Plant populations in corn have certainly changed over the course of my career. We have noticed in our Five Acre Corn Club an increase of about 400 plants per acre per year in plant populations. Last year plant populations on these high-yielding fields averaged 31,363 plants per acre, with an average yield of 262 bushels per acre. If you assume a 5 percent mortality, this is a seeding rate of about 33,000 seeds per acre. This is probably a reasonable reflection of what some of our top corn growers think is needed to grow top yields.
A good question is how these populations compare with some recent research? Purdue has conducted research on more than 80 locations on field scale trials that have ranged from 30 to 100 acres. The average percent stand in these trials was 95 percent but ranged from 79 percent to 100 percent. Of these trials, about 60 of them were considered normal growing conditions with minimal to moderate stress. Optimum plant populations varied across the trials from 24,000 to 44,000 plants per acre but averaged across all these trials, maximum yields occurred at an average plant population of 32,000 plants per acre or a seeding rate of 34,000 seeds per acre.
The optimum economic seeding rate was about 2,000 plants per acre less or about 32,000 seeds per acre. This is because the benefit of extra seed is small near the optimum and at some point the added seed cost is not justified. They did not find a great relationship between the yield of the trial and the optimum seeding rates. In other words, the highest yielding sites did not always need the highest seeding rates. They also compared “responsive” and “non responsive” hybrids in population trials in the same field and found that they don’t always follow the expected pattern.
In the 14 other Indiana locations, the corn was stressed fairly severely and yields averaged 116 bushels per acre. On these sites the optimum plant density was 22,800 which would translate into a seeding rate of 24,000.
The University of Illinois also has been conducting population trials for the past five years. They found that in productive fields optimum plant densities range from 30,000 to 40,000 per acre. Over all their testing across 32 fields, they found that the optimum economic plant density was 33,400 plants per acre. They also found that the optimum plant population was not highly correlated to yield, but yields in this study were mostly at or above 200 bushels per acre. Another finding from the Illinois data is that seeding rate responses are fairly flat around the optimum and small (1,000-2,000) differences in population don’t make that much difference in yield.
Based on this and other work, recommended seeding rates in the Penn State Agronomy Guide are 35,000 for fields with yields over 175 bushels per acre and 30,000 for fields in the 125 to 175 bushel per acre range. For fields less than 125 bushel per acre yield potential, 25,000 seeding rates are recommended. These are fairly general but based on the recent results they appear to be fairly representative of most fields.
Seed companies have developed hybrid specific recommendations for different yield levels, either in charts or even in online calculators. In general, they seem to reflect what university research is showing. The calculators allow you to enter corn and seed prices. I would just caution that these are estimates of the average optimums for hybrids and yield levels and there is likely a fair amount of variation in what you might see compared to the predicted optimum in any field.
Here are a few final thoughts on populations. The optimum range in any field is often fairly broad, like 4,000-5,000 plants per acre and our recommendations seem to be in line with industry and research results. Underplanting will cost a few bushels, but save a few dollars on seed. Overplanting may bump yields but has a higher seed cost. Based on the research, there is some rough relationship with yield potential, but it’s not as close as you might think. Lower-yielding fields or parts of fields that experience drought stress often are good candidates for reduced populations. When it comes to variable rate seeding, returns to this will be best if you have dramatic differences in soil types and yield potential. There, cutting populations on low-yielding areas could save seed costs with little impact on yield. Lastly, when discussing populations in corn be sure to specify plant or seed populations as there is a difference of about 5 percent based on the Purdue study.
Editor’s note: Greg Roth is the Penn State Extension grain specialist and professor of agronomy.