Improved soil health, reduction in chemical and fertilizer needs, field workability and yield are all points of discussion with cover crops. For some farmers in Iowa, cover crops have been adopted and thrive as a management practice. Other farmers still have questions about the overall benefits and risks surrounding the practice.

Quantifying Results

Given these questions, the Iowa Soybean Association (ISA) Research Center for Farming Innovation (RCFI) team established a multi-year project in attempts to quantify some of the changes from continued use of cover crops in fields across the state. Today, we have 15 trial locations around the state that have continued with support from ISA. Our longest running trials will be entering their eighth year of the project in 2024. These trial sites consist of cover crops seeded in the same strips each fall, with uncovered check strips for comparison.

Improved Yield Stability

When looking at the yield differences, changes have been small and slow to show. No matter the cash crop, yields have been variable especially in the first few years of cover crop implementation. From 108 sites years analyzed, yield differences for both corn and soybean hovered near even when looking at years in cover (Fig. 1), with soybeans seeing a slight increase in yield during the later years. Average responses in corn ranged from approximately -1.25 to 1.25 bu/acre and soybeans between approximately -1.5 to 1.5 bu/acre. When looking across years in cover, yield variability is minimized with continued use of a cover crop beyond three to four years, indicating improved yield stability. The important takeaway from this is that while we don’t see a significant yield increase, we don’t see significant losses either.

Soil Health Indicators

Soil health sampling was completed in 2022 and 2023 in accordance with the NRCS CEMA 216 standard with partial funding from USDA NRCS. In addition to the basic suite of soil measurements, this testing standard included organic carbon, aggregate stability, soil respiration, POX-C and ACE protein. Of these soil health test measurements, our results showed little or no change between the cover and no-cover strips across sites.

Of the 19 sites sampled, five locations had positive significant differences based on the treatment (see Fig. 2). These positive changes were seen in aggregate stability, indicative of erosion resilience and water infiltration, and POX-C, indicative of available carbon. While it was encouraging to see these results, most fields sampled showed no change in these soil health indicators. It is important to note that many of the sites were already managed with no-till or minimal tillage that would promote soil health benefits on its own.
While biomass sampling was not done on a routine basis, our team used NDVI imagery collected from each site each year as a proxy for biomass accumulation. When these values were compared to calculated yield differences from the cover crop, we found that as the NDVI value increased (cover crop biomass increased) the yield differences in both cash crops trended down, with a greater impact on corn. Although goals for a cover crop are going to change from farmer to farmer, the data shows that our yields can be reduced with increased cover crop growth. In our experience, early cover crop termination or strip tillage could be practices that help mitigate yield losses, especially in the first few years of the practice. Farmers did note, however, that having the increased biomass provided a blanket on the ground that allowed for additional days suitable for field work during wet times in late spring and early summer.

Get Involved

Overall, the project has provided a wealth of information for our team to pull from, but it has also left unanswered questions that we hope to target in the coming years. ISA is leading a project called the Improved Cropping System. With support from Iowa State University, Iowa Corn Promotion Board and funding from USDA-NRCS, this project will continue to explore this group of long-term cover crop study locations. We hope to answer our additional questions and examine the benefits of a cover crop blend instead of a single species. Additional in-field measurements will include soil moisture sensing as a proxy for workability, weed counts to determine suppression, and additional soil health sampling incorporated every 2-3 years. Want to learn more about this cropping system? Contact us to be a part of the trial or to dive into the research further.

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