We’ve all seen advertisements for corn hybrids that show a row of corn with an ear on every plant that is filled to the tip. The implication is that well-filled ears are an indication of high yield.

 This makes sense because ears that are filled to the tip are an indication of good growing conditions during pollination and kernel set. Good growing conditions mean high yields.

Much of the variation in corn yield is related to kernels pr acre. Many kernels usually mean high yields. The number of kernels is, in turn, related to the productivity of the crop during pollination and kernel set (around growth stages VT/R1). Any stress, such as high temperatures or a lack of water or both, during this critical period will interfere with pollination or cause abortion of immature kernels, reducing kernel number and yield potential. Well-filled ears may be an indication of a lack of stress during this critical period and potentially high yields.

Are well-filled ears always an indicator of high yields? Unfortunately not! Well-filled ears can also be an indication that the population was too low, and yield was left in the field. How can that be?

A basic problem with corn is that over the years, it lost its flexibility – it lost much of its ability to adjust the number of kernels per plant in response to changes in the productivity of the environment. Most modern corn hybrids produce only a single ear, even though the wild species (Teosinte) that gave rise to modern corn produced ear-bearing tillers and multiple ears per plant – it was very flexible. Over the years, corn breeders got rid of this flexibility and limited the ability of the plant to increase kernel numbers in response to increases in plant productivity. Producers have to supply flexibility by adjusting population.

In comparison, soybean is a very flexible plant that can easily adjust the number of seeds to match changes in productivity. Soybean plants branch and increase flowers per node to produce more seeds. 

The size of the ear (florets per ear) and ears per plant sets the maximum number of kernels the corn plant can produce. If the productivity of the environment exceeds that capacity, the number of kernels will limit yield. The plant had enough photosynthesis to produce more kernels, but there weren’t enough florets; in other words, the plant was limited by the number of kernels, and the yield was left in the field. This situation can be avoided by increasing the population, which increases the number of florets and the number of potential kernels per acre, and yield will no longer be limited by kernel number.

This relationship explains why corn populations increased from roughly 8000 plants per acre when producers were growing open-pollinated varieties in the 1930s to 30,000 plants per acre or more today. As breeders developed higher-yielding hybrids, they did not necessarily increase ear size. So, populations had to increase to provide enough florets to match the increase in productivity and prevent the number of kernels from limiting yield. In contrast, soybean populations remained the same or decreased as yield increased. The difference is just a matter of flexibility.

If the population in a particular field is too low relative to productivity, all the ears will be filled to the tip, and the number of kernels will limit yield. These well-filled ears are not an indication of high yield; instead, they signify unrealized yield. There weren’t enough florets to translate all the productivity of the plant into yield. Increasing population would have increased yield.

Well-filled ears can be an indication that yield has been left in the field because the population was too low, or an indication of no stress during tasseling and silking, which sets the potential for high yields. One indicator but two outcomes – now isn’t that a kick in the head.

How should we respond to this dilemma? If last year’s population resulted in a field of well-filled ears, instead of celebrating high yields, the prudent approach may be to increase population to capture lost yield by avoiding a limitation by the number of kernels per plant. On the other hand, a field of unfilled ears may be a cause for rejoicing; we got the population right and maximized yield. Of course, it may also be an indication of stress during pollination and seed set that reduced yield. All that can be done then is to hope for better weather next year. Unfortunately, ears don’t tell the whole story. As Abraham Lincoln (1809 – 1865, 16^th President of the U.S.) once said, “We can complain because rose bushes have thorns, or we can rejoice because thorn bushes have roses."

Adapted from Egli, D.B. 2021. Applied Crop Physiology. Understanding the Fundamentals of Grain Crop Management, pp. 103-111. CABI.