Timely planting is a critical management practice for optimizing yields. Long-term studies conducted in Illinois show that the optimum planting date for corn is mid to late April (Figure 1). Yields declined gradually with planting delays in early May, reaching around 93% of the maximum by May 15. However, yield loss became more rapid thereafter, dropping to 84% of the maximum by May 31 and 78% by June 8. There were a few instances where planting before April 10 lowered yield potential, and a few trials where planting in late May or early June produced the same yield as planting in mid to late April.

Another important consideration when deciding when to start planting is the soil condition and forecasted weather for the next 7 to 10 days. Planting into wet soils or “mudding in” can increase the risk of sidewall compaction and poor root development, especially if the weather turns hot and dry after planting. This can result in reduced plant stands, delayed emergence, and restricted root growth, all of which can negatively affect yield.

Seeding rate is the number of seeds planted per acre, and plant population density (or stand) refers to the number of plants established per acre. The goal is to establish enough plants to maximize light interception and yield potential, but not so many that yields actually decline, or rise so slowly with added plants that economic return is lowered. Improvements in corn genetics have increased stress tolerance and standability, which allows plant populations high enough to maximize yield without much fear of having too many plants. This does not mean that corn plant populations need to keep increasing; in fact, USDA-NASS survey data show that corn plant populations in Illinois increased by about 430 plants per acre per year from 2000 through 2014, but by only 110 plants per acre per year from 2015 through 2024, averaging around 32,000 plants/acre in recent years.

Figure 2 shows examples of yield responses to plant population in trials conducted in Urbana. Yield typically increases with increasing plant population up to a point, then levels off (“plateaus”) as plant numbers increase further. Plant populations much higher than those needed to maximize yield can occasionally lower yield, for reasons that include lodging and stress-related to lowering of seed numbers or seed weight. This has become rare with newer hybrids and varieties, especially in productive soils. In the examples shown in Figure 2, corn yield followed a “quadratic+plateau” pattern up to 42,000 plants/acre, but going to 48,000 plants increased yield a little in 2018 but decreased yield in 2027, a year with more late-season dryness.

Much like we do with nitrogen for corn (the MRTN approach), we can fit curves to population response and then determine the economic optimum plant density (EOPD) — the population density that maximizes profitability with regard to seed. The EOPD takes into account both the value of the crop (yield multiplied by grain price) and the cost of seed, and so varies according to input prices. The EOPD is reached when the last thousand seeds (or plants) increase the yield just enough to cover the added cost of that seed. The EOPD is always lower than the agronomic maximum, because adding seed past the EOPD produces less yield than needed to cover the cost of that seed. In the examples shown in Figure 2, the EOPD was 32,400 plants per acre in both years.

Figure 3 below illustrates the response of corn yield to plant population in 17 sites across Illinois from 2016 to 2018. Each trial included four to six hybrids, with planted populations (which closely matched final stands) ranging from 18,000 to 48,000 plants per acre. It seems logical that achieving high corn yields requires having a large number of plants per acre. However, our trials show that the optimum plant population density is not correlated to yield level; sites with the highest yields didn’t necessarily require the highest populations (Figure 3). Data from the NCGA National Corn Yield Contest supports this, showing no correlation between harvest population and yield. Over the past four years (2020-2024), high-yield entries (> 300 bushels per acre) had harvest populations ranging from about 27,000 to 50,000 plants/acre, with an average of 35,400 plants/acre (Pioneer, 2025).

Overall, the most consistent economic return to seed for corn was reached at harvest populations of 34,000 to 36,000 plants per acre. If there were a reliable method to adjust seeding rates field by field, profits could have been slightly improved. However, these trials were conducted on similar soils at four or five sites over three years, and the responses to yield and population varied more by year than by location (soil productivity). This highlights the challenge of identifying a consistent basis for varying seeding rates, especially in fields with uniform soil types and productivity.

Ideal planting depth varies with soil and weather conditions. Emergence is more rapido from relatively shallow-planted corn, so early planting should not normally be as deep as later planting. For most situations, though, corn should be planted 1-1/2 to 1-1/4 inches deep. Later in the season and under dry conditions, planting as much as 2-1/2 inches into moisture may be advantageous, especially if the forecast is for continued dry weather. Depper planting tends to result in reduced stand due to crusting or wet soils and an increased chance of uneven emergence, which can cause yield losses.