Has The Corn Population Treadmill Ended?
DR. DENNIS B. EGLI
LEXINGTON, KENTUCKY
Have corn populations reached a plateau? They increased steadily from 4000 to 8000 plants per acre when producers were growing open-pollinated varieties to around 29,000 to 32,000 plants per acre today. Yield contest winners report populations as high as 50,000 plants per acre. Corn producers were on a treadmill – always wondering if their populations were high enough for maximum yield.
Populations in the heart of the corn belt, however, haven’t increased since 2015, plateauing at 29,000 to 32,000 plants per acre (Fig. 1), according to the National Agriculture Statistics Service. This plateau was also observed in field trials in Illinois (Dr. Emerson Nafziger) and Indiana (Dr. Dan Quinn). Yields in 2025 were 25 bu/acre higher than the 2014 – 2016 average with no increase in population. This plateau is a drastic change from the treadmill we were on for the past 60-some years.
What’s going on? How can yield increase without an increase in population? One way to address this question is to ask - why was it necessary to increase population in the past to get higher yield?
The increases in yield that started with the introduction of hybrid corn in the1930s and 1940s were associated with more kernels per acre. Since yield equals kernels/acre x weight per kernel, higher yields must come from more kernels and/or larger kernels, there is no other option. As U.S. plant breeders increased yield over time, ear size (maximum kernels per ear) did not increase, so population (ears per acre) had to increase to supply enough kernels to support higher yields. The productivity of the crop increased (better hybrids and/or improved management) until all the flowers on the ear developed into kernels and then higher populations (more ears and kernels) were needed to produce the extra kernels need for higher yield.
The fundamental basis for the population treadmill was inflexibility of the corn plant. Its ability to increase the number of kernels per plant in response to higher productivity was limited. The original corn plants were flexible, they could produce tillers that bore ears or several ears per plant, but historically, corn breeders selected against these characteristics. This forced producers to ‘create’ flexibility by increasing population to increase kernels per acre. While kernels per acre were increasing there was very little change in weight per kernel (kernel size).
Thinking about the response of soybean to population helps us understand the corn response. Soybean, a very flexible species, can increase seeds per plant in response to higher plant growth rates by increasing pods per node or increasing nodes by branching. Consequently, soybean produces the same yield over a wide range of populations and populations remained the same or declined as historical yields increased. The opposite response to that exhibited by the inflexible corn plant.
The key point to understanding the population plateau is that increasing corn yield doesn’t absolutely require higher populations and more kernels per acre – there are other options. Increasing ear size or ears per plant (both are heritable characteristics) would increase kernels per acre. Another option is to increase the length of the kernel-filling period. If the kernel grows for a longer time, it will be larger, and yield will increase without any change in population. Kernel-filling period is also a heritable characteristic and there are research results showing that higher yields can be a result of longer kernel-growth periods.
Eliminating the need for ever higher populations to get higher yields solves one of the problems that would face producers if populations continued to increase. There are only so many plants that you can jam into a 30-inch row, so continually increasing population would eventually require narrow rows or a twin-row system and expensive equipment changes.
The plateaus in population (Fig. 1) suggest that modern hybrids are using different strategies to produce higher yield. So far, these new strategies (larger ears, longer-kernel filling periods) have successfully continued the increase in yield we are used to (Fig. 1). We will have to wait and see how far into the future these new strategies will be successful. ∆
DR. DENNIS B. EGLI
UNIVERSITY OF KENTUCKY