Many agronomic practices and standards need periodic evaluation. Soybean population studies are being evaluated across the United States and observing that lower populations do not necessarily result in lower yield. While Delaware results may be similar to those of other regions, our climate and soil types will still have an interaction with plant population.
This study will observe yield results from 5 different planting populations between 60-180,000 seeds per acre in double-crop soybean systems. Double crop populations are typically planted at higher rates than full season beans to make up for the loss of vegetative growth with later plantings. While full-season beans have shown resilience to lower populations under irrigated conditions, we still have to evaluate double-crop planted soybeans for the best recommendations.
Additionally, double crop soybeans typically follow a small grain, similar to full season beans following a cover crop. At this stage, wheat residue has a high C:N and breaks down slowly. How this breakdown interacts with population, row spacing, and canopy closure may provide additional information to those who would like to add residue management to planting decisions.

1) Plant double crop soybeans at five rates between 60-180,000 seeds per acre.
2) Plant double crop soybeans at two row spacings (15 + 30”).
3) Evaluate yield response to planting populations and row spacings.
4) Evaluate wheat residue breakdown under different populations and row spacings.

Soybeans will be planted at the Carvel Research and Education Center in Georgetown, DE into a June harvested wheat field. Soybeans will be planted at five densities (80, 100, 120, 140, 160 thousand seeds per acre) and two row spacings (15 and 30 inch) and irrigated throughout the season. Previous drone flights over the wheat will be compared to biomass and yield data to determine potential interactions by plot and block. Subsamples of wheat fodder will be collected from outside the plot boundaries. Biomass will be separated into decomposition bags for each plot (30 rye and 30 corn fodder), weighed, and placed back into the planted plots in the center of a row. Six subsamples of each will be dried and saved to determine the initial carbon (C), N, and moisture content of the biomass. Six more decomposition bags will be setout side the plots as controls to measure breakdown without soybean canopy. At the end of the season decomposition bags will be collected from the plots, dried, weighed, and analyzed for C, N, and the biomass loss. In three population plots (80, 120, 160), logging sensors will be placed in the 15 and 30” rows to measure EC, moisture, and temperature throughout the season. Yields will be collected
with a plot combine in the late fall.

Data will be analyzed in SAS as a randomized complete block design structured by a factorial including biomass loss, changes in C and N, as well as yield. Yield will also be correlated to various predictors from the study.

Updated July 24, 2024:
Following wheat harvest in June, 2024, soybeans were planted at populations ranging from 60,000 to 180,000 seeds per acre at both 15 and 30" row spacings. Drone flights have been performed for both the wheat growing season and soybean emergence. Deer damage has been more persistent in this field than in the past, but plots are beginning to pull away from the foraging and appear to be in good shape. Irrigation has continued and been performed earlier than normal due to the persistent drought conditions in southern Delaware.

Updated January 3, 2025:
Soybean plots were harvested in October 2024 using a plot combine, ahead of schedule due to the drier conditions. Even with irrigation, plots dried down faster than usual. Yield data has been analyzed and included on an article in the Delaware Agronomy Blog and for presentations at Delaware Ag Week.

Updated April 3, 2025:

View uploaded report

Double-cropping, where soybeans are planted after a winter small grain crop like wheat, is a common practice in the Mid-Atlantic region. The longer growing season and the ability to grow winter grains make this practice especially beneficial. However, optimizing planting methods for soybeans can help improve yields. In southern Delaware, we tested a range of planting populations, from 40 to over 180 thousand plants per acre, along with two row spacings—15 inches and 30 inches between rows. These combinations were evaluated to see how they influenced soybean yield under double-cropping conditions.

For planting population, the results showed that higher planting populations (greater than 120 thousand seeds per acre) produced higher yields. While all populations (from 40 to 180 thousand plants per acre) had some impact on yields, the densest plantings consistently outperformed the lower populations. This suggests that increasing the number of plants per acre helps optimize soybean growth and improve yields in double-cropping systems. For row spacing, narrower rows (15 inches) resulted in better yields compared to wider row spacing (30 inches). Soybeans planted in narrower rows had better sunlight exposure and more room to grow, which enhanced plant development and overall productivity.

For double-cropped soybeans in the Mid-Atlantic, increasing planting population beyond 120 thousand seeds per acre and using 15-inch row spacing are key practices for boosting yields. While lower populations (down to 90 thousand plants per acre) have maintained yields in irrigated, full season systems, double crop requires higher populations. These recommended planting rates are still lower than previous University recommendations for our region.