Diversification of Corn-Soybean Rotations with Cereal Rye/Red Clover: Impacts on Nitrogen Availability in Corn

Final Report for GNC14-193

Project Type: Graduate Student
Funds awarded in 2014: $9,144.00
Projected End Date: 12/31/2015
Grant Recipient: Iowa State University
Region: North Central
State: Iowa
Graduate Student:
Faculty Advisor:
Dr. Matt Liebman
Iowa State University
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Project Information

  • Cultivation of cereal rye for cover crop seed offers the possibility of frost-seeding a legume into the standing rye crop in early spring, where the legume can produce biomass and fix N following rye harvest.
  • Farmer-cooperators Tim Sieren and Dick Sloan grew corn in rotation following cereal rye frost-seeded with red clover.
  • Iowa State University graduate student Will Osterholz quantifed N uptake by corn as well as two measures of N release from soil organic matter: net N mineralization and gross ammonification.
  • Key findings
    • Red clover did not improve corn growth, N content or grain yield compared to synthetic N fertilizer.
    • Soil N mineralization rates in August tended to be higher with red clover compared to synthetic N fertilizer, but differences were not statistically different.
    • Fertilization with supplemental N at planting could provide corn with early season N before clover decomposition can provide sufficient N to the corn crop in late summer.


Corn (Zea mays) and soybean (Glycine max) rotations are the major land use in Iowa.  While these systems are highly productive, conventionally managed corn-soy systems usually require large applications of nitrogen fertilizer during the corn year to achieve high corn yields.  Most corn-soy systems lack a growing crop in late winter and early spring, which elevates the risk of nutrient losses to the environment, which in turn impacts farmer profitability and environmental quality.  Adding an overwintering small grain mixed with a frost-seeded legume to the corn-soy rotation may reduce mineral N fertilizer requirements and mitigate negative environmental impacts of this cropping system.  The small grain provides cover in the late fall to early spring period to reduce N losses, and the legume acts as an N source to the following corn crop.  Cereal rye (Secale cereale) has gained popularity in Iowa as a winter cover crop following soybean in corn-soybean rotations due to its ability to overwinter successfully.  Consequently, the increased demand for cereal rye seed has increased the price of rye grain, which may favor the economics of growing cereal rye for grain.  Cultivation of cereal rye for grain offers the possibility of frost-seeding a legume into the standing rye crop in early spring, where the legume can produce biomass and fix N following rye harvest.  Red clover (Trifolium pratense) is well adapted to frost-seeding into a small grain and can produce a large amount of biomass with significant N content following small grain harvest.   The clover biomass will decompose following termination, gradually releasing N during corn growth which could potentially replace part of the fertilizer required for corn. 

While crops have traditionally been thought to utilize only mineral N that soil microbes fail to immobilize (i.e.net mineralization and nitrification), recent findings suggest that they can also successfully compete with microbes for actively cycling mineral N (i.e. gross mineralization and nitrification).  This suggests that N availability is controlled by both the pool size of mineral N in the soil as well as the gross rates of N cycling.  Diversified cropping systems that utilize legumes as a major N source could maintain high rates of gross N cycling, producing a stream of mineral N from a large organic N pool which is rapidly taken up by plants or microbes.  We proposed that including cereal rye/red clover in corn and soybean rotations will enhance both net N mineralization and gross N mineralization, and thus increase N supply to a subsequent corn crop.

This project quantified N uptake, net N mineralization, and gross N mineralization in a corn crop following cereal rye/red clover.  Utilizing two established on-farm trials in eastern Iowa, we measured soil and plant N in the corn year of corn-soy-rye/clover rotations.  Results of this project were shared with farmers and researchers to improve awareness of the feasibility of utilizing small grain/legume mixes to diversify corn-soybean cropping systems and attendant effects on N cycling.  This information will contribute to the development and implementation of cropping systems that will enable farmers to better optimize N fertilizer application rates in diversified cropping systems, providing high levels of productivity with reduced input costs while simultaneously improving environmental quality in Iowa.


Project Objectives:

Objectives met by this project included the development of knowledge by regarding the effect a cereal rye/red clover crop on N cycling in a subsequent corn crop. This project increased farmer and researcher understanding of the feasibility and desirability of diversification of corn-soybean rotations with a small grain/legume crop, and should encourage further research into this topic by farmers and researchers interested in this topic. The project outputs will contribute to the long-term goal of increasing Midwestern farmer adoption of diversified cropping systems in order to improve the environmental and economic performance of agroecosystems in the region.

Primary outputs completed as a result of this project included a presentation at the 2015 Practical Farmers of Iowa cooperators meeting and a PFI research report that described our methods, results, and inferences made about the data collected. The research presentation shared results and conclusions drawn from this project, and was presented to 30+ farmers interested in innovative agricultural approaches from Iowa and neighboring states.  The research report has been made available to the public through the PFI website and has been printed for distribution at PFI events including the annual conference that draws over 800 attendees from across the Midwest.  Additionally, the results were presented by the graduate student to university researchers at a sustainable agriculture research symposium at Iowa State University.


Click linked name(s) to expand
  • Dr. Matt Liebman
  • Tim Sieren
  • Richard Sloan


Materials and methods:

We utilized an on-farm research project that was initiated by by two corn-soybean producers in Iowa in association with Practical Farmers of Iowa (PFI). This research project was established in 2013 by the cooperating farmers and PFI to evaluate the feasibility and profitability of growing cereal rye for grain, with red clover frost seeded into the standing rye and used as a green manure for a subsequent corn crop.

Measurements were taken at Iowa farms operated by Dick Sloan and Tim Sieren. Dick Sloan farms in Buchanan County in NE Iowa and Tim Sieren farms in Washington County in SE Iowa. The experimental treatments on the farms consisted of a diversified rotation with corn following cereal rye/red clover. Cereal rye was planted after grain harvest, and red clover was frost-seeded into the cereal rye stand in March.  Cereal rye was harvested for grain in July, and red clover was left to grow as a cover crop/green manure until the following spring, when it was terminated prior to corn planting by herbicide application. The experimental treatment was compared against a control treatment that differed by farm. At the Sloan farm the control treatment was no-till corn following a year of soybean/rye cover crop. The control treatment at the Sieren site was corn following cereal rye for grain without a legume cover crop. Soil nitrogen levels in the corn year of all treatments was measured using the Late Spring Nitrate Test (LSNT) according to Iowa State University recommendations. Mineral N fertilizer was side-dressed in June at the recommended rates.

The farms employed side-by-side strips between 200 and 600 m in length to compare treatments.  The Sieren farm contained 4 randomized replicates of the treatments, while the Sloan farm consisted of a single side-by-side comparison of the treatments.  In order to increase confidence in our results, the single comparison at the Sloan site was divided into 4 replicates.  The sampling scheme thus entailed sampling 4 replicates of 2 treatments at 2 sites, for a total of 16 experimental units for each of the measurements.

We characterized N cycling by measuring both net N mineralization and gross N mineralization in a laboratory at Iowa State University (ISU) on soil samples taken in August 2014. Net N mineralization measurements followed a 7-day anaerobic incubation technique. Gross N mineralization measurements were completed using the 15N pool dilution technique described. Additionally, N uptake in the corn crop was quantified by sampling corn plants at the time of soil sampling as well as after plant maturity, separating the corn into grain and stover, and analyzing for biomass and N content in both grain and stover.  We also performed cornstalk nitrate tests to gauge N supply sufficiency.  Our measurements were combined with additional measurements taken by the farmer participants including grain yields of corn, and red clover biomass and N content in the spring before termination.

Research results and discussion:

See attached PFI report for Tables and Figures.

Red clover biomass

At the Sieren farm, the above ground red clover biomass averaged 5,210 lb/ac prior to termination in Spring 2014, with a total N content of 113 lb N/ac. At the Sloan farm, clover was sampled in Fall 2013 only, prior to a killing frost. Aboveground biomass averaged 6,670 lb/ac with a total N content of 176 lb N/ac.

Corn biomass and N content

Over the two sampling dates both corn biomass (Fig. 2) and corn N content (Fig. 3) were greater in the synthetic N strips compared to the red clover strips at both farms. Similarly corn grain yield was greater in the synthetic N corn compared to the corn following red clover (Table 2), which suggests that corn following red clover experienced N limitation. Yield results presented in an earlier PFI research report from the same trial at the Sieren farm supports this hypothesis. This earlier report considered an additional treatment where the corn following red clover received 100 lb N/acre in June (this treatment was not included in the current study). Tim found corn from this treatment was able to yield ~30 bu/ac greater than the corn following red clover without supplemental fertilizer, suggesting N limitation in the corn following clover without N (Gailans and Sieren, 2014).

The corn plant N content results (Fig. 3) suggest that reduced corn yield following red clover may be due to limitations in plant N supply early in the growing season N. Between the two sampling dates, the increase in corn total N between early August and October was possibly greater in the corn following red clover compared to the synthetic N corn at both farms, although this was not statistically significant (Fig. 4). This result suggests that the red clover treatment was able to provide available N to the corn late in the growing season as the red clover was decomposed and N was released. Growing red clover before corn has been shown to replace a significant portion of the N fertilizer needed by a following corn crop in northeast Iowa, and previous findings at the Sieren farm showed that the red clover was able to replace at least 43 lb N/acre (Gailans and Sieren, 2014).

Soil N cycling

Soil N cycling was assessed in early August at both farms. Both gross ammonification and potentially mineralizable N measurements provide an estimate as to the amount of soil N becoming plant-available on a daily basis and are expressed as lb N/ac/day. Gross ammonification was >2x higher at the Sieren farm compared to the Sloan farm, which may have been caused by greater precipitation and soil moisture at Sieren’s enhancing the breakdown of red clover biomass. The N cycling measurements were not statistically significantly between the red clover and synthetic N treatments, due in part to high variability in the measurements (Figs. 5 and 6). While the N mineralization rates in the red clover treatments tended to be higher than in the synthetic N treatments, the differences were not large enough to provide support for the hypothesis that  red clover treatment would enhance soil N mineralization rates. The results do suggest, however, that any reductions in yield in the corn following red clover (Table 2) are not due to lack of soil N later in the summer. Rather, N limitation likely occurs early in the growing season before the decomposition of the clover biomass can release substantial plant available N.


Gailans, S. and T. Sieren. 2014. Nitrogen Replacement Value of Red Clover. Practical Farmers of Iowa Cooperators’ Program. Ames, IA. http://practicalfarmers.org/farmer-knowledge/research-reports/2014/nitrogen-replacement-value-red-clover/ (accessed Oct. 30, 2015).

Participation Summary

Educational & Outreach Activities

Participation Summary

Education/outreach description:

Results of the project were presented at the Practical Farmers of Iowa Cooperator’s Meeting to a group of approximately 30 farmers who are actively involved in PFI’s on-farm research projects. The group was composed primarily of corn and soybean farmers, and included many individuals with experience testing and adopting innovative farming techniques. Thus the project results were presented to farmers with an open-minded, yet critical view towards innovative practices such as the practices examined in this project.

A PFI research report titled “Timing of Nitrogen Supply to Corn from Spring Terminated Red Clover” was written and published in conjunction with the PFI research coordinator Stefan Gailans and the cooperating farmers, Tim Sieren and Dick Sloan. The research report highlighted important results of this research, as well as the insights of the cooperating farmers regarding the pros and cons of the farming practices investigated by the project. This report will be made available in paper format at future PFI events including field days and the PFI annual conference that is attended by approximately 350 farmers, researchers, and other interested parties. Additionally, the report is currently available for free on the PFI website, and was highlighted in a blog post written by Stefan Gailans on December 16, 2015.  


A poster presentation of research results was given at the Iowa State University Graduate Program in Sustainable Agriculture’s Research Symposium on April 20, 2016. The symposium highlights research done by graduate students in the GPSA program, and is attended by researchers and students interested in sustainable agriculture research at ISU.

Osterholz N supply red clover_poster

Project Outcomes

Project outcomes:

Economic Analysis

This project did not include an economic component. Future work focused on similar cropping systems should investigate the N replacement value of legume cover crops, and analyze this value relative to the cost of legume seed, labor, and yield impacts.

Farmer Adoption

No information was gathered regarding farmer adoption of interseeded red clover in rye or other small grain crops. Similarly, adoption of spring termination management of legumes prior to corn was not investigated. However, efforts are being made by Practical Farmers of Iowa and other organizations to encourage more widespread cultivation of small grains, opening up opportunities for farmers employ spring-terminated red clover in corn-based cropping systems.


Areas needing additional study

Spring terminated red clover did not enhance N cycling in a succeeding corn crop on the two cooperating farms. Corn yields in this study were less when following a spring terminated red clover cover crop compared to conventionally managed corn. Corn plant N results, however, suggest that the decomposing red clover was able to provide significant late season N to the corn. 

Management that attempts to address early season N limitation following spring-killed red clover may help overcome the corn yield hit observed in these on-farm trials. Research from Canada showed that the corn yield response to N fertilizer may have been greater in spring-killed red clover compared to autumn-killed red clover, suggesting that N fertilization is particularly important when a red clover cover crop is killed shortly before corn planting. Building on the insights of this on-farm study, we propose that earlier fertilization with supplemental N, possibly at corn planting, could provide corn with early season N during the period of N limitation before clover decomposition can provide sufficient N to the corn crop in late summer.

Future agronomic research should focus on optimizing timing and rates of N fertilizer application in corn crops following spring-terminated legumes. Additionally, adjusting the timing of red clover termination will likely influence N availability in subsequent corn crops and could be investigated as a method to improve early season synchrony between corn N demand and soil N supply. With further adjustments of agronomic practices, we believe that spring terminated red clover could prove to be a successful strategy to reduce synthetic N fertilizer requirements and N losses while still maintaining high yields in a following corn crop.

In addition to further agronomic optimization of the this cropping system, the economic viability of this system compared to the dominant corn-soybean cropping system should receive attention.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.