Angie Peltier, UMN Extension crops educator, Northwest Research & Outreach Center, Jeff Vetsch, UMN research supervisor, Southern Research & Outreach Center and Axel Garcia y Garcia, UMN Extension, sustainable cropping systems specialist, Southwest Research & Outreach Center
The following information was provided during a 2025 Strategic Farming: Field Notes session. Use your preferred podcasting platform or listen online to a podcast of this Field Notes session hosted by UMN Extension crops educator Liz Stahl.
Note: Join us this week on Wednesday when we welcome Dr. Dean Malvick, Extension plant pathologist, and Brad Carlson, Extension educator, to discuss What to expect after heavy June rains in terms of crop diseases and nutrient management.
Nitrogen content in rye at termination varies depending upon the growth stage and height. When rye is 24-inches or taller, N content would be approximately 2.5%. However, the N content in 12-inch tall rye crop is greater, about 3.5%. There are tools available to estimate above ground dry matter biomass from rye height. These tools allow a farmer to estimate the amount of N in their rye cover crop.
The ISU study also looked at the carbon (C) to nitrogen (N) ratio in both above- and below-ground biomass. C:N ratio is an important measure of nitrogen availability. When this ratio is high (30:1 or greater), the saprophytic microbes in the soil that break down plant material scavenge nitrogen from the soil itself to gather the amino acids needed to grow and develop, reducing the amount available for the crop to take up. In biomass with lower C:N ratios, nitrogen is more biologically available through mineralization and so tie-up of soil nitrogen doesn’t tend to take place. ISU’s study found the above-ground C:N ratio was 15 to 20 and the below-ground ratio 45 to 50, meaning that the nitrogen in the above-ground biomass is much more likely to be mineralized during the first growing season than the nitrogen in the below-ground root biomass.
Depending upon how long they are allowed to grow, legume cover crops may be able to add some N to the system through biological nitrogen fixation. However, enough time is needed in the growing season for N fixation in the cover crop to take place. Establishing a legume cover crop in the late summer after small grains or canning crops would allow enough time for N fixation in the cover crop and might just provide some additional N to your crop. Rather than specific legume cover crops providing a hard and fast nitrogen credit, it is a safer bet to select a nitrogen rate toward the lower end of the recommended range after a legume cover crop.
While it is possible that one may be able to reduce N rates when seeding a crop after a legume cover crop, or alternatively, one may have to add additional N to compensate for N immobilization (tie-up) after a nitrogen-scavenging cover crop like spring terminated rye, UMN research has not yet reached the level of certainty needed to provide hard and fast recommendations.
Thanks to the Minnesota Soybean Research and Promotion Council and the Minnesota Corn Research and Promotion Council for their support of this program.
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| Cereal rye cover crop seeded at 50 lb/a in the fall and terminated at planting. Photo: Liz Stahl, UMN Extension |
Note: Join us this week on Wednesday when we welcome Dr. Dean Malvick, Extension plant pathologist, and Brad Carlson, Extension educator, to discuss What to expect after heavy June rains in terms of crop diseases and nutrient management.
With Cover Crops, Flexibility is a Must
Liz visited with the guests who had just presented some of their research at the June 17 Agronomy Tour at the Southern Research and Outreach Center in Waseca. Much of the discussion focused on cover crop management and practices need to adapt to a given location, with different soils, weather conditions and objectives. At the Southwest Research & Outreach Center (SWROC), the 2025 growing season began warm and dry, moved to cool and wet and is forecast to turn hot. These conditions forced early termination of cover crops, highlighting just how important flexibility and timing are when it comes to managing cover crops. Much of the discussion focused on research investigating cereal rye as a cover crop, given its ability to survive the harsh Minnesota winters. As one of the most winter-hardy species, rye is well suited for establishment after full-season crops and remains a reliable option across different systems.Termination Timing Drives Biomass
Cover crop biomass — mainly aboveground — is the amount of plant material that the cover crop produces. Later termination results in more biomass. This can have implications related to both planting and nutrient uptake of the cash crop. Research by Garcia y Garcia has shown that termination timing is a much stronger driver of biomass than seeding rate. Early results from his multi-site, multi-year trial show that drilled cereal rye planted at ~ 30 lb/A produces very similar biomass to biomass of rye at higher seeding higher rates.Cover Crop Water Use
Fall growth and water use of winter rye and other cover crops are typically minimal due to shortening days and cooling temperatures. However, spring water use can be substantial, especially when termination is delayed and biomass accumulates. Water use is affected by species, management practices (e.g., termination timing), soil type (water holding capacity), and weather conditions (rainfall, temperature, wind). In dry springs, delaying termination increases the risk of soil moisture depletion, which could affect the establishment of the main crop. Timely termination is recommended if limited rain is forecasted and soil moisture is marginal.Cover Crop Nutrient Use
One audience member asked about how much nitrogen (N) a one-foot-tall rye cover crop could sequester. Garcia y Garcia’s research has shown 20-60 lb of N per acre in aboveground biomass. Vetsch added that 60% or more of that N could become available within 10 days. This value is based on research at Iowa State University (Sawyer et al. 2017) that showed 60 and 77% of the nitrogen in cereal rye was released 105 days after spring termination, with more being released when corn followed soybeans than when corn followed corn in the cropping system.Nitrogen content in rye at termination varies depending upon the growth stage and height. When rye is 24-inches or taller, N content would be approximately 2.5%. However, the N content in 12-inch tall rye crop is greater, about 3.5%. There are tools available to estimate above ground dry matter biomass from rye height. These tools allow a farmer to estimate the amount of N in their rye cover crop.
The ISU study also looked at the carbon (C) to nitrogen (N) ratio in both above- and below-ground biomass. C:N ratio is an important measure of nitrogen availability. When this ratio is high (30:1 or greater), the saprophytic microbes in the soil that break down plant material scavenge nitrogen from the soil itself to gather the amino acids needed to grow and develop, reducing the amount available for the crop to take up. In biomass with lower C:N ratios, nitrogen is more biologically available through mineralization and so tie-up of soil nitrogen doesn’t tend to take place. ISU’s study found the above-ground C:N ratio was 15 to 20 and the below-ground ratio 45 to 50, meaning that the nitrogen in the above-ground biomass is much more likely to be mineralized during the first growing season than the nitrogen in the below-ground root biomass.
Cover Crops Differ in Nitrogen Scavenging/Fixing Potential
Rye has a tremendous capacity to scavenge nitrogen that would otherwise be subject to environmental losses. Radish and turnip cover crops have been reported to do well at scavenging phosphorus and potassium in addition to nitrogen. The Ruark Lab at the University of Wisconsin showed that N scavenged by radish could negatively impact a following corn crop’s yield potential without additional nitrogen being added.Depending upon how long they are allowed to grow, legume cover crops may be able to add some N to the system through biological nitrogen fixation. However, enough time is needed in the growing season for N fixation in the cover crop to take place. Establishing a legume cover crop in the late summer after small grains or canning crops would allow enough time for N fixation in the cover crop and might just provide some additional N to your crop. Rather than specific legume cover crops providing a hard and fast nitrogen credit, it is a safer bet to select a nitrogen rate toward the lower end of the recommended range after a legume cover crop.
While it is possible that one may be able to reduce N rates when seeding a crop after a legume cover crop, or alternatively, one may have to add additional N to compensate for N immobilization (tie-up) after a nitrogen-scavenging cover crop like spring terminated rye, UMN research has not yet reached the level of certainty needed to provide hard and fast recommendations.
Rules of Thumb for Rye Before Corn
Terminate when rye is less than 8 inches tall to minimize the risk of N immobilization and reduced corn yield potential. If rye biomass reaches 1,000 lb/A, consider adding starter nitrogen and/or sulfur applied near the row at planting. Annual ryegrass terminated by a fall frost or brassica mixes are less riskier for N tie-up, but each strategy must be evaluated individually.Audience questions
Garcia y Garcia and Vetsch addressed key farmer questions, including “How much nitrogen can a 12 inch rye crop sequester?”, “Which cover crop is best at nutrient recycling?”, “How can one use cover crops for weed suppression?”, “Should cover crops cause one to change their nitrogen management strategy?”, “How is terminating cover crops working in a really wet spring?”, “If we find ourselves in a situation where a summer cover crop can be planted, which annual is best for soil compaction?”.Thanks to the Minnesota Soybean Research and Promotion Council and the Minnesota Corn Research and Promotion Council for their support of this program.
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