The idea is to initiate research on the continuous soybean cropping system in Iowa to accumulate relevant knowledge to aid decision making. Currently, there is no soybean after soybean cropping system in Iowa, but this may change in the coming years, and we need to be prepared. For instance, in Argentina, soybean after soybean is a very common. Farmers adopted the soybean-soybean system because of the increased profitability (no nitrogen fertilizer input, less field operations).
We propose to establish long-term plots at three locations in Iowa (TBD). In each location we will establish four systems: 1) soy-soy, 2) soy-corn, 3) corn-soy, and 4) corn-corn, to have all crop phases every year. Each system will be replicated 3-4 times and will have large plots (e.g., 24 rows x 200 ft). This way, different teams can work on different research questions on the same plots (so we connect agronomy – plant pathology – soil science), while have enough area to measure crop yields. The experiments will be designed by getting input from the multi-disciplinary team and will be instrumented with sensors to support system level assessments and extension demonstration efforts.

There are many questions associated with the continuous soybean system. For example, what is the yield penalty, how does this penalty vary from year to year and from location to location? What are the causes for the yield penalty and how can we close it? Is it profitable? How does the continuous soybean system impacts soil hydrology, N cycling, soil carbon, and overall sustainability?
We expect to answer some of the above questions, in particular the yield penalty which is the foundation to determine profitability. Further, we will leverage the established research plots to develop future proposals to continue soybean research. Lastly, the established plots will support extension efforts and field-days.

*set up (at least) two long term experiments with multiple cropping systems within each experiment. The cont-soy system will be the dominant system and we will add other systems (with corn and cover crop) for comparisons
*collect, analyze and report productivity data
* collect, analyze and report soil water and nitrogen data

Updated February 2, 2024:
The Continuous Soybean experiment was established in May 2023 in two locations: at the Agricultural Engineering and Agronomy farm in Boone, IA and at the Northwest Research Farm in Sutherland, IA. Both locations are characterized by highly productive soils, soil with low soybean cyst nematode (SCN) counts, and shallow water table. In each experiment we investigated seven cropping systems, all planted in no-till, were established in both locations in fields that were planted with soy in the 2022 season: 1) Continuous maize, 2) Maize-soybean rotation 3) Soybean-maize rotation, 4) Continuous soybean with 30 inch row spacing, 5) Continuous soybean with 15 inch row spacing, 6) Continuous soybean with winter cereal rye cover crop and 7) Continuous soybean with winter cereal rye cover crop and Fall applied manure. We established large plot sizes to allow extensive data collection by different groups (Fig. 1). Plots are 40ft X 100ft in Boone and 40ft X 80ft in Sutherland, with four replications. Soybean plots were split-plots, so that two varieties with different SCN resistance were planted. The total number of plots per experiment was 7*4*2=64 plots. In Boone, crops were planted May 5th, 2023; maize was a Dekalb hybrid 110-day, and soybean varieties were XO2963E (2.9MG, Peking SCN resistance source) and XO2832E (2.8MG, PI88788 resistance source). In Sutherland, crops were planted May 17th, 2023; maize was a Dekalb hybrid 105 RMG, and soybean varieties were P25A16E (2.5MG, Peking SCN resistance source) and P21A53E (2.1MG, PI88788 resistance source). For plots receiving treatments (6) and (7), winter cereal rye (Hazlet variety) was seeded using a high clearance cover crop seeder at a rate of 80lb/a in soybean plots at start of soybean senescence on both locations. Crops harvested October 10th in Sutherland and October 9th in Boone. For plots receiving treatment (7), chicken manure was broadcasted at a rate of 2 ton/a on November 6th in Sutherland. In Boone, manure source was beef compost, which was broadcasted at a rate of 10ton/a. All chemical control applications were carried out to reflect farmers’ practices. At the Boone location, a control plot of 40ft X 220ft was planted to have a measure of the yield penalty of continuous soybean in the 2023 season. The control was planted the same day as the main experiment; with same soybean varieties XO2963E and XO2832E, but at nearby field that was cultivated with maize in the 2022 growing season.

Measurements were taken along the growing season to characterize crop growth and development, soil, soybean cyst nematode pressure, and field hydrology. Aboveground biomass was collected at stages V14, R2 and R6 for maize and R2, R5 and R6.5 for soybeans. Grain yield was determined by machine harvesting the center 6 rows in Sutherland and center 4 rows in Boone of each variety and plot. Soil and aboveground residue were collected before planting at 0-12 and 12-24inch depth for initial condition assessment. Each subplot was sampled for soybean cyst nematode counts both before planting, and after harvest. To better understand soil hydrology dynamics, a total of 24 sensors were installed in each location in the continuous maize and continuous soybean plots. Tubes down to 8ft were installed to allow access to TDR TRIME-PICO IPH/T3 and SOP 503 HydroProbe-Moisture Neutron Gauge to measure soil moisture at 12inch intervals. Sentek-BT sensors were installed in selected plots to measure soil moisture and temperature down to 40inch depth at 2inch intervals. Solinst 3001 Edge Leveloggers were installed to measure soil temperature and water table depth at different points of the field. Additionally, throughout the season, drone flights with RGB and NDVI cameras happened weekly to observe canopy closure. Currently, the experiment is ongoing, winter cereal rye is developing in the field ahead of the 2024 experimental year, and hydrology sensors are performing measurements aiming to get a whole year profile of soil water dynamics in two Iowa environments.
Through field data and crop modelling, the experiment established in 2023 has already allowed us to gain some insight into the continuous soybean system, and how its productivity and environmental performance compares to the other cropping systems established. In Boone, there was no yield gap associated with growing second year soybeans in 2023 (Fig. 2). In both locations, soybean varieties with different SCN resistance sources had the same performance (Fig. 3), that was expected given the low Spring SCN egg counts. Studies of the hydrology data obtained from the sensors in this study have given us important information on the importance of water tables to crop production. Regardless of a year with greater potential of water deficit in many regions of Iowa, we found the system to be resilient, as subsoil water was able to support normal yields in both locations. The observed water table dropped approximately 3ft at time of peak crop growth, agreeing with previous studies from the Archontoulis lab. Through measurements of volumetric soil water content at different depths and times of the growing season, we observed that as the season became drier, crops relied on soil water stored 4 to 6ft in the soil profile for growth (Fig. 4).

For doing a preliminary assessment of environmental performance of the cropping systems established in this experiment, APSIM 7.9 was used to build long-term simulations representing each treatment. Simulations were run without reset, using 30 years of local weather data for Boone. The model provided the following outputs: grain yields, biomass production, grain N uptake, soybean Nfixation, N2O emissions, N leaching, net mineralization, soil organic carbon, plant transpiration, soil water evaporation, water table depth, runoff, and subsurface drainage. We calculated N balance as inputs (N fertilizer + N fixed) – outputs (N in grain). Cropping systems were compared based on these variables. Overall, we found that soybean-based systems had higher net N mineralization, and lower N balance compared to corn-based systems. Continuous maize had higher N-leaching and N2O emissions than all other systems. Inclusion of cereal rye cover crop was important to aid in reducing the rate of soil organic carbon loss in the topsoil when soybean is the main crop. These are all important findings on alternative ways to reduce N loss to the environment and design more sustainable systems.

Throughout the year 2023, we had opportunities to disseminate results and insights gained from the first experimental year of the continuous soybean project. On August 2nd ,2023 we received the Iowa Soybean Association (ISA) Experience Class, a group of around 35 farmers and staff, on the experiment site in Boone, IA. There was productive conversation around soil moisture measurement methods and importance, weed management in a continuous system, cover crop benefits and choice. At the 2023 TriSocieties Meeting held in St. Louis-MO on November 1st, we presented a poster titled “Evaluating environmental and economic performance of soybean-based systems using APSIM”, where we contrasted the cropping systems’ profitability and Carbon, Nitrogen, and water balances using model outputs. Additionally, soil hydrology results from both experimental sites were presented in two sessions at the Iowa State University Integrated Crop Management conference held at Ankeny, IA on December 4th, , the talk was titled “What do deep soil moisture measurements tell us about water stress in Iowa?”. The public consisted mostly of farmers and industry professionals, with a turnout of around 100 attendees. The Continuous Soybean experiment is currently ongoing, and next steps include winter cereal rye biomass collections, Spring initial soil conditions sampling, initial soil water measurements, and similar data collection in the 2024 experimental year.

See attached document for figures and graphs.

View uploaded report

Updated June 24, 2025:
Project summary

Objectives: the main research questions we answer with this study are:
- What is the magnitude of the soybean-soybean yield penalty and how does it vary from year to year?
- Can management practices such as use of cover crops close the yield gap of continuous soybeans?
- Does the pest/disease pressure build up in the soybean monocropping systems?
- Is continuous soybean a profitable cropping system?
- How does the continuous soybean system impacts soil hydrology, N cycling, soil carbon, and overall sustainability in the long term?

Benefit to Soybean Farmers: Iowa soybean farmers could potentially benefit economically from planting soybeans after soybeans (with/without rye cover crop in between) in time periods when input prices are high or in the scenario of higher soybean demand to meet crushing capacity. Soybeans require no N fertilizer inputs and have a lower cost of production compared to corn. Also, current policies that aim to offset the cost of increasing biodiesel production while adopting conservation practices (e.g. 45Z) could increase demand for more soybean production. There are risks associated with introducing a new cropping system, so it is important to test and optimize the management of the continuous soybean system, and quantify its risks, benefits, and economics at the university level before it is implemented in farmers’ fields.

Progress update

Field experiment establishment: the continuous soybean project was established in 2023 in two IA locations, Boone and Sutherland. It explores 7 cropping system treatments (plot), with 2 soybean cultivars (PI88788 and Peking resistance, subplots), each replicated 4 times. Plots are 16 rows by 100ft to allow for extensive data collection and collaboration across multiple labs. The cropping systems explored are: 1) soy-soy in 30” rows, 2) soy-soy in 15” rows, 3) soy-rye cover crop-soy, 4) soy-rye cover crop-soy + manure, 5) corn-corn, 6) corn-soy, and 7) soy-corn. The data collected covers a wide range of agronomy research areas, from drone imagery, hydrology to crop physiology, soil, and soybean pathology. The Continuous Soybean experiment is currently ongoing in year 2025, we will carry similar data collection as in the 2024 experimental year to build capacity and gain confidence on conclusions and recommendations.

Field experiments results: there was no yield gap associated with growing third year soybeans in 2024 (Fig. 1). In both locations, we observed a trend of increase in soybean cyst nematode (SCN) pressure in plots of continuous soybean. There is large variability in the measurements, and this trend was not statistically significant (Fig. 2). Studies of the hydrology data obtained from the sensors in this study have given us important information on the importance of water tables to crop production, and patterns of water use for both corn and soybeans. Through measurements of volumetric soil water content at different depths and times of the growing season, we observed that in 2024 crops relied on water stored at the top 90cm of the soil profile, with similar uptake amounts and pattern for corn and soybeans (Fig. 3).

Modelling study results: data from field experiments also support modelling studies for long-term assessments of cropping systems’ performance. We have calibrated and validated the APSIM cropping systems model in the continuous soybean plots and already performed a systems evaluation in collaboration with ISA personnel (Freitas et al., 2025). Some highlights of our findings are:
• Intensifying soybean-based systems offers the opportunity for higher and more stable yields (Fig. 4)
• Inclusion of a winter rye cover crop to soybean-based systems improved system nitrogen use efficiency, and reduced N2O emissions (Fig. 6)
• Water use efficiency can be improved through management compared to current soybean management practices (Fig. 6)
• Soybean systems had similar profitability to maize monoculture using current production cost estimates and market prices (Fig. 5)

Freitas, C., Hart, C., Litch, M., Hubber, I., Castellano M., Helmers M., McClure, J., Herman, M., Grassini, P., Northrup D., & Archontoulis, SV. (2025). Economic and environmental evaluation of soybean-based cropping systems in the US Midwest. Agricultural Systems Journal – in-review

Outreach: throughout the year 2024, we had opportunities to disseminate results gained from the continuous soybean project. During July 2024 we hosted ~60 farmers and Iowa Soybean Association (ISA) staff on the experiment site in Boone, IA during the ISA Field Day and the ISA Experience Class. Also in July, we welcomed a group of ~20 scientists who were attending the APSIM workshop. Throughout the month of August, we hosted at the plots ~65 soybean producer farmers from Brazil and Argentina who were attending the Farm Progress Show. Insights from modelling studies, supported by the field data collected at the continuous soybean trials, were presented at the 2024 Tri-Societies Meeting held in San Antonio -TX on November 10th and summarized in a scientific article currently under review.

See attached document for figures and graphs.

View uploaded report

This research will bring new data and knowledge on the continuous soybean that may increase overall farm level profitability. Despite yield penalties, the soy-soy system is profitable in Argentina. We hypothesize that there will be cases in Iowa (location x management practices) that this system will be more profitable that current systems.