Timely planting is a critical management practice for optimizing yields. Key yield benefits of planting soybeans early are the potential to produce more nodes on the main stem and early canopy closure, which enables more light interception to drive photosynthesis. Illinois producers should consider several planting best practices to maximize grain yield and economic returns.
Planting date
Figure 1 presents a summary of 30 planting date trials conducted in central and northern Illinois. The target planting dates were set for mid-April, with subsequent plantings occurring approximately every two weeks until mid-June. Each trial involved four planting dates. The highest yields were achieved when planting occurred between the second and last week of April. 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 87% of the maximum by May 31, 79% by June 15, and 76% by June 20. While we did not plant before April 10 in these trials, the fact that yields were no higher from mid-April planting than from planting at the end of April indicates that the “early planting” advantage was generally maximized if planting was done by the end of April. We see more variability with planting in late May and June because conditions later in the season can add to or diminish potential problems from late planting.
Planting soybeans in June often leads to shorter plants with fewer nodes and reduced yield potential. To counter some of these morphological changes, late-seeded soybeans can be planted in narrow rows and at a higher seeding rate than what is typically used for early planting. Double-crop soybeans, which are planted after wheat harvest and thus always sown late, may also benefit from narrow rows and slightly higher seeding rates. If soybean planting extends into July, producers might consider selecting a variety with a maturity group (MG) 0.5 to 1.0 earlier to reduce the risk of fall frost damage. However, this may reduce yield potential, which may or may not balance out the advantages of earlier maturity.
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
Research in Illinois and elsewhere has shown that soybeans attain maximum yield when the harvested plant population is about 100,000 plants per acre when the crop is planted at normal times. In some cases, only 50,000 plants have produced a yield as high as plant populations 2 to 4 times that high. This illustrates the capacity of soybean plants to produce more branches, pods, and seeds per plant when resources are available. Most data also show a very wide “plateau” over which yields respond little, if at all, to increasing or decreasing population. In rare cases, plant population can be high enough to reduce yield, but this seldom occurs unless conditions are dry, and having more plants causes faster water loss.
From 2015 to 2018, researchers evaluated soybean yield response to seeding rate in Illinois. In these studies, the average planted (% emerged) seeds per acre were: 50 (92% emerged), 100 (89%), 150 (89%), and 200 (86%) thousand seeds per acre. Overall, we found no discernible correlation between yield and stands. Across all sites, the stands that maximized yield (and maximum yield) ranged from about 60,000 (88.6 bushels per acre) to 201,000 (89.9 bushels per acre) (Figure 2).
Because soybean seed costs account for about 40% of the variable costs in soybean production, another approach is to look at the economic return to seeding rate, which is the rate at which the last thousand seeds added increase the yield just enough to pay for itself. When averaging all 25 trials together, our data shows that 115,000 to 120,000 plants (not seeds) per acre at harvest are often needed to produce the highest dollar return on the seed investment. If we plant good seeds into good conditions, we can expect 85% stand establishment, in which case we should plant about 140,000 seeds per acre, which for most seed companies today is one unit of seed. We can tinker with that based on conditions at planting: lowering it by maybe 10 or 15 thousand if conditions and forecast are both favorable, expecting about 90% stand establishment. If the forecast is for heavy rainfall after planting, raising the seeding rate in anticipation of more stand loss may not be as effective as waiting to plant until the threat of heavy rainfall is passed.
Inoculation
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.
Seed treatment
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.
Planting depth
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.
Row spacing
Averaged over the past five years (2018-2022), Illinois producer 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.
Giovani Preza Fontes, Extension Agronomist
Reviewed in 2024