About a year ago I declared in a post, “Cover crop best bet is monoculture, not mix.” It stirred up quite a few comments and discussion, but no scientific evidence that countered my assertion. Nevertheless, research continues, so I have followed results as they have been published over the last year. Here is an update.
Do cover crop mixtures produce more biomass than the best monoculture species?
This is called transgressive overyielding and is important because many studies (including four in 2017) find that many of the benefits of cover crop, including weed control, nutrient scavenging, and effects on soil, are related to biomass quantity.
Again, the answer is no. Out of 10 published papers, one project report, and one PhD dissertation (see full list below), only one paper found that mixtures produced more than the best monoculture (Wendling et al 2017).
Weed control by cover crops was a major focus of four of the papers I reviewed and three of them found biomass directly related to weed control, and more important than the number of species (Bybee-Finley et al. 2017).
Do cover crop mixtures provide more benefits (ecosystem services) than monocultures?
This “multifunctionality” of mixtures is one of their purported benefits.
Here again, recent results are similar to what was found in the past. Mixtures can indeed provide more services than monocultures, but at the expense of lower levels of individual services compared to higher levels of fewer services provided by monocultures.
Instead of having a clear advantage over monocultures, researchers have found that there are tradeoffs. Finney et al. (2017) and Blesh (2017) both found a tradeoff between biomass production and nitrogen supply (including legumes in the mix). Similarly, White et al. (2017) observed a tradeoff between N supply (including legumes) and nutrient retention (scavenging), finding that in many circumstances “only one service or the other was provided at a high level.”
The PhD dissertation (Florence 2016) found that mixtures did not improve weed control, soil nutrient retention, or soil microbial biomass. Barel et al. (2017) found that mixtures did not increase soil nitrogen nor suppress parasitic nematodes better than monocultures. Appelgate et al. (2017) concluded “Cover crop mixtures did not provide benefits beyond cover crop monocultures.” And a report on a statewide project in Iowa (Juchems, 2017) concluded “single species are the way to go in Iowa for corn and soybean producers”.
Increased seeding rates relative to monoculture did not increase mixture biomass production (Murrell et al. 2017)
The high producing species in monocultures often dominate stand in mixtures (Holmes et al. 2017 and others)
Again, 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.
Another study that failed to find transgressive overyielding by cover crop mixtures, “On average, spring-planted mixtures yielded 30 to 60% more forage than the average of spring-planted monocultures when planted in spring. Mixtures, however, did not yield more forage than the most productive monoculture (triticale).”
Sanderson, M., H. Johnson, and J. Hendrickson. 2018. Cover Crop Mixtures Grown for Annual Forage in a Semi-Arid Environment. Agronomy Journal 110(2): 525–534.
Does it just take a lot of time for the benefits of multi-species cover crops to kick in? This study hints that it does. After three years of no differences in corn and soybean yields following monoculture and multi-species cover crops, the soybean yields after the fourth cycle of cover crops were significantly higher after the multi-species cover crop relative to the monoculture covers. Soil organic matter/carbon did not differ under the various treatments.
Chu, M., S. Jagadamma, F.R. Walker, N.S. Eash, M.J. Buschermohle, and L.A. Duncan. 2017. Effect of Multispecies Cover Crop Mixture on Soil Properties and Crop Yield. Agricultural & Environmental Letters 2(1). doi: 10.2134/ael2017.09.0030.
Appelgate, S.R., A.W. Lenssen, M.H. Wiedenhoeft, and T.C. Kaspar. 2017. Cover Crop Options and Mixes for Upper Midwest Corn–Soybean Systems. Agronomy Journal 109(3): 968–984.
Baraibar, B., M.C. Hunter, M.E. Schipanski, A. Hamilton, and D.A. Mortensen. 2017. Weed Suppression in Cover Crop Monocultures and Mixtures. Weed ScienceAvailable at /core/journals/weed-science/article/weed-suppression-in-cover-crop-monocultures-and-mixtures/A64B7FFE8A491B56C94385E8CF232A5C (verified 10 October 2017).
Barel, J.M., T.W. Kuyper, W. Boer, J.C. Douma, and G.B. De Deyn. 2017. Legacy effects of diversity in space and time driven by winter cover crop biomass and nitrogen concentration. Journal of Applied EcologyAvailable at http://onlinelibrary.wiley.com/doi/10.1111/1365-2664.12929/full (verified 12 June 2017).
Blesh, J. 2017. Functional traits in cover crop mixtures: Biological nitrogen fixation and multifunctionality. Journal of Applied EcologyAvailable at http://ntserver1.wsulibs.wsu.edu:2139/doi/10.1111/1365-2664.13011/full (verified 20 November 2017).
Bybee-Finley, K.A., S.B. Mirsky, and M.R. Ryan. 2017. Crop Biomass Not Species Richness Drives Weed Suppression in Warm-Season Annual Grass–Legume Intercrops in the Northeast. Weed Science: 1–12.
Cardinale, B.J., K.L. Matulich, D.U. Hooper, J.E. Byrnes, E. Duffy, L. Gamfeldt, P. Balvanera, M.I. O’Connor, and A. Gonzalez. 2011. The functional role of producer diversity in ecosystems. Am. J. Bot. 98(3): 572–592.
Couëdel, A., L. Alletto, H. Tribouillois, and É. Justes. 2018. Cover crop crucifer-legume mixtures provide effective nitrate catch crop and nitrogen green manure ecosystem services. Agriculture, Ecosystems & Environment 254: 50–59.
Finney, D.M., E.G. Murrell, C.M. White, B. Baraibar, M.E. Barbercheck, B.A. Bradley, S. Cornelisse, M.C. Hunter, J.P. Kaye, D.A. Mortensen, C.A. Mullen, and M.E. Schipanski. 2017. Ecosystem Services and Disservices Are Bundled in Simple and Diverse Cover Cropping Systems. Agricultural & Environmental Letters 2(1)Available at https://dl.sciencesocieties.org/publications/ael/articles/2/1/170033 (verified 13 November 2017).
Florence, A. 2016. Cover Crop Mixture Diversity and Function. Florence, A. 2016. Cover crop mixture diversity and function. Ph.D. dissertation. University of Nebraska, Lincoln. Crop Watch Available at https://cropwatch.unl.edu/2016/cover-crop-mixture-diversity-and-function (verified 15 November 2017).
Hodgdon, E.A., N.D. Warren, R.G. Smith, and R.G. Sideman. 2016. In-Season and Carry-Over Effects of Cover Crops on Productivity and Weed Suppression. Agronomy Journal 108(4): 1624–1635.
Holmes, A.A., A.A. Thompson, and S.E. Wortman. 2017. Species-Specific Contributions to Productivity and Weed Suppression in Cover Crop Mixtures. Agronomy Journal 0(0)Available at https://dl.sciencesocieties.org/publications/aj/abstracts/0/0/agronj2017.06.0309?access=0&view=article (verified 17 October 2017).
Juchems, L. 2017. Exploring whether cover crop mixtures make sense on Iowa farmland. Iowa Learning FarmsAvailable at https://iowalearningfarms.wordpress.com/2017/08/10/exploring-whether-cover-crop-mixtures-make-sense-on-iowa-farmland/ (verified 15 November 2017).
Murrell, E.G., M.E. Schipanski, D.M. Finney, M.C. Hunter, M. Burgess, J.C. LaChance, B. Baraibar, C.M. White, D.A. Mortensen, and J.P. Kaye. 2017. Achieving Diverse Cover Crop Mixtures: Effects of Planting Date and Seeding Rate. Agronomy Journal 109(1): 259–271.
Villalobos, L., and J.E. Brummer. 2017. Yield and Nutritive Value of Cool-Season Annual Forages and Mixtures Seeded into Pearl Millet Stubble. Agronomy Journal 109(2): 432–441.
Wendling, M., L. Büchi, C. Amossé, B. Jeangros, A. Walter, and R. Charles. 2017. Specific interactions leading to transgressive overyielding in cover crop mixtures. Agriculture, Ecosystems & Environment 241: 88–99.
White, C.M., S.T. DuPont, M. Hautau, D. Hartman, D.M. Finney, B. Bradley, J.C. LaChance, and J.P. Kaye. 2017. Managing the trade off between nitrogen supply and retention with cover crop mixtures. Agriculture, Ecosystems & Environment 237: 121–133.