Why did the Kernels near the Ear Tip not Fill?

Incomplete kernel set is something you can’t ignore when you start peeling back husks to evaluate your corn crop this time of year. Often times we 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 optimal, then plants wouldn’t abort those kernels near the ear tip. In other words, various environmental factors do indeed influence crop growth and interact with our cultural management of the crop. Therefore, if we want to improve our yields, we need to investigate 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 a couple of primary reasons.  Silks responsible for receiving pollen emerge last from kernels near the ear tip. Thus, fertilization failure is more likely, because these silks may not emerge in time to be receptive to pollen. Severe drought stress is well known to delay silk development, often promoting this synchrony issue and resultant pollination failure. However, we commonly see plenty of unfilled ear tips in both our irrigated and good dryland fields. This is primarily due to kernels which abort, because the plant cannot support them all. I believe this is generally the far more important component of kernel tip fill.

Corn prioritizes its available energy first to developing kernels at the base of the ear, so any shortage is likely to promote kernel abortion near the ear tip. Kernel development is extremely sensitive to current energy production (photosynthetic rate) during the first few weeks after pollination, because tiny kernels have very poor ability to draw stored energy from mature vegetation. Therefore, relatively modest issues may significantly reduce yield potential at this time. It is during the 18-20 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:

Drought stress is a common factor known to reduce transpiration and photosynthesis.  Water deficit causes leaf stomates to close, directly restricting CO₂ uptake needed for photosynthesis.

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.

Photosynthesis requires light and its rate is closely correlated to light intensity. Thus, lots of overcast days shortly after pollination, or shading from excessive plant population or leaf canopy will limit photosynthetic rate and kernel development. Prevalent cloudy weather, particularly when combined with soil saturation, can be a limiting factor in some cases.

Nutrient deficiencies reduce growth or the vegetative “plant factory,” and restrict resources needed to optimize physiological processes.  Beyond not supplying adequate nutrients to support a good yield level, we often see wet soils and soil compaction severely restrict root development and limit nutrient uptake.  Rainy weather can also delay fertilizer application timing and increase nutrient losses, so plants do not have sufficient fertility throughout the season. For example, soils saturated by excessive rainfall over an extended time can promote substantial nitrogen loss.

High night temperatures increase plant respiration rates and expend energy which could otherwise support kernel development. Night temperatures above 70 deg F are known to increase wasteful respiration and reduce corn dry matter accumulation and grain yield. Therefore, cool night temperatures strongly support high corn yield potential. This year’s night temperatures have generally been warmer (30 day avg beginning at tassel of 71.5 deg F at Stoneville) than normal, so this factor will be a more significant limitation than last year (69.4 deg F), but less than 2016 (73.5 deg F).

When planting conditions are challenging we may see variable corn seedling emergence resulting from low temperatures and generally unfavorable conditions for seedling growth. The plant growth disparity resulting from variable emergence puts late plants at a distinct competitive disadvantage for resources. Late plants will likely be spindly, have smaller ears and may even fail to successfully pollinate, because their development lags behind the neighboring plants.