Mixing the Perfect Cover Crop Cocktail

posted in: Science | 12

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.

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Andrew McGuire is an Irrigated Cropping Systems Agronomist for Washington State University Extension. He works with farmers in the Columbia Basin of Central Washington in improving soils through cover crops and high residue farming systems.