Research HighlightsIt Takes Time: Long-Term Cover Crop Results
Highlights:
- Years of strip trials comparing cover crops and no-till provide insight on yield and soil health over time.
- Soybean and corn yields can be maintained while using cover crops when termination is managed to prevent excess cover crop growth.
- In Iowa soils, cover crops reduce soil nitrates, improve aggregate stability and address soil compaction.
By Laura Temple
For years, farmers have heard about using cover crops.
And for years, a handful of Iowa farmers conducted cover crop strip trials. These long-term trials gather data and experience that deepen understanding about how cover crops work in a soybean-corn rotation and influence yield and soil characteristics.
The Iowa Soybean Association has been investing in on-farm cover crop strip trials since 2016 through the Research Center for Farming Innovation, or RCFI. Over time, similar long-term trials initiated by the Soil Health Partnership have been incorporated into this statewide study, providing a unique dataset. In 2024, Iowa Soybean Association partnered with the Iowa Corn Promotion Board to maintain and strengthen this long-term work focused on conducting specific experiments to supplement the data collected through the years. In addition, this research has leveraged Soy Checkoff and corn promotion board investments to secure funding from USDA-NRCS and industry partners like BASF.
The replicated strip trials compare a no-till system to no-till plus cover crops. Cooperating farmers seed cover crops over the same strips each year. In 2025, 14 locations participated throughout the state. Trials primarily focus on yield, but the research team gathered soil health data in 2022 and 2023, as well.
“Soil health takes a long time to change,” says Matt Carroll, Ph.D., analytics and insight lead for the Iowa Soybean Association. “And farmers adopting cover crops have found it takes three to five years to figure out how to make the system work for them. These trials provide information to help other farmers succeed with cover crops more quickly.”
Maintaining Yield
Strips with cover crops don’t out-yield no-till.
“Most years, there is no notable difference in yield,” Carroll reports. “The exception is when cover crops aren’t terminated before they add high levels of biomass.”
Analysis of years of data showed that in both 2017 and 2024, cover crops caused notable yield drops. Carroll says that wet springs in both of those years made timely termination difficult and allowed cover crops to grow taller. The additional biomass challenged early season crop growth, and ultimately, crop yield.
“We now encourage farmers to have a backup termination plan for cover crops,” he says. “We’ve correlated revenue loss to when cover crops get taller than roughly calf-height.”
Building Soil
In gathering soil data, the team conducted all the soil tests outlined in the USDA Conservation Evaluation and Monitoring Activity, or CEMA, protocol on samples gathered for testing for consistency moving forward.
Those tests found very little difference in soil organic matter, microbial activity and plant-available carbon between no-till and cover crop strips. The autoclaved citrate extractable protein, or ACE protein test, which is associated with soil nitrogen levels, also showed very little difference between the strips. Carroll attributes these findings to the relatively high levels of organic matter in many Iowa soil types.
However, soil tests found that cover crops significantly improve aggregate stability and soil compaction.
“Aggregate stability relates to soil structure,” Carroll explains. “As aggregate stability improves, soil resilience to erosion and water infiltration also improves. This can help soils manage water better during drier summers and wetter springs.”
Managing Nutrients
The research team followed up those soil tests with sampling for nutrient stratification in 2024. Stratification occurs when immobile nutrients like phosphorous and potassium build up in some soil layers, instead of being distributed evenly. This tendency often occurs in no-till systems because the soil isn’t disturbed.
“While we didn’t see differences in nutrient distribution at different soil depths between no-till and cover crop strips, we did find that prior to planting, cover crops reduced soil nitrate levels 44%,” Carroll says. “That benefit lingers. In June, soil that had cover crops contained 36% less soil nitrates 7 to 14 inches deep.”
These real-world results demonstrate that cover crops scavenge residual nitrogen and reduce the potential for nutrient loss, especially during wet weather.
Ongoing Research
Years of data have prompted questions that the ongoing trials aim to address. For example, cooperating farmers used cereal rye as the cover crop before both soybeans and corn for years. After seeing the soil health data, they seeded a blend of cover crop species in the fall of 2023 to see how that might influence soil characteristics like microbial activity and nutrient availability.
“We also plan to take a closer look at sulfur management in soybeans and nitrogen management in corn when using cover crops,” Carroll adds. “These trials have raised questions about the availability of these nutrients to the crop at the right time following cover crops.”
Data patterns that lead to such recommendations emerge because these trials have been repeated for so many years. Carroll and his colleagues are excited about how farmers can apply what they have learned, and they plan to explore questions spurred by ongoing results.
“We appreciate participating farmers allowing us to continue these trials,” he adds. “They have learned what they need to know about cover crops, but they continue doing these strip trials for the greater good.”
Additional Resources
Long-Term Research Incorporates Cover Crop Questions to Uncover Solutions – SRIN article
Cover Crops in Iowa Long Term Study Results – Iowa Soybean Association guide
2024 Cover Crop Impact on Yield – Iowa Soybean Association publication
2024 Cover Crop Impact on Nutrient Stratification and Soil Compaction – Iowa Soybean Association publication
2023 Cover Crop Impact on Yield and Soil Health – Iowa Soybean Association publication
Published: Oct 13, 2025
The materials on SRIN were funded with checkoff dollars from United Soybean Board and the North Central Soybean Research Program. To find checkoff funded research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.
