernels may have the appearance of translucent bubbles but they will eventually wither away. Normal kernels adjacent to the affected ones will grow larger and the normal rows and columns of kernels will appear more jumbled.
Ineffective Pollination / Fertilization
Poor kernel set may be caused by ineffective pollination (the transfer of pollen from the tassel to the silks) and/or the subsequent failure of the pollen's male gametes to fertilize (i.e., unite with) the female gametes of the ovules on the cob. Ineffective pollination/fertilization is characterized by a total absence of obvious kernel development. In other words, all you see is cob tissue where there should be kernels. Pollination and/or fertilization problems may be due to several stress factors, sometimes working together to influence kernel set.
Severe drought stress, aggravated by excessive heat, can delay silk emergence to the extent that pollen shed is complete or nearly complete by the time the silks finally emerge from the husk. Without a pollen source, ovule fertilization cannot occur. Without the capture of pollen, silks continue to elongate until they deteriorate with age.
Persistent severe silk clipping by insects such as the corn rootworm beetle (Diabrotica species) or Japanese beetle (Popillia japonica) throughout the active pollen shed period can also limit the success of pollination by restricting viable exposed silks. The simultaneous effects of severe drought stress on silk emergence can easily amplify the consequences of severe silk clipping.
Severe drought stress coupled with excessive heat and low humidity can desiccate emerged silks to the point that they become non-receptive to pollen grain germination. I suspect this is low on the list of possible stressors for Indiana most years (because of our typically high humidity levels), but may play a role in some fields once in a while. Similarly, I suspect that pollen viability is usually NOT an issue for Indiana cornfields because summer air temperatures in the low to mid 90's (F) common to Indiana are usually not great enough to kill pollen.
Consecutive days of persistent rainfall or showers that keep tassels wet for many hours per day over several days can delay or interfere with anther exsertion and pollen shed. Such weather does not typically occur in Indiana, but the remnants of Hurricane Dennis that visited many parts of Indiana in early July of 2005 influenced kernel set in some fields that were trying to pollinate during that week as a result of the many days of showery humid weather (coupled with the excessive cloudiness and its negative effect on photosynthesis).
Exceptionally long potential ears that often result from good weather during the pre-tassel ear size determination period sometimes fail to pollinate the final kernels near the tip of the cob. Remember, butt silks emerge first and tip silks emerge last. With oversized ears, sometimes tip silks emerge after all the pollen has been shed.
An increasingly common hybrid trait in recent years is an aggressive silking habit associated with drought tolerance that lessens the risk of delayed silk emergence in response to severe drought stress and, thus, silk/pollen synchrony is better maintained. However, favorable weather during silk elongation with such hybrids tends to result in silks emerging from the husk leaves several days prior to the availability of pollen from the tassels. Such unusually early silk appearance can result in silk aging / deterioration prior to the availability of pollen. The typical kernel set pattern associated with this situation is blank cob tissue near the basal end of the cobs.
An unusual form of rapid growth syndrome described as "Tassel Wrap" was documented throughout many corn growing areas of the U.S. in 2025 (Jeschke et al., 2025). Rapid growth syndrome itself is not uncommon and usually appears in young corn at about the V6 stage of development (6 leaves with visible leaf collars) and disappears by about V10 (Nielsen, 2019b). The variation of this phenomenon termed as "tassel wrap" is unusual because it manifests itself at later leaf stages and results in the uppermost whorl leaves becoming tightly wrapped around the tassel as it begins to emerge from the whorl. The worst case scenario is when tassels of a large percentage of plants in the field are unable to emerge from the tightly wrapped whorl leaves and shed pollen in sync with the emergence of the silks on the ears. The phenomenon is thought to be correlated with the previously described aggressive silking habit and so silks emerge early before sufficient pollen is available, resulting in varying degrees of ineffective pollination and poor kernel set.
Another factor that affects the availability of pollen in commercial corn fields is the relatively common use of hybrids produced using cytoplasmic male sterility (CMS) technologies (Nielsen, 2025). Seed produced using CMS technologies is itself typically male sterile (i.e., inviable pollen) and must be blended with fertile seed in about a 1:1 ratio before it is marketed. The pollen load in fields planted to such seed is automatically about half that of a normal hybrid but is still adequate for successful pollination. On rare occasions, mistakes occur in the seed blending process that result in fields exhibiting extremely low percentages of plants with fertile tassels that are incapable of providing enough pollen to adequately pollinate all the silks in the field, thus resulting in varying degrees of poor kernel set.
Kernel Abortion
Poor kernel set can also be caused by kernel abortion following successful fertilization of the ovules on the cob. In contrast to ineffective pollination or fertilization, initial kernel development precedes kernel abortion, so the symptoms are usually shriveled remnants of kernels that are whitish- or yellowish-translucent.
The causes of kernel abortion are generally those stresses that greatly reduce the overall photosynthetic output of the plant during the first several weeks after the end of pollination as the kernels develop through the blister (R2) and milk (R3) stages of development. The risk of kernel abortion decreases significantly after the R3 stage of kernel development. Obvious photosynthetic stressors include severe drought and heat stress, consecutive days of excessively cloudy weather and significant loss of photosynthetically active leaf area (e.g., hail damage, leaf diseases, insect damage, nutrient deficiency).
Warm nights during pollination and early grain fill may indirectly affect survival of developing kernels. Research suggests that the increased rate of kernel development due to warmer temperatures lowers the available amount of photosynthate per unit of thermal time; which then becomes a stressor to kernel development particularly at the tip of the ear, leading to kernel abortion (Cantarero et al., 1999).
Final Food for Thought
A plethora (meaning a whole lot) of blank cob tips can quickly ruin the joy of walking a cornfield in the middle of August. Before getting too bent out of shape over the missing kernels, remember to count the number of harvestable kernels on those ears. Sometimes, ears exhibit 1 to 2 inches of blank tips; yet still contain 16 rows by 30 to 35 harvestable kernels per row. Those are perfectly acceptable ear sizes in a year where dry weather has been a concern.
Citations and Related Reading
Cantarero, M.G., A.G. Cirilo, and F.H. Andrade. 1999. Night temperature at silking affects kernel set in maize. Crop Sci 39:703-710.
Jeschke, Mark, Lucas Borrás, and Brent Myers. 2025. Tassel Wrap in Corn. Pioneer Agronomy Sciences, Crop Insights Vol. 35 No. 8. https://corteva.showpad.com/share/BHwnIR3jGfDtTLscbr2jB [URL accessed July 2025].
DuPont Pioneer. 2014. Corn Ear Injury Risk with Off-Label Glyphosate Applications. DuPont Pioneer Agronomy. https://www.pioneer.com/CMRoot/Pioneer/US/Non_Searchable/agronomy/cropfocus_pdf/off_label_glyphosate_app_corn.pdf [URL accessed July 2025].
Nafziger, Emerson. 2017. Corn, in the Illinois Agronomy Handbook. http://extension.cropsciences.illinois.edu/handbook/pdfs/chapter02.pdf [URL accessed July 2025].
Nielsen, R.L. (Bob). 2018. Effects of Severe Stress During Grain Filling in Corn. Corny News Network, Purdue Univ. http://www.kingcorn.org/news/timeless/GrainFillStress.html [URL accessed July 2025].
Nielsen, R.L. (Bob). 2019a. The "Zipper" Pattern of Poor Kernel Set in Corn. Corny News Network, Purdue Univ. http://www.kingcorn.org/news/timeless/Zipper.html [URL accessed July 2025].
Nielsen, R.L. (Bob). 2019b. Yellow Tops and Twisted Whorls in Corn. Corny News Network, Purdue Univ. http://www.kingcorn.org/news/timeless/TwistedWhorls.html [URL accessed July 2025].
Nielsen, R.L. (Bob). 2020. Silk Emergence. Corny News Network, Purdue Univ. Available http://www.kingcorn.org/news/timeless/Silks.html [URL accessed July 2025].
Nielsen, R.L. (Bob). 2021a. Estimating Corn Grain Yield Prior to Harvest. Corny News Network, Purdue Univ. http://www.kingcorn.org/news/timeless/YldEstMethod.html [URL accessed July 2025].
Nielsen, R.L. (Bob). 2021b. Grain Fill Stages in Corn. Corny News Network, Purdue Univ. http://www.kingcorn.org/news/timeless/GrainFill.html [URL accessed July 2025].
Nielsen, R.L. (Bob). 2022. A Fast & Accurate "Pregnancy" Test for Corn. Corny News Network, Purdue Univ. http://www.kingcorn.org/news/timeless/EarShake.html [URL accessed July 2025].
Nielsen, R.L. (Bob). 2025. Tassel Emergence & Pollen Shed. Corny News Network, Purdue Univ. http://www.kingcorn.org/news/timeless/Tassels.html [URL accessed July 2025].
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