Mixing the Perfect Cover Crop Cocktail

Last year, at a Washington State University (WSU) Building Soils for Better Crops conference, farmers from Kansas, North Dakota, and Colorado all spoke on the benefits they were seeing from using multi-species cover crops. These cover crop “cocktails” consist of 8 or more species chosen to maximize diversity. Cocktail mixers aim for at least 1 entry from each of the following categories: warm season broadleaf species, cool season broadleaf species, warm season grasses and cool season grasses. In addition to the benefits regularly associated with cover crops, farmers using these cocktails often point to increased crop yields and reduced inputs as the reasons they are using them. These cocktails also seem to give rise to a passion for cover crops not seen in farmers using a single-species.
So, what is going on here?
First of all, these farmers are ahead of the science and are likely to remain there. There is very little research on cover crop mixtures of more than 5 species, which is the point where some NRCS personnel say the benefits of the diverse cocktails are first seen. Where such research has been done, it has used 3 or at most, 4 species. An example from WSU research on a 2-species cover crop mixture shows why research using cover crop cocktails will be difficult. In this study, various proportions of rye and vetch were studied to see which might be most beneficial or if the mixtures were better than the monocrops of either rye or vetch.

Grass mix cover crop in Pierre, SD. Photo by Michael Stephens of USDA-NRCS via Flickr.
Grass mix cover crop in Pierre, SD. Photo by Michael Stephens of USDA-NRCS via Flickr.

The treatments were

  1. 100% rye
  2. 50% rye, 50% vetch
  3.  38% rye, 62% vetch
  4. 25% rye, 75% vetch
  5. 100% vetch

Using this logic, a 2-species mix produces at least 5 treatments, which is reasonable for field research, but extend this thinking to a cocktail of 5 or 8 or more species and the number of treatments makes for a cumbersome and expensive study.
An alternative strategy, used in a PennState study, disregards the proportion question in favor of including more species, mixed to provide specific ecosystem services:

  1. Red Clover
  2. Oat
  3. Cereal Rye
  4. Radish
  5. Winter Canola
  6. Winter Pea
  7. 3-species mix designed for weed suppression
  8. 3-species mix designed for N retention and supply
  9. 4-species mix including pollinator-friendly species
  10. 6-species mix
  11. A commercially available mix with 8 cultivars

Periodic Table of Cover crops, developed by USDA-ARS NGPRL. Click for a PDF.
Periodic Table of Cover crops, developed by USDA-ARS NGPRL. Click for a PDF.

Even here, the large number of treatments makes it likely that the results will be inconclusive, confounded by the sheer complexity of cocktails with 5+ species. The periodic table of cover crops (from North Dakota where some very active cocktail proponents farm) contains 47 species – you can see the difficulty facing researchers! Even if there were sufficient funding for such daunting research, the “right mix” would probably vary with climate, soils, and cropping system.
Given all this, what can be done?
When faced with this level of complexity, a good strategy is to rely on principles. “The more diversity the better” seems to be the guiding principle of cocktail farmers. The limitations are agronomic (planting mixtures with varying seed sizes can be challenging), economic (some seed is very expensive) and climatic (the right mixture will depend on when it is being planted and the succeeding temperature and precipitation/irrigation).
Once a mixture is chosen, the next step is to monitor the results. Here, the yield and quality of the crop following the cover crop cocktail will best demonstrate the overall effects and give the economic payback of the practice. While this “black box” approach (i.e., treatment is applied and end results are measured but what happens in between is unknown) is practical, it does not give any information on what actually happens to the soil. This is where researchers and farmers can collaborate to gain a better understanding of how cover crop cocktails work with the goal of improving their benefits and consistency. With cover crop cocktails, well-designed on-farm research will likely result in more usable information than more expensive studies at Research Centers alone.
To learn more about cover crop cocktails, start with these online resources ATTRA, Gabe Brown (North Dakota farmer), Brendon Rockey (Colorado farmer), and Ray Archuleta (NRCS soil scientist).

9/16/15 Update. For more on the ecological theories behind the use of polycultures, see here. For more on the results of research using cover crop polycultures, see here and here.

Andrew McGuire

Written by Andrew McGuire

Andrew McGuire has been with Washington State University Extension since 1999. He works with farmers to implement solutions to irrigated farming challenges in the Columbia Basin. He is currently evaluating soil health measurements and developing high-frequency green manure rotations for soilborne disease control. He thinks, then writes about agriculture at the Center for Sustaining Agriculture and Natural Resources.

12 comments

  1. Thanks for the post. We’ve been cover cropping for a whopping two years now. We had 200 acres seeded in fall of 2012 and we have 400 growing right now which is about 20% of the farm. The mixes we have are one field in oats and tillage radishes, and another is split between just annual ryegrass, and a a cocktail of ryegrass, clover, and radish.
    One thing I need to think about doing when we seed covers this fall is to leave a strip in each field with no cover so I can compare cash crop yields at harvest. Also performing strip trials of different combinations would be a good idea.
    It’s so tricky to quantify the benefits because every farm is at least just a little bit different and soil and management.

  2. Thanks for this article. I’m an amateur farmer trying to bring 40 acres of starved sandy soil back to life. Been planting successions of cover crops for about 4 years, cocktails for the last 2 years (no-till grow and mow, repeat).
    The root exudate argument makes a lot of sense to me – each plant leaks a unique set of compounds into soil to stimulate a targeted set of beneficial soil microbes. So a large variety of plants growing at one time means a large variety of soil bugs will be stimulated, and lots of soil bugs doing their soil-service jobs can get soil functioning again, the theory goes.
    Organic matter has increased from 1% to 3% in some of my fields. Here are pics showing earthworms and other soil bugs returning to soil where they weren’t living before: http://farmingsweetbay.wordpress.com/2013/07/31/see-what-a-cover-crop-cocktail-did-to-our-farm-soil/

  3. That’s really exciting. Congrats on your organic matter increase and good luck with keeping it up! Please let us know if you’re ever interested in a Biofortified guest post on your quest to improve organic matter 🙂

  4. Kelly, I agree with you, the exudate argument seems to make sense. Since we don’t know what exudates each species has, nor what those exudates stimulate, it makes sense to increase the number of species, although there is probably a point of diminishing returns as the distance between plants of the same species becomes so far that the following crop may not benefit from the effects of that species.
    I also like that you said “starved soil,” instead of dead soil. And your starved soil is probably not so starved anymore! I read on your blog that your soil is at 2.8% organic matter, high for a sandy soil, at least in my experience – nice work.

  5. Thanks for the link, Andrew. We are purchasing a new anhydrous ammonia applicator this year for our farm which will be setup to do a proper job with variable rate application. Before I go VRT on all our corn acres we are going to plant a field or two of corn with a single planting population and do some test strips of N rates and VRT. Once we learn from that we’ll start combining both variable seeding and N.

  6. Kelly, that’s an impressive jump in OM! The fields we’ve put in the cover the last two years will be due for a soil test soon so I’m anxious to compare the new results with past tests. We generally test about a third or half of the farm each year.
    I’ve been reading that organic matter increases are coming on faster in no-till/cover situations faster than many would have guessed. Just a few years as opposed to a decade or two. If I get nothing else but more OM out of covers, then I think they will be a success for us. Better with water and more available nutrients and holding capacity.

  7. Brian, research has shown that no-till by itself may not lead to increased levels of soil organic matter; it just slows down the decline. However, adding cover crops to no-till may change the direction of the organic matter change, at least in some soils. I will look to see if there is any research on this.

  8. I agree with your point about diminishing returns. But the more diverse the cocktail, the more beautiful it is, especially when flowering. Neighbors love it 🙂 Why don’t we know more about exudates? Have we just not done the studies?
    Starved soil – I read your article about that, a very good point. Thanks for the compliment. I’m really interested in what plants can do for degraded soil. I haven’t fertilized much yet in the interest of seeing how plants would behave. One field grew a very lush stand of sorghum sudan grass this past summer without any added N. It followed a legume-heavy winter cocktail cover crop though. And then I have some poorer fields that are not showing much response.

  9. Thanks Brian! OM probably increased so quickly b/c we used cover crops that produce the largest amount of biomass tons/acre and we’ve taken nothing off the fields for 5 years now. Previously, fields were in soybeans on soybeans for about a decade and tobacco for possibly a couple of centuries, so I think the cc cocktails are a nice wake-up for the soil.

  10. I hear the same can be said for compaction. No till alone doesn’t do a lot to alleviate soil compaction, but cover crops can work with no till to help out.

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