Squeezing more bushels by seeding above optimum corn plant populations doesn’t work

So how thick should you plant your corn? That’s a consideration that grows in importance each year, given the cost of seed corn. Below, Bob Nielsen, Purdue University Extension agronomist, gives his take on this question backed by nearly 100 field-scale trials that he and Jim Camberato, a Purdue soil scientist, have conducted. You can link here for the story in its entirety.

This story deviates from normal style in that PLANT POPULATION is capitalized. Nielsen does this to point out that corn responds to the actual plant population in the field, not directly to the seeding rate at planting. Percent stand success is rarely 100%. In Purdue field trials, the average percent stand is 95%. For others, though, that number may be 90% or 98% or 85%.

Seed corn represents the single-most expensive variable input cost for Indiana corn growers, so choosing the most economical seeding rate is important for maximizing that dollar return to seed at harvesttime.

Choosing the most economical seeding rate involves balancing the cost of the seed corn and the price you expect to receive for the harvested grain when you sell it. Just as importantly, the most economical seeding rate depends on the yield response of corn to final PLANT POPULATION.

Jim Camberato (jcambera@purdue.edu) and I conducted nearly 100 field scale trials around Indiana from 2008 through 2019 to document the yield response of corn to PLANT POPULATION. The complete summary of that research is available online.

The accompanying table below comes from that summary and illustrates the average yield response of corn to final plant population for 83 trials that represent a range of growing conditions that we characterize as “normal” for Indiana. In particular, those 83 trials were not subject to severe drought conditions. Mathematically, maximum corn yield occurred at a final plant population of about 32,000 plants per acre.

Purdue University plant population chart

Note how “shallow” or nearly “flat” the yield response curve is for those 83 trials. That flatness reflects how tolerant today’s hybrids are to higher populations, much more so than hybrids were 30 years ago. However, also note how tolerant today’s hybrids are to lower populations. The tolerance to both low and high populations results in the shallow response curve.

The upshot of such a shallow yield response curve is that squeezing one more bushel from a field by increasing seeding rate, when the POPULATION is near the optimum to start with, requires more seed than you can afford.

In fact, our data suggest that potential yield at final PLANT POPULATIONS ranging from about 28,000 to 35,000 PLANTS per acre at harvest varies by only +/- 1 bushel per acre!
The dollars-and-cents upshot of the shallow yield response curve is that the economical OPTIMUM PLANT POPULATION for corn throughout most of Indiana is amazingly low. The table above, also from our complete online summary, provides estimates of ECONOMIC PLANT POPULATIONS for a range of seed corn costs and market grain prices.

The astute reader will have noticed my use of the capitalized term “PLANT POPULATION”. The reason is that corn responds the actual plant population in the field, not directly to the seeding rate because percent stand success is rarely 100%. In our own field trials, we know that average percent stand is 95%. For other folks, that number may be 90% or 98% or 85%.

Assuming you know your typical percent stand from past field scouting, then you can calculate the seeding rate that targets a desired economic optimum plant population by simply dividing the target PLANT POPULATION by the percent stand. For example, if you are aiming for a final stand of 30,000 PLANTS per acre and your average percent stand success is 95%, then the seeding rate to achieve that target would be 30,000 divided by 0.95, which would equal a seeding rate of about 31,600 SEEDS per acre.

The final comment I would make is that optimum PLANT POPULATION for drought-prone fields is obviously lower than for fields with adequate soil moisture. Our data suggests that the optimum population for droughty fields is 5,000 to 10,000 fewer PLANTS.