Soybean seed has become more costly in recent years due to advances in biotechnology, higher-yielding germplasm, and a variety of available seed treatments. These advances have enabled lower soybean seeding rates than those of a few decades ago, yet seed remains a major expense—about 35% of soybean direct production costs under current practices. Optimizing seeding rate is therefore an important strategy for improving profitability.

Seeding rate is the number of seeds planted per acre, while plant population (or stand) refers to the number of plants per acre that successfully establish. The goal is to settle on the number of plants (stand) needed to maximize sunlight interception and yield potential, then to choose seeding rates to get to this stand. Because we can’t precisely predict the percentage of planted seeds that will produce healthy plants, and soybeans show a fairly wide range of stands needed to produce highest yields, choosing a seeding rate always involves some guesswork.

Soybean exhibits high levels of phenotypic plasticity relative to other crops, allowing plants to adjust their growth and yield components (e.g., number of pods and seeds per plant) through branching in response to different plant population levels. We’ve seen in our research that fields with 80,000 plants/acre can look and yield similarly to those with 125,000 plants/acre, provided stands and spacing between plants are uniform, such that sunlight interception is high during seed fill. Other crops such as corn also exhibit phenotypic plasticity, but their ability to compensate for lower population densities is more limited, making yield loss at lower stands more predictable.

Yield response and economic optimum

Figure 2 illustrates soybean yield response to plant population in trials conducted at Urbana. Typically, soybean yield increases with plant population until a point at which yield plateaus and does not increase despite additional plants per acre. In some cases, particularly under severe stress such as drought or disease pressure (e.g., white mold), increasing plant population beyond the point of maximum yield can lower yield. In the examples shown in Figure 1, soybean yield followed a “quadratic-plus-plateau” (leveling off with no decrease up to the highest stand in the trial) response in 2017 and 2018, but in 2016, yield at the highest stand (~160,000 plants/acre) was lower than the yield at 110,000 plants/acre.  

The Economic Optimum Plant Density (EOPD) takes into account the ratio of the price of seed to the value of a bushel of soybean, thereby maximizing return on seed investment. The EOPD is achieved when the last increment of seed increases yield just enough to offset its cost; unless seed has no cost, the EOPD is always lower than the population that maximizes yield (Pmax) (Figure 1). Figure 2 shows the relationship between EOPD and the yield at the EOPD from 32 trials conducted in Illinois between 2015 and 2025. There was essentially no correlation between EOPD and yield at the EOPD, which ranged from 54 to 90 bushels/acre. In addition, soybean yield responses to population were highly variable: the two highest yields in the study came at 70,000 plants per acre (93 bushels per acre) at a site in 2015, and at 200,000 plants per acre (92 bushels per acre) at another site in 2018. Across trials, the Pmax exceeded EOPD by an average of 38,600 plants per acre, but yield at the EOPD was only 1.3 bushels/acre less than the maximum yield. Note that the cost of this number of seeds is about the same as the value of the 1.3 bushels of yield not realized when the seeding rate is optimized.

Determining seeding rate

Table 1 provides estimated EOPD values for a range of soybean prices and seed costs, based on the average of the response curves across 32 field trials. Higher soybean seed quality and better seed treatments have improved emergence in recent years, but stands can still be reduced by poor conditions after planting, especially wet soils. Using 85% as the default for stand establishment is often reasonable, although this percentage should be adjusted downward (and seeding rate increased) if planting into poor conditions or if standard (warm) germination is less than 90%. Factors such as excessive or insufficient soil moisture, suboptimal temperature, improper planting depth, soil crusting, insect damage, or disease can all lower seed germination and emergence.

For example, at a seed bag cost of $90 per 140,000 units and a soybean price of $11.00 per bushel, the EOPD is about 110,100 plants/acre. To calculate seeding rates from these values, divide by your expected percent establishment. For instance, 110,100 plants/acre divided by 85% establishment equals about 129,500 seeds/acre. Many companies sell 140,000 soybean seeds as a unit, and in this case, planting a unit or slightly less than a unit per acre should produce enough plants to maximize return to seed.

In Illinois, inoculants containing Bradyrhizobium japonicum are the most commonly used. Most inoculants on the market today are formulated to be applied to seeds during the seed treatment process. In most Illinois fields, seed inoculants generally result in little to no yield increase, as the bacteria can persist in the soil for several years after a well-nodulated crop. However, using a high-quality inoculant is recommended for fields new to soybean production, fields that have been out of soybean production for five years or more, or fields with a history of poor nodulation. Although rare in Illinois, the failure of soybeans to form nodules can significantly reduce yields, unless the soil has unusually high nitrogen levels.

Most seed companies offer fungicide and insecticide seed treatment packages to provide protection for emerging seedlings. The most significant benefit from seed treatments is likely to occur in no-till soils or early-planted situations (e.g., cold and wet soils, damped seedbeds, etc.) that slow germination and encourage seedling disease and damage caused by insect feeding, and when planting into a field with a history of problems with stand establishments.

Studies on soybean planting depth typically show that the depth that produces the best stand and yield varies greatly between seasons and soils. In general, though, emergence will be more rapid and stands more uniform if soybeans are planted at consistent depths of 1-1/4 to 1-3/4 inches. When planting into warm and dry conditions, there might be an advantage to planting deeper than normal to reach soil with enough moisture to get seeds to germinate; this should probably be no deeper than 2 1/2  to 3 inches, and maybe a little shallower in heavier-textured soils. Alternatives include planting at normal depth then letting the seed wait until it rains to germinate; or to wait to plant until after it rains. There is seldom a clear answer to which approach is best. In light soils such as sandy loams, planting up to 3 inches deep to reach moisture might be a good strategy since rainfall after planting on such soils poses little problem for emergence. In heavier soils, heavy rainfall after planting can restrict oxygen and cause emergence to fail. Waiting to plant until after it rains can result in considerable delays in planting and emergence.

Averaged over the past five years (2018-2022), Illinois producers reported planting 64% of their soybeans in 15-inch rows, 30% in 30-inch rows, and 6% in rows narrower than 15 inches. Much research has been done on row width in soybeans, with most studies showing that soybean yields increase when row width is decreased from 30 inches to less than 30 inches. In one set of trials across 40 Illinois sites from 2010 to 2018, 15-inch rows outyielded 30-inch rows at about half the sites; the average difference over all 40 sites was just over 2 bushels per acre in favor of 15-inch rows. Other row spacing trials have shown that the narrow-row advantage over 30-inch rows tends to diminish as rows reach 20 inches or less. In environments where water limits yields, row spacings of 10 inches or less may yield more than 15- or 20-inch rows. Drilling soybeans in 7.5-inch rows is still done, but drilled acreage continues to decrease.