So, what is going on here?
- 100% rye
- 50% rye, 50% vetch
- 38% rye, 62% vetch
- 25% rye, 75% vetch
- 100% vetch
Using this logic, a two-species mix produces at least five 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:
- Red Clover
- Cereal Rye
- Winter Canola
- Winter Pea
- 3-species mix designed for weed suppression
- 3-species mix designed for N retention and supply
- 4-species mix including pollinator-friendly species
- 6-species mix
- A commercially-available mix with 8 cultivars
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. Consider that the periodic table of cover crops (from North Dakota where a number of very active cocktail proponents farm) contain 47 species to choose from and 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).
Here are more recent posts on this topic:
- 5/26/2015, Ecological Theories, Meta-Analysis, and the Benefits of Monocultures
- 6/8/2015, Monoculture vs. Polyculture Part I: “Straight up” beats “cocktails” for cover crop productivity
- 6/11/2015, Monoculture vs. Polyculture Part II: “Straight up” beats “cocktails” for cover crop ecosystem services
- 12/21/2016, Cover crop best bet is monoculture, not mixture