Missouri corn farmers might have an unpleasant surprise when corn harvest begins. There have been reports of poor corn pollination and reduced ear size, especially in west and southwest Missouri. Unfortunately, these pollination problems are hidden under several layers of husks and may not be apparent until combining begins.
In general, there are three broad causes of corn ears with fewer than expected kernels: fewer female flowers produced on the ear, poor synchronization between pollen shed and silk receptivity, and aborted kernels. For a deeper explanation of these, please read the three part series “Corn Pollination: the Good the Bad and the Ugly” (part 1, part 2, and part 3).
It is not clear which of the three causes occurred in Missouri this year, and it is likely that all three may have been involved, at least to some extent. Most reports involve what is often called “tip dieback”. Unfilled kernels at the tip of the ear are common even with excellent growing conditions. In fact, we ought to manage corn planting rates such that at least some empty or small kernels are observed at ear tips. Distinguishing between unfertilized kernels and aborted kernels (true tip dieback) can be difficult at corn maturity, so checking fields before maturity instead of at harvest will be helpful for diagnosis.
Unfertilized kernels means that pollen was not present when silks were receptive to pollen or something interfered with the growth of the pollen tube inside the silk. Because silks from flowers at the ear tip begin elongation last and their growth rates are slower than on other areas of the ear, they are often the ones that miss pollen. Before physiological maturity, these unfertilized kernels will appear white and blank - no growth of the kernel. Poor synchronization is mostly related to slow silk growth. Although water stress is the most common cause, anything that slows silk growth without delaying pollen shed can lead to unfertilized kernels.
True tip dieback is caused by kernel abortion or arrested development. The kernels were fertilized and at least some growth had occurred. These aborted kernels will vary widely in appearance even on the same ear. Some aborted kernels may look similar to unfertilized kernels because the kernels aborted within a few days after fertilization. Other kernels will have nearly normal shape (tip kernels are normally round and not flat) and color except they are noticeably smaller than normal. This wide variation in appearance is due to variation in the timing of when kernel development stopped. Most of what we call tip dieback happened early in kernel development, so the kernels remain very small. At maturity, aborted kernels will appear chaffy or will be so small they are difficult to see (Figure 1).
Developing kernels need water, sugar and mineral nutrients to gain weight. Any stress that limits any of these requirements can cause kernel abortion. Unfortunately, kernel abortion is permanent; growth will not resume if the stress is relieved. The majorly of kernel weight is starch, which is manufactured from sugars produced during photosynthesis. Because sugar is important to continued kernel growth, conditions that reduce photosynthesis may lead to increased tip die back. Even cloudy days, if frequent enough can increase tip dieback.
Weather stations located at agebb.missouri.edu/weather/ measure light energy and record it as total solar radiation for each day. I collected data from three of these stations in an attempt to determine if cloudy weather during kernel filling may have resulted in the reported increased of tip dieback in 2013. Data from the three weather stations are provided in Figure 2. I selected two stations in west Missouri (Buchanan and Barton counties) and one from central Missouri (Columbia). The graph begins on July 1 and continues through August. For Figure 3, I divided each day’s total radiation by a constant equal to an estimated maximum. This allows for easier determination of the magnitude of light reduction by clouds. Large fluctuations among days for sunlight are apparent in figures 2 and 3.To smooth the curves, I calculated a moving 5-day average. These data are presented in figure 4. Averaging light energy over a few days is more meaningful than data for individual days because periods of sustained low light should affect plants more than a single day of clouds.
Corn planting dates in 2013 varied widely among Missouri fields, so silking dates also varied. If fields were planted on a normal date, silking occurred in early July. Because of wet spring weather, corn planting was often delayed. Southwest Missouri experienced the longest wet period and greatest delay. So, silking occurred in the areas represented by these three weather stations from mid to late July. Sugar availability for developing kernels is critical during the kernels entire life, but disruptions within several weeks after fertilization is often related to early kernel abortion and what we call tip dieback. So, light levels from late July through mid-August may be important. Figure 4 shows an apparent reduction in light energy during that period for all three weather stations.
Daily light energy normally decreases after the first day of summer. Day length shortens and sun angle increases. These changes reduce the amount of energy impinging on corn fields. From late June to late August this normal reduction in light energy is about 15%. All three weather stations show light reductions of 40% or more in numerous 5-day periods in early August – a time when continued growth of kernels is easily impacted.
Light energy in 2012 is a convenient reference because it seemed like the sun shone brightly every day last summer. Figure 5 presents daily total solar radiation at Columbia in 2012 and 2013. Even in 2012 some days were cloudier than others and total light energy fluctuated among days. On August 31, a hurricane approached Missouri and sunlight was dramatically reduced because of heavy clouds. Data in figure 6 are the result of division by the constant, 26, and smoothing with a moving 5-day average. Clear differences between 2012 and 2013 are apparent. Light energy in mid-July through mid-August was greatly reduced in 2013 compared to 2012. That difference between the two years disappeared in late August.
The data I presented are just observations and not part of a controlled experiment. If there is more tip dieback than normal this year, the cause or causes may continue to be unexplained. Kernel development depends on current (daily) photosynthesis, because corn plants do not store large pools of carbohydrates. Reduction in light energy can decrease photosynthesis and the amount of sugar available to kernels. Finally, tip kernels are more susceptible to interruptions in kernel sugar supply because of ear structure, and they are further away for the sugar source. The unusually cloudy weather in 2013 during early stages of kernel filling may have contributed to increased kernel abortion of ear tip kernels.
REVISED: October 1, 2015