The 2026 growing season has presented a wide range of challenges across Iowa. Some areas have received excessive rainfall, while others have struggled with limited precipitation. As the corn crop advances toward maturity, it is important to assess how those environmental conditions may have influenced corn development and yield potential. Most corn fields in Iowa are now in the mid to later reproductive stages of development. At this point in the season, we can evaluate kernel development and begin making yield estimates.
This year, some fields are showing atypical or incomplete kernel set on ears. While most ears have a normal kernel set, others are missing kernels. These missing kernels may be near the butt, on the underside, near the tip, or more sporadic throughout the ear. Additionally, some ears contain kernels at different developmental stages. For instance, some kernels may be at the R4 (dough) stage and others are still in the R3 (milk) stage. This indicates that there was a wider window for pollination and fertilization of the kernels on those ears.
To better understand why we are seeing this, we need to look back at what happens during the growing season, from ear development through early grain fill in the ICM Encyclopedia Article “The Basics of Ear Formation and Kernel Development”.
Possible Reasons for Poor Kernel Set
If you noticed issues with kernel set, below are some possible reasons on why:
- Temperatures and moisture: Many affected fields were planted that first week of May and would likely have been pollinating during the week of July 13. Looking back at weather data, both daytime and nighttime temperatures were high. These conditions can reduce pollen viability and silk receptivity. Additionally, accompanying moisture stress could also be a contributing factor.
- Wind: Looking at wind data during the timeframe of July 10 to July 20, when many fields would have been pollinating, winds were calmer in general. For instance, looking at wind data specifically for Ames, the average windspeed from July 10 to July 20 was 3.4 mph, and 44% of the time it was calm (winds less than 2.0 mph). This is far less than what we typically see, as on average the wind speed the during this same timeframe between 2015 – 2025 was 6.9 mph and the average percent calm was 16%. This brings up the question, could the lack of wind have reduced the spread of pollen in some fields? Look at windrose data for other locations in the state on the ISU Mesonet website.
- Hybrid characteristics: A common hybrid trait in recent years is aggressive silking. While this trait can improve pollination success during drought stress, it may result in silks emerging before pollen shed. Consequently, silk receptivity may decline before pollen is available and resulting in poorer kernel set. If you notice more kernels missing near the butt or at the base of an ear, this could be a likely cause as those silks would have emerged first.
- Tassel wrap: While it didn’t necessarily appear to be an issue this year like last it was last year, “tassel wrap” could cause incomplete fertilization (Jeschke et al., 2025). This would have been noticeable in the field around pollination and silking.
- Pollen availability: While weather conditions can influence pollen availability, hybrids produced using cytoplasmic male sterility technologies are typically male sterile and need to be blended with fertile seed, but the pollen load in those fields would be less than normal, but typically still enough for pollination to be successful. (Nielsen, 2025).
Insect pressure: Persistent silk clipping from insects, specifically Japanese beetles and/or corn rootworm beetles can affect pollination success in fields. If this is the case in the field, you’d see that the silks have been clipped closer to or below the husk.
Corn ear that had silks clipped due to insect feeding. In this case, the ear had already been pollinated, but if clipping happens prior to or during pollination it can really affect pollination success. Photo by Rebecca Vittetoe, ISU. - Pesticide applications: Not following the growth stage cutoffs on pesticide products like herbicides is another consideration. Some examples include glyphosate and ALS-inhibitors, but care should be taken to apply all products within label parameters.
In many cases, affected plants represent only a small percentage of the field, and neighboring kernels may partially compensate for lost kernels. However, the severity and distribution of the issue varies among fields. It’s likely that there were several factors that contributed to kernel set issues in fields this year. Scout your own fields to confirm the success of pollination and kernel set.
If you have questions about kernel set in your fields, contact your Extension Field Agronomist, or if you are noting lack of kernel set is more prevalent in one hybrid over another, contact your seed dealer.
Resources:
Nielson, RL. 2023. Ear Size Determination in Corn. Corny News Network, Purdue Univ. Agronomy Extension. https://www.agry.purdue.edu/ext/corn/news/timeless/EarSize.html [URL accessed August 7, 2026]
Nielson, RL. 2025. Causes of Poor Kernel Set in Corn. Corny News Network, Purdue Univ. Agronomy Extension. https://www.agry.purdue.edu/ext/corn/news/timeless/kernelset.html [URL accessed August 7, 2026]
Jeschke, M., L. Borrás, and B. Myers. 2025. Tassel Wrap in Corn. Pioneer Crop Insights Vol. 35 No. 8. Corteva Agriscience. Johnston, Iowa.
Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on August 10, 2026. The information contained within may not be the most current and accurate depending on when it is accessed.
- Field Agronomist in SW Iowa
- Field Agronomist in EC/SE Iowa