Optimizing Corn and Soybean Seeding Rates

By Giovani Preza Fontes and Emerson Nafziger

The first half of April has been a little cooler than normal, with a very wide range in rainfall totals, from an inch or more below normal in northwestern Illinois to 6-8 inches above normal in south-southeastern Illinois. There is some rain in the forecast over the next week, but some progress in fieldwork and planting is likely. While it’s too early to consider planting in wet soil, there is no reason not to plant as soon as conditions allow.

Seed costs for corn and soybeans have risen considerably in recent years, along with genetic yield potential and improved ability of planted seeds to emerge and establish productive plants. This improvement, along with better planting technology, has decreased seeding rates for soybean over the past two decades. Still, seed accounts for 34% of direct costs to produce soybeans in 2025, and 26% of direct costs to produce corn (Paulson et al, 2025). While there’s little gain in risking yield loss from having too few plants in the field, we want to avoid having more plants than needed to maximize the economic return to seed.

Seeding Rate and Plant Population Density

Seeding rate is the number of seeds planted per acre, and plant 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 needs to keep increasing; in fact, 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.

With their ability to “flex” to fill in space between neighboring plants, soybean plants can adapt to varying population levels a little better than corn can. So, a soybean field with only 80,000 plants per acre may look and yield about the same as one with 125,000 or more plants per acre. Their ability to do this, though, depends on having plants close enough to one another to minimize the area where sunlight hits the soil during seed-filling.

Yield Response to Plant Population Density

Figure 1 shows the responses of corn and soybean yield to plant population in trials conducted in Urbana. Yield typically increases with 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 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 Fig. 1, 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. The responses for soybean also differed by year.

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. As the number of plants per acre increases, corn kernel or soybean seed number per acre tends to increase, as weight per seed decreases; maximum yield is at the plant population where these changes just offset each other. 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.

It seems logical that high corn yields require having a lot of plants per acre. Our trials show, though, that the optimum plant density is not correlated to yield level. High-yielding sites do not always require very high populations (Fig. 1). And low-yielding sites can sometimes require fairly high populations; for example, seed number per ear can be limited by stress at pollination, but if that stress is relieved after pollination, kernels may reach normal size, and yield might be higher when there are more ears. 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).

Trials in Illinois from 2015 to 2018 showed that the most consistent economic return to seed for corn was reached at harvest populations of 34,000 to 36,000 plants per acre. For soybeans, maximum return occurred at harvest populations of about 115,000 to 120,000 plants per acre. For both crops, these are the actual stands at harvest, not seeding rates.

Final Thoughts

While 100,000 soybean plants per acre (or even fewer) may achieve over 95% of yield potential in many cases, our data indicate that over a wide range of conditions and yield levels, we should aim for establishing more plants than that. This will help when field conditions at or after planting lower emergence and stand establishment, and in those cases – which are not well-understood and can’t be predicted – when soybean yield responds to higher plant populations. In one of our most unusual trials, 109,000 plants produced 84 bushels per acre, and 201,000 plants produced 92 bushels per acre. In another case, 70 thousand soybean plants produced 93 bushels per acre and 164 thousand plants produced 88 bushels per acre. We can’t know what might happen in any given field, but can use results over a range of trials to make our best guess.

Dry harvest conditions and low seed moisture at harvest lowered germination in some seed lots, so check the germination percentage when calculating your seeding rate. A number of factors—seedling disease, insects, slugs, soil crusting, or other stressors—can lower stand establishment. Typical stand establishment ranges from 80 to 90%, but can of course be lower, especially when it rains a lot after planting or where warm germination was lower than normal.

Optimizing Corn and Soybean Seeding Rates was originally published by Farmdoc.