Cover Crops: Taking a Closer Look at Legumes

In the previous weeks, we’ve showcased grasses and brassicas as great options for cover crops that can be readily integrated into corn and soybean cropping systems across Iowa. Now it’s time to show some love to the legumes!

Last month, Liz shared the Top 10 Cover Crops for Iowa in 2016 identified by plant scientists, agronomists, and other researchers focused on cover crops, as well as you, our blog readers. Two of the top ten cover crops from this poll are legumes: red clover and hairy vetch.

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What makes the legumes special, and why consider them as potential winter cover crops in your farming operation? The biggest difference between legumes and the other (non-leguminous) cover crops is their ability to fix nitrogen. Particular bacteria in the soil (rhizobium species) form nodules on the roots of the legume plant. It’s a natural symbiotic relationship that allows for the capture of nitrogen from the atmosphere (N2 gas) and conversion of this nitrogen to forms that are plant usable. This process is referred to as biological nitrogen fixation.

1280px-Nitrogen_Cycle.svg

A very common example of this nitrogen fixation process happens in our soybean fields. The reason nitrogen fertilizer historically has not been added to soybean crops is because they are able to pull the nitrogen they need from the atmosphere as well as from mineralized nitrogen in the soil.

LegumesKnowing that legumes have the potential to add nitrogen back into the soil, this offers another “tool in the toolbox” when it comes to cover crops!  Hairy vetch and red clover were mentioned above, but there are a number of other legume cover crops to consider as well, including common vetch, crimson clover, white clover, kura clover, sweet clover, cow peas, winter lentils, and alfalfa.

When it comes to getting your cover crop planted in the fall, legumes and brassicas need more heat units than small grains to be effective. Thus, timely planting is of the essence!  In the Cover Crops: A Guide for Iowa Producers recommendations from USDA-NRCS, legumes should optimally be seeded between August 1 – September 15.

Seeding rates vary based on the specific legume you’re using – check out Practical Farmers of Iowa’s Cover Crop Recommendations and the Midwest Cover Crops Council webpage, which includes information on a variety of cover crop research trials as well a robust cover crop selector tool.

PFI-CCRecommendations

Do legume cover crops survive over the winter? It depends – it depends on the intensity of the winter, when the legume cover crop was planted, fall weather conditions (how much fall cover crop growth was attained?), and where geographically you are located in the state! From the Iowa Learning Farms’ perspective, the only experience we’ve had with legumes overwintering in Iowa came from one of our farmer-partners in the southern part of the state, and that was only described as being moderately successful.

Legumes on their own can offer many benefits, including fixing atmospheric nitrogen, providing a nitrogen source for the soil to be used by future crops, as well as protection from soil erosion along with building soil structure and organic matter. However, this is ultimately dependent upon how much growth is achieved, which can be a big challenge – weed control abilities are less, and legumes do not increase soil organic matter as much as other cover crops.

However, legumes can be used in a cover crop mixture with grasses or brassicas, which offers the ability to harness some of the benefits of different types of cover crops. From SARE’s Managing Cover Crops Profitably: Third Edition, “Mixtures of legume and grass cover crops combine the benefits of both, including biomass production, N scavenging and additions to the system, as well as weed and erosion control.”

CCMixLewis-ThreeSpecies

We are using one legume, hairy vetch, as part of our USDA-NRCS Conservation Innovation Grant investigating cover crop mixtures. It is being utilized in a three species grass-brassica-legume mixture, including oats, radish, and hairy vetch (this specific mixture is seeded in the fall following soybeans). All three species can be seen distinctly in the above photograph from November 2015; the hairy vetch is the small plant with fern-like leaves in the foreground.

Fall 2015 offered fantastic conditions for cover crop growth, including hairy vetch, so it will be interesting to see what impact that has as we carry out soil testing as well as look at the nitrate-nitrogen data in the spring. Will there be observable differences between the single species plots (containing oats only) compared to those with a mixture of oats, radish, and hairy vetch? Stay tuned…

Additional Cover Crop Resources:
Cover Crops in Iowa: A Quick Guide (Iowa Learning Farms)
Cover Crops: A Guide for Iowa Producers (USDA-NRCS)
Cover Crop Recommendations (Practical Farmers of Iowa)
Cover Crop Business Directory (Practical Farmers of Iowa)
Overview of Legume Cover Crops (SARE)
Managing Cover Crops Profitably: Third Edition

Ann Staudt

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