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) is the number of plants that successfully establish. The goal is to establish enough plants to maximize light interception and yield potential, then choose seeding rates to achieve this. Because we can’t precisely predict the percentage of planted seeds that will produce healthy plants, choosing a seeding rate to get to a particular stand always involves some guesswork.

Improvements in corn genetics have increased stress tolerance and standability, which means that we can aim for populations high enough to maximize yield without much risk 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.

Yield response and economic optimum

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 reductions in kernel numbers or kernel weight. This has become rare with newer hybrids, though, especially in productive soils. In the examples shown in Figure 2, corn yield followed a “quadratic+plateau” pattern in 2025, but going to 48,000 plants lowered yield under late-season dryness in 2027.

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 number of plants that maximizes profitability rather than absolute yield. The EOPD takes into account both seed cost and crop value (price per bushel); it is the point at which the last increment of seeds (or plants) adds just enough yield to offset the cost of that amount of seed. Unless the seed is free, the EOPD is lower than the population needed to maximize yield. But because yields rise slowly with population up to the maximum, the yield at the EOPD is only slightly less than the maximum yield. Across 23 Illinois trials between 2016 and 2025, the population needed to maximize yield exceeded the EOPD by an average of 4,000 plants per acre, but yield at the EOPD was only 1.7 bushels per acre less than the maximum yield.

Contrary to what we might expect, high corn yields do not require extremely high plant populations. Illinois trials show no correlation between optimum plant density and yield level, even with yields ranging as high as 280 bushels per acre (Figure 3). High-yielding sites often achieved maximum returns at moderate populations, while low-yielding sites sometimes needed higher populations; for example, when stress limits kernel number per ear at pollination, but kernels then fill normally under better conditions. NCGA National Corn Yield Contest data support this: between 2020 and 2024, entries over 300 bushels per acre had harvest plant populations ranging from about 27,000 to 50,000 plants per acre, with an average of 35,400 plants per acre (Pioneer, 2025). Many seed company recommendations are in line with these rates, not because higher populations may hurt yields, but because they aren’t needed to maximize yields of newer hybrids.

Determining seeding rate

Table 1 provides estimated EOPD values for a range of corn prices and seed costs, based on the average of the response curves across 23 Illinois field trials. Corn seed quality and the equipment used to plant corn seed have improved substantially over recent decades, resulting in greater consistency between the desired plant population and seeding rate. Research in Illinois and other Midwest states shows that corn stand establishment success typically ranges from 90 to 95%. For example, at a seed bag cost of $300 per 80,000 units and a corn price of $4.00 per bushel, the EOPD is about 34,100 plants per acre. To calculate seeding rates from these values, divide by your expected percent establishment. For instance, 34,100 plants per acre divided by 95% establishment equals about 35,900 seeds per acre.

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.