Soybean Row Spacing and Population Studies

Modern soybean varieties can tolerate lower seeding rates and still produce good yields, providing there are no other issues with stand loss or stress. In southern Delaware two studies (2022 and 2023) observed no yield difference when planting full season beans (MG 4.2, late May planting date) when planted between 60,000 to 180,000 seeds per acre (Figure 1). You may consider dropping your seeding rates, considering your regional weather, planting date, and soil conditions.

Figure 1: Soybean yields by a) population, row spacing, and irrigation in 2022 (average yields below the legend) and b) irrigated yields by population and row spacing in 2023. Soybeans were planted in a coastal southern- Delaware climate.

Over both years, 15” row spacing provided an 8-to-10-bushel advantage compared to 30” rows. A narrower row spacing helps soybeans canopy faster, providing more leaf area per acre to increase yields. Water is consistently a major yield limiting factor, where irrigation boosted yields by 26 bushels in 2022. All these yield differences were statistically significant.

Planting Date Studies

Between 2020-2022, University of Delaware performed planting date trials for soybeans sponsored by the Delaware Soybean Board. All planting dates were at least two weeks apart, typically starting in mid-April (weather and field conditions permitting) and the final planting date in mid to late May (Table 1).

Table 1: Planting dates for each site and year of the study for early (1), mid (2), and later planting dates.
Year123
20208-May21-May4-Jun
202112-Apr28-Apr10-May
202213-Apr26-Apr11-May

Although earlier planting (April) did produce earlier canopy closure and sometimes earlier flowering, yields did not vary in any of the years. While there was no benefit to planting earlier, but there was also no yield penalty (Figure 2). In fact, we purposely did not apply a seed treatment to provide a more suitable environment for disease pressure. At the UD site in 2021, one row unit planted too deep in April, but yields remained similar to May planting. Freeze damage at UD in 2022 cause the death of some leaf and cotyledon tissue, but the soybeans recovered and matched later planted yields.

Figure 2: Soybean yields based on early (1), mid (2), and later (3) planting dates in Georgetown, DE. No differences were found to be statistically significant.

As there was no clear benefit to planting in mid to late April for soybeans, planting decisions can be made based on field suitability, disease presence, or tillage and soil temperatures. As noted in the population studies above, these decisions should be matched with local knowledge of yield potential, planting dates, and potential freeze damage.

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