Since my 2016 post, “Cover crop best bet is monoculture, not mix,” I have continued to follow the research on cover crop mixtures looking for evidence contrary to my prior conclusions. However, I found much the same as when I wrote the 2017 update of the post. In this 2018 update, I cover all the research I found published during the past year, give some guidelines for conditions under which mixture research should be done to match those where the most benefits are being claimed, and in response to a suggestion, review the Jena biodiversity experiments in relation to our cover crop question, “is monoculture or mixture better?”
My criteria for when a mixture should be favored is still transgressive overyielding, where a mixture’s biomass surpasses that of the best monoculture. Although Bybee-Finley and Ryan (2018) suggest that transgressive overyielding is too high a bar for polycultures/intercrops, I believe that to justify the extra expense (mainly seed), time (modifying and setting up planting equipment) and especially the extra management needed to get a diverse stand, mixtures should either out-yield the best monocultures, or have some other similarly large benefit over monoculture to justify their use.
The research on cover crop mixtures seems to have peaked and is now decreasing as I found fewer papers to review in 2018. Sanderson et al. (2018) looked at annual forage mixtures and concluded “Mixtures, however, did not yield more forage than the most productive monoculture…” so no transgressive overyielding.
Couëdel et al. produced three papers on crucifer-legume bi-mixes, which do show benefits of mixtures over monoculture. These, however, fall in the exception that I have mentioned before, that of grass-legume mixtures on low nitrogen soils. In the studies, crucifers took the place of grasses in the mixtures. The mixtures benefit from the nitrogen fixed by the legumes, while the crucifer monocultures did not have their nitrogen needs met.
The Bybee-Finley and Ryan paper mention above provides an interesting discussion of intercropping in general. The authors observe that mixtures of perennials will more often result in transgressive overyielding than will mixtures of annuals. This is because it often takes several years or more for the benefits of diversity to become evident. With the shorter growing seasons of annual crops, and especially of cover crops, the benefits of diversity in terms of biomass production are rarely observed. Even in mixtures of perennials, overyielding is only found in 37-53% of research trials (Fort & Segura, 2017) so it is not a given, even in the long-term. Furthermore, maintaining a diverse stand of perennials can be difficult in the long-term as many managers of pasture have found. However, although diversity-in-space of mixtures may not be much benefit for annuals, diversity in time, something which nature cannot do, is highly beneficial for annuals; we call it crop rotation.
The Jena Biodiversity Experiment
The Jena biodiversity experiment is an impressive endeavor. It was suggested to me as an example of mixtures benefits over cover crops. And it is, but it has some limitations in its application to cover crops. The experiment did not look at mixtures of annuals but did find that mixtures of perennials, in long-term grasslands, produce better than monocultures on average. However, as mentioned before, these benefits often take years to appear, an option not available to a 90-day cover crop.
While looking through the research that has come from the Jena project, I found a paper that explains why there is such a disparity between ecologists and agronomists like myself on the topic of the use of diversity in plants. Schulze et al. (2018) points out that the two groups have different criteria for measuring the success of polycultures vs. monocultures. It gets back to the different ways of measuring overyielding that I covered here. “Two approaches mark the difference between the “ecological” and “agricultural” view of the biodiversity/ growth relation. In ecology the trend is averaged by taking monocultures of all species as baseline to evaluate mixtures. This contrasts the ‘agricultural’ view focusing on the most productive species or species combination as baseline to evaluate mixtures.” Evaluated by the ecological view, the Jena experiments do indeed show the value of diverse species mixtures. However, by the agricultural view, the best yielding monoculture out-yields the mixtures.
Finally, in a review of the Jena experiment results, Buchmann et al. (2018) find that the relationship between diversity in grasslands and productivity is mainly determined by nitrogen, either from legumes in mixtures or from fertilizer.
Research conditions to capture the benefits of cover crop mixtures
I would like to suggest how research should be done to test the claims of regenerative agriculture proponents regarding the benefits of cover crop mixtures. Based on the solid regenerative agriculture principles, I think this would be the minimum:
3+ year crop rotation
6-8 species in the cover crop mixture
Long growth period
I suggest the longer growth period because many of the cover crops used in regenerative agriculture are grazed once, or often multiple times. This longer field time gives the legumes in the mixtures time to become established and fix significant nitrogen giving them an advantage over shorter season cover crops that are being squeezed between cash crops and which mainly grow in cool conditions from fall to early spring. The latter conditions do not allow legumes to fix a significant amount of nitrogen.
Much of the research I have reviewed has not been done in this combination of conditions, so I am still open to new research results on this question. However, it has been suggested to me by regenerative agriculture proponents that the cover crop research I reviewed did not find benefits of mixture because it was done on degraded soil. This is curious, because in regenerative ag and elsewhere cover crops are a tool to improve the soil. If they do not function on degraded soils, and if the benefits are only found when they are used on already healthy soils, then why use them?
As I have written before, any cover crop can be beneficial, but given the accumulated weight of evidence going back over 40 years (Cardinale et al. 2011), I conclude that if there are benefits of cover crop mixtures over monocultures, they are not easy to achieve. Add to this the extra expense for seed and planting difficulties, and the case for mixtures becomes difficult to make.
Buchmann, T., Schumacher, J., Ebeling, A., Eisenhauer, N., Fischer, M., Gleixner, G., … Roscher, C. (2018). Connecting experimental biodiversity research to real-world grasslands. Perspectives in Plant Ecology, Evolution and Systematics, 33, 78–88. https://doi.org/10.1016/j.ppees.2018.06.002
Bybee-Finley, K. A., & Ryan, M. R. (2018). Advancing Intercropping Research and Practices in Industrialized Agricultural Landscapes. Agriculture, 8(6), 1–24.
Cardinale, B. J., Matulich, K. L., Hooper, D. U., Byrnes, J. E., Duffy, E., Gamfeldt, L., … Gonzalez, A. (2011). The functional role of producer diversity in ecosystems. American Journal of Botany, 98(3), 572–592. https://doi.org/10.3732/ajb.1000364
Couëdel, A., Alletto, L., & Justes, É. (2018). Crucifer-legume cover crop mixtures provide effective sulphate catch crop and sulphur green manure services. Plant and Soil, 1–16. https://doi.org/10.1007/s11104-018-3615-8
Couëdel, A., Alletto, L., Kirkegaard, J., & Justes, É. (2018). Crucifer glucosinolate production in legume-crucifer cover crop mixtures. European Journal of Agronomy, 96, 22–33. https://doi.org/10.1016/j.eja.2018.02.007
Couëdel, A., Alletto, L., Tribouillois, H., & Justes, É. (2018). Cover crop crucifer-legume mixtures provide effective nitrate catch crop and nitrogen green manure ecosystem services. Agriculture, Ecosystems & Environment, 254, 50–59. https://doi.org/10.1016/j.agee.2017.11.017
Fort, H., & Segura, A. (2017). Competition across diverse taxa: quantitative integration of theory and empirical research using global indices of competition. Oikos, 127(3), 392–402. https://doi.org/10.1111/oik.04756
Sanderson, M., Johnson, H., & Hendrickson, J. (2018). Cover Crop Mixtures Grown for Annual Forage in a Semi-Arid Environment. Agronomy Journal, 110(2), 525–534. https://doi.org/10.2134/agronj2017.04.0228
Schulze, E. D., Bouriaud, O., Weber, U., Roscher, C., Hessenmoeller, D., Kroiher, F., & Schall, P. (2018). Management breaks the natural productivity-biodiversity relationship in forests and grassland: an opinion. Forest Ecosystems, 5(1), 3. https://doi.org/10.1186/s40663-017-0122-y