Cover crops are great. If I thought I could get away with it, I would just grow cover crops in my garden. They protect the soil, feed microbes, build soil structure, add root channels, and support beneficial insects. I think they look cool too. When cover crop mixtures got popular a few years ago, I got excited and grew a 17 species mix. It looked really cool, I mean, diverse, with all sorts of seeds that became all sorts of plants. I took pictures, showed my kids, and even had a neighborhood open garden event! (Well, maybe not that last one) Then I grew some vegetables after the cover crop. They did OK. Just OK. I wanted it to be the best tomato/squash/cucumber/lettuce crop ever, but I could not tell the difference between these vegetables and those I had grown after many previous un-biodiverse cover crops. Recent research results may explain this.
Research thus far (summarized in these previous posts, 1, 2, 3) has consistently found that cover crop polycultures are not necessarily better than cover crop monocultures. This is now reaffirmed by a large study, done in Pennsylvania, published this year (Finney et al. 2016). The study had 18 treatments replicated four times (these studies are a lot of work). Eight species were grown as monocultures. Seven 4-species mixes, and two 8-species mixes, were the polyculture treatments, and they included a no cover crop check treatment. All this was planted in August, for two years (different field each year).
The big idea behind cover crop mixtures is that the increased biodiversity will result in increased productivity, increased ecosystem services, or both. The Finney group tested both hypotheses. They found that the mixtures produced less biomass than the best monocultures (here, canola and cereal rye). They also found that mixtures did not provide increased ecosystem services (here, weed control, nitrogen scavenging, nitrogen storage, and effect on following crop) over the best monocultures. These findings are related. Finney et al. found that most of the ecosystem services which we want cover crops to provide are related to biomass production. Because a few of the monocultures produced the most biomass, they also provided more services. From this they concluded that “a mixture may not be necessary” and “a single cover crop species may be sufficient and more economical than a mixture.” (I attended several sessions on cover crop mixtures at this year’s meeting of the Agronomy, Crop Science and Soil Science Societies, and did not hear any results that went against these findings.)
Mixtures do have one advantage, they can provide more services (multifunctionality) than a monoculture. However, in mixtures, the level of individual services provided is less than with a monoculture. For example, cereal rye is a great weed suppressor, but it does not fix nitrogen. If we mix hairy vetch, which fixes nitrogen from the air (an ecosystem service), with rye, we get both services, but the added vetch dilutes the weed suppression of the rye. There is a tradeoff in using mixtures to obtain multiple services (multifunctionality). In another strike against mixtures, Finney and Kaye (2016, same study, different paper) found that this multifunctionality was only weakly related to the number of species in a mixture. Their study, “does not support the hypothesis that increasing the number of species in a mix will lead to predictable increases in multifunctionality at levels that are agronomically or ecologically relevant.”
Why don’t cover crop mixes work better than monocultures? Well, first, some ecological theory. The idea that biodiversity is better than monoculture comes from ecologists studying natural habitats. In nature, they observe niche differentiation (Connor et al. 2011). The idea is that a diverse mix of organisms can better use the available resources because of their different use characteristics. When their resource use does not overlap much, they are complementary. Take a field of wheat. If some plant species needed different resources than wheat, then we could expect that adding that species to the wheat field might result in more production in the same area. Better resource use means better productivity. That is the theory.
Niche differentiation is seen most clearly with animals, but plants are different. First, they do not move. They are stuck where chance happens to put them. Second, plants require the same resources; sunshine, CO2, water, and nutrients (nitrogen for legumes is the exception). For these reasons, plants, especially annual crops, have much less opportunity for niche differentiation. And this is what we see in cover crop research results, little evidence of complementarity. Even in more natural systems, support for complementarity in plants is rare (Cardinale et al. 2011). The authors of this paper ask, “how can species be ‘complementary’ in their use of resources and production of biomass, and yet, a diverse community not perform processes any more efficiently than its most efficient species?” The simple answer is that there is no complementarity in these diverse mixes. Cardinale and colleagues cannot go this far, but rather think it “warrants more investigation.”
Rather than complementarity, there are simple tradeoffs. When plant species compete for the same resources, there are winners and losers. Mixing a less productive species with a high productive species reduces total biomass production (with the exception of legumes in infertile soils) Winners dominate losers in mixed stands, to the point where the losers are suppressed by the canopy, or larger root system of the winner. This is what we see in cover crop polyculture research. Dominant species, which happen to be our most productive crops, tend to dominate. If we control them by reducing their seeding rate, the less extreme species grow better, but not enough to make up for the lower population of the dominant species.
Finney and Kaye mention possible reasons why cover crop mixtures may not live up to ecological theory. Whereas polyculture advantages have been seen mainly in perennials growing together for several years, cover crops are annuals growing for just a short time. In natural systems, the number of species present is very much greater than in agriculture where we select dominant plant species for our crops.
This demonstrates the basic problem with attempts to make agriculture more like nature; agriculture is not like nature. Whether it be crop rotation, cover crops, or the need to supply farm fields with inputs, agriculture is not like nature. Therefore, we should not be surprised when one of these principles, here the diversity-productivity relationship, does not apply.
There are other reasons not to use mixtures in cover crops. First, because monoculture powers crop rotation benefits, planting a cover crop mixture increases the risk that a pest will find something in the mix it likes. Add to this the difficulty in seeding multiple species, finding the right timing for planting a diverse mix, and the increased cost of seed blends…if these mixtures do not give extra benefits, why grow them?
What might be a feasible and possibly beneficial option is planting multiple varieties of one species. Wenatchee ARS researcher, Mark Mazzola (2002), has found significant differences in the effects of different wheat varieties on the soil microbial community. Similar effects may occur with varieties of other cover crop species. Adding this kind of diversity to a cover crop comes without all the problems of species mixes and may prove beneficial.
I think that any cover crop can do some good. If you like planting polycultures, do it. But don’t let the appeal of the silver bullet, of the secret solution, cloud your judgement. Novelty entices the most sober-minded of us into thinking “this is it.” I still sometimes grow cover crop mixes, but also monocultures. Both are good, but as I found, and as science is confirming, cover crop mixes are not the restore-everything-to-as-it-should-be final solution we hope for.
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
Connor, D. J., Loomis, R. S., & Cassman, K. G. (2011). Crop Ecology: Productivity and Management in Agricultural Systems. Cambridge University Press.
Finney, D. M., & Kaye, J. P. (2016). Functional diversity in cover crop polycultures increases multifunctionality of an agricultural system. Journal of Applied Ecology, n/a-n/a. https://doi.org/10.1111/1365-2664.12765
Finney, D. M., White, C. M., & Kaye, J. P. (2016). Biomass Production and Carbon/Nitrogen Ratio Influence Ecosystem Services from Cover Crop Mixtures. Agronomy Journal, 108(1), 39. https://doi.org/10.2134/agronj15.0182
Mazzola, M. (2002). Mechanisms of natural soil suppressiveness to soilborne diseases. Antonie van Leeuwenhoek, 81(1–4), 557–564.
Added 2-21-17
Further evidence that mixes are not necessarily better than monoculture cover crops: http://cropwatch.unl.edu/2016/cover-crop-mixture-diversity-and-function
Update 3-27-17
OK, here is a study that has found transgressive overyielding in mixes that are not grass+legume. Although most of their overyielding plots were grass, or mustard, plus a legume, and most occurred in unfertilized plots (both of which I mentioned as the exception in the above and previous posts), this study also found that mixtures of mustard+oat outyielded plots of monoculture mustard or oats, but only when fertilized and only when specific proportions of each species was in the mix. They used unreplicated plots in a response surface design, so the statistics are not something I am familiar with. The authors did mention what I have pointed out, that besides their study, there are very few studies that have found transgressive overyielding in cover crop mixtures. The two studies they cite both used grass+legume mixtures with no added nitrogen.
The paper: Wendling, M., Büchi, L., Amossé, C., Jeangros, B., Walter, A., & Charles, R. (2017). Specific interactions leading to transgressive overyielding in cover crop mixtures. Agriculture, Ecosystems & Environment, 241, 88–99. https://doi.org/10.1016/j.agee.2017.03.003
Update 4-10-17
Here is my question. Is a forage crop seeded after harvest of a main crop just a forage crop, or is it a cover crop, or both? Villalobos and Brummer stick with “forage crop” in their recent paper. They planted grass monocultures (triticale, wheat, and barley) and various mixtures of the grasses with brassicas and legumes. The mixtures did not produce more than the best producing monoculture which was triticale (no transgressive overyielding). If we were after biomass yield, the monoculture would have been the easiest way to obtain it. Although it was not reported, the soil must have been well fertilized because the legume mixes did not do any better than those without legumes. Similar to other such research, V. and B. reported that the mixtures were dominated by a particular species, in this case one of the Brassicas. Finally, as with the yield, so with forage quality; the mixtures did not do any better than the monocultures (if this is the focus, then I would argue this is a forage crop, not a cover crop).
What all this research tells us is that, if there are mixtures that are superior to monocultures, it appears that they will take a lot of work to find. Just mixing up a cocktail from diverse groupings of species does not seem to be a consistently successful strategy.
Villalobos, L., & Brummer, J. E. (2017). Yield and Nutritive Value of Cool-Season Annual Forages and Mixtures Seeded into Pearl Millet Stubble. Agronomy Journal, 109(2), 432–441. https://doi.org/10.2134/agronj2016.06.0324
Update 5-9-17
Another attempt to find advantages of cover crop mixtures. This time at five locations across Iowa, over two winters, using cover crops planted after soybeans. From the abstract: “Cover crop mixtures had no advantages over monocultures except for increasing fall stand density.”
Appelgate, S. R., Lenssen, A. W., Wiedenhoeft, M. H., & Kaspar, T. C. (2017). Cover Crop Options and Mixes for Upper Midwest Corn–Soybean Systems. Agronomy Journal, 109(3), 968–984. https://doi.org/10.2134/agronj2016.08.0453
Update 6-12-17
More mixed results. Overyielding was found in a mixture of a legume with oilseed radish, but not with a mixture of a legume with a grass. Contrary to their hypotheses, the authors found that cover crop mixtures did not increase soil nitrogen nor suppress parasitic nematodes better than monocultures. Cover crop mixtures did not predictably increase following crop yields.
Their conclusion, “The specificity of spatial and temporal diversity effects warrants consideration of plant species choice in mixtures and rotations for optimal employment of beneficial legacy effects.” Or, given our results, we don’t know what to tell you about choosing cover crop species for rotation effects, but we didn’t want to say that.
Barel, J. M., Kuyper, T. W., Boer, W., Douma, J. C., & De Deyn, G. B. (2017). Legacy effects of diversity in space and time driven by winter cover crop biomass and nitrogen concentration. Journal of Applied Ecology. Retrieved from http://onlinelibrary.wiley.com/doi/10.1111/1365-2664.12929/full
Update 8-07-17
The title of this paper says it well: Crop Biomass Not Species Richness Drives Weed Suppression in Warm-Season Annual Grass–Legume Intercrops in the Northeast. And their conclusion, “…if weed suppression is the top priority, our results suggest that monocultures of high biomass–producing grasses will provide more effective suppression at a lower seed cost than functionally diverse intercrops…”
Bybee-Finley, K. Ann, Steven B. Mirsky, and Matthew R. Ryan. 2017. “Crop Biomass Not Species Richness Drives Weed Suppression in Warm-Season Annual Grass–Legume Intercrops in the Northeast.” Weed Science, July, 1–12. doi:10.1017/wsc.2017.25.
Update 8-29-17
“Rye and oats provide the best biomass return on seed investment! Single Species are the way to go in Iowa for corn and soybean producers.” This is the conclusion from a multiple location study in Iowa. Results are not yet published, https://iowalearningfarms-wordpress-com.cdn.ampproject.org/c/s/iowalearningfarms.wordpress.com/2017/08/10/exploring-whether-cover-crop-mixtures-make-sense-on-iowa-farmland/amp/
Comments
Try Gabe browns 30 years of research followed by natures 4 billion and maybe the you will find diversity is the key in nature but it might not be the most productive. ” So having a mono culture has every plant sending out the same messages and requires the same nutrients at the same time” this is a laymen statement from fiona hubbard. My wife who has just introduced herself to bio agriculture. Pretty solid pick up on soil biology.
I have not seen Gabe Brown’s research where he directly compared mixtures to monoculture cover crops. He, and it sounds like you too, assume that nature is the best model for agriculture. Nature does not provide for us what we get from agriculture. Because it doesn’t, we do agriculture. So if nature does not do what agriculture does for us, why should it be a good model for agriculture? Nature often does not have a choice except be biodiverse. We have a choice in agriculture, and so far the research supports monoculture cover crops.
I would argue you are also making assumptions against ‘nature’ in a very narrow minded concept of agriculture. For the parameters you have decided to include you may be correct. However I argue your scope is too narrow to include a
decent argument against nature and biodiversity in agriculture. Certainly knowing the rate of extinction occurring now. It is not a black and white issue, as you have portrayed.
Gretchen, yes, I am narrowing what I think we should take from nature as beneficial for agriculture, but I hope that does not make me narrow minded. We can learn from nature, but not everything in nature is necessarily beneficial for agriculture, because nature is not optimized for the goals of agriculture. I think we can learn much more from the adaptations of individual species, for example, but less perhaps from how we see all the species in nature organized (or unorganized, depending on your perspective). I am a proponent of diversity in agriculture, through crop rotation. The best way to lower agriculture’s impact on wild species is to maximize production on existing farmland so that we do not have to use more land to produce food.
What part of nature is not beneficial to agriculture? When I see a natural system (one that has had little influence from man), I see a system that is resilient to climate, not reliant on inputs, yet abundant with healthy food. Nowhere have I seen where our increased production has been beneficial to wild species.
Hi Gail. I am not sure what natural system you are thinking of, but none of them produce abundant human food. If they did we would not need agriculture. Because they don’t, two things are apparent. First, we need agriculture. And second, agriculture, to do what we need it to do must be fundamentally different than nature. Nature does not need inputs because it has very low exports, ~10% at the most. Compare that to the year-after-year export of ~50% of the productivity from farm fields and you see why farming requires inputs.
Outstanding compilation and summary of cover crop options that is applicable to both farmers and gardeners. Your writing style is easy to read and follow, summarized the results in layman’s terms that was down-to-earth, and was clear and concise!!
Well put together summary Andy. Thank you for sharing.
I think the jury is still out on biodiversity for me. I think it has merit. Think of planting a legume like alfalfa, for example, along with a “grass”, or a variety of grasses, for a forage mix. Both plant varieties will benefit in most cases. And the overall feed quality of the feed stock improves too. It provides a more balanced ruminant diet, with less chance of bloating, etc.
I HAVE seen Gabe Brown’s support for biodiversity of cover crops, and it’s pretty compelling. But you should be aware that he is also advocating for biodiversity of intended use of the crops grown as well. He strongly believes in grazing animals as being highly beneficial to a cropping system, including row crop cash crops. That won’t give you a corn crop every year however, or even every other year, and if cash grain crops are to be your primary means of support, you will in general be left with “a shorter end of the stick” using his plan, if you have to take a “loss year” or two simply to grow cover crops to replenish the soil, without the benefit of grazing animals as a cash crop in those years.
Serious cover croppers don’t ever want to put themselves in a position where they need to “replenish” the soil. That inherently indicates that we have somehow scavenged the soil, and reduced it’s viability and micro-biology… so it needs to be recharged and replenished to get it back to where it once was. They hope to only be in a “soil building” mode, even if they have very high soil organic and fertility levels. Always, every year, continue to build to better condition… but never reduce or deplete… so never have to “replenish”.
That’s why they advocate for never tilling the soil. Never leaving the soil bare. Never leave the soil without a growing, living root of something in it… even through a very cold winter. There’s still benefit to that root being there, even if the plant above the ground appears to be “dormant”.
I think this is a field that still needs more investigation.
But have you seen Gabe Brown’s comparison of his mixes with the best monocultures? I ask because I don’t think such a comparison exists (let me know if it does!). Grazing cover crops is also a different purpose – the cover crop is really a forage crop, and forage mixes have advantages.
I agree with adding livestock, no-till, keeping the soil covered…just saying that the research on cover crop mixes suggests that monocultures are often better.
Andrew, I spent way too many years practicing the “monoculture mindset.
What it left me with was a severely degraded ecosystem which required high use of synthetic fertilizers, pesticides, fungicides and herbicides. I have followed this with over two decades of comparing monoculture production to one which focuses on regenerating the resource by following natures principles which includes diverse mixes. I am not going to argue with you when you say a monoculture may produce higher biomass, may give better weed control, etc., etc.. BUT what ramifications do those monocultures have on the resource and PROFITABILITY? For example you say a diverse mix may attract pests, true but you fail to mention that a diverse mix WILL attract and provide a home for the predator insects which would control that pest. hence the reason I have not used an insecticide for over 16 years. You mention that by adding hairy vetch to a rye mix weed control was less effective, however you did not mention that hairy vetch can easily supply all of the nitrogen requirements for the rye. I could go on and on and on. One of my favorite quotes is from Don Campbell who said, “If you want to make small changes, change the way you do things but if you want to make major changes, change the way you SEE things.”
Gabe, thank you for your comments. When I look at your system and the systems being studied in the research I summarized above and elsewhere, the big difference is grazing. Adding livestock, as I think you would agree, changes everything in a cropping system. Your “cover crops” are really annual pastures, and as I mentioned in a comment above, there are advantages of forage mixes. I also mentioned in my post above, and elsewhere, that grasses with legumes (your rye with hairy vetch), are the exception to the monoculture advantage, especially in the absence of nitrogen fertilizer, like in your system. If my goal was to do without synthetic nitrogen, I would certainly grow cover crop mixes of grasses and legumes.
Andrew, When practicing a monoculture cover, your c/n ratio is set in stone. As a farmer, it is critical that I’m thinking of impact 1,2,3 steps ahead. Polycultures allow me to do many things to set up for success in the future. I can tailor my CC to any ratio that I am needing, all while accomplishing different tasks that would otherwise take me years or decades in a monoculture. As Gabe and many others that have commented here well know, trial data can be nice, but the ramifications of what we experience on our own farms over years and years of trial, failure, success, and observation is invaluable.
Andrew, I have 400 acres of cropland that I am not able to graze any livestock on. I have been able to advance soil health on those acres to the point we no longer need synthetic fertilizers, pesticides or fungicides also. I have done this by using diversity including diverse polyculture cover crops either before, along with or following a cash crop. You said if your goal was to eliminate synthetic nitrogen you would grow mixes of grasses and legumes, why would your goal not be to advance soil health to the point you can significantly reduce/eliminate N? Nitrogen if free, is not there something like 32,000 tons of atmospheric N above every acre? To use fossil fuels to produce nitrogen is just plain wanton waste. I challenge every researcher to have to make a living from their production, it would open their eyes.
Gabe, if you are not adding any nutrients to your 400 acres, and you are continuing to harvest and export crops from those fields, then your soils are being depleted of nutrients. Perhaps not nitrogen because of your legumes, but certainly all the other nutrients that leave your fields in the harvested crop. The microbes may be allowing you to continue to do this but that doesn’t make it sustainable. This is where farming differs significantly from nature. Nature exports little of its production so almost all of its nutrients are recycled. THAT is why nature continues without external inputs.
Nitrogen from air is not free. Legumes require time and space in a field, time and space that could be growing food crops. Nitrogen is not even free for the legumes – they must pay the price of nitrogen by giving over some of their photosynthetic products to the bacteria fixing the nitrogen in the nodules. This reduces the yield potential of legumes compared to grasses which do not have to pay this price.
Andrew, very good question to ask whether agriculture can be sustainable like nature without inputs. The products agriculture produce like corn, rice, beets and so on, requires it to be agriculture and not nature.
When we I’m thinking of using nature in agriculture, it’s the principles. Conventional agriculture in more geologic then biology, and we need to take advantages of the biology to produce good and profitable food.
About the inputs. A crops in mostly Carbon, Nitrogen and water, those are all free inputs in unlimited quantities. The nutrient part is very very small compared to the many many tons of biomass, and the soil/subsoil is one big pile of them. If we have the biology go extract it they are almost also unlimited, but of course, I don’t know how many years we can go on without applying any.
For now I adore the production system people like Gabe Brown and David Brandt uses. More of that.
About mono or poly culture. There are many cases with results showing that is the way to go.
I found this video with Dr. Christine Jones making things clear, and thought you should see it. Man can she tell us how it works.
https://vimeo.com/149989170
Gabe, nice response. Dr. Elaine Ingham has done great work in showing the limitations discussed as well. One issue is the test itself. A mono vs poly mix test/trial in Penn will not be an accurate read for a producer in Texas. I have read research from Iowa State. Great school. Great people. But narrow minded in regards to the research performed. We have to understand our goals! A successful mix in ND might not work for me in Missouri. Also, are we only looking at short term results? Gabe, your research is over a long period of time. Very valid. Trial and error in a given local will determine which mix to use. And, some goals might be best achieved via a mono crop.
Dave, thanks for your comments. The post above is a summary that added the results of the Pennsylvania research to research done in New Hampshire, Nebraska, Michigan and Japan. In all these places, the results were similar (see the link above under “research thus far”). Perhaps research in Missouri and North Dakota would give different results, but I don’t think we have to have research in all 50 states before we can start to generalize the results. If you know of any examples of published research that goes contrary to what I state, please post it here or send it to me, andrew.mcguire@wsu.edu and I will gladly post it.
Watch the last 5 min. of this talk with Gabe, more nutrients in soil where nothing is added https://www.youtube.com/watch?v=tuwwfL2o9d4
Hi Andy, very good summary of these concepts. If you lived closer, I’d arrange to bring you in for a presentation! (I’m at Cooperative Extension in Rhode Island). I do want to address the idea that some people actively seek to use as much legume-fixed N as possible. In that case my understanding is that Rye-Vetch is the #1 combo. In the BIG picture of things, it seems reasonable to assume that using legumes to gather N (between cropping seasons) is always going to be better, greenhouse gas-wise, than exclusively relying on manufactured N fertilizers. In that case, there is another trade-off that you are not addressing, which is biomass production vs. CO2 footprint. How, in your opinion, does this sort out?
I am no expert on the greenhouse gas tradeoffs of cover crops, and I am not against using legumes to fix nitrogen, but as I said in another comment, legume-fixed nitrogen is not free, so those costs must be factored in. In many cases it will depend on how much N is in the soil when the cover crop is planted; whether your cover crop will be able to scavenge enough to produce maximum biomass, or whether it will be limited by N availability. Another assumption is that we have to use fossil fuels to produce N fertilizer. Here are some other possible options that may be viable in the future, http://thebreakthrough.org/index.php/issues/decarbonization/ammonia-is-everest-base-camp-for-clean-energy.
One advantage of a polyculture versus the monoculture is balancing the C:N ratio which is effectively feeding the microbes in the soil. High C:N monocultures, such as ryegrain, can stunt the following cash crop as the microbes utilize applied N to breakdown the organic material. Legumes, or a low C:N should always be utilized with a high C:N cover crop.
The new clover varieties developed by my company can supply substantial amount of nitrogen as well as biomass. Our FIXatioN Balansa clover produced over 40 tons of green matter in trials in Illinois this past year, more than enough to offer complete weed suppression.
It would seem the most simple solution is to plant single-species bands (monoculture) on alternating rows (e.g., one for weed suppression, another for nitrogen fixation).
There are multiple ways to introduce biodiversity at even small farm scales.
Ken, that is a very interesting idea. There is some work looking at precision cover cropping where different covers are planted in different rows, and then the cash crop is precision planted right over, or between, specific covers. This is intriguing but I have not seen any results so far.
One of the problems with a mix that I see is that once you get so many species in it (6, 8, 10+?) each plant of your following crop is going into soil that has been affected by just a subset of those many species. It is just a matter of space. So in terms of adding diversity to the soil, there is a randomness as to what diversity your cast crop plant experiences.
So I’m confused. Agriculture isn’t linked to nature. And there is no way to replenish land
By using covers?
Oh I see you got your data from PSU research farm?? Hmmm that’s sad. Hard to get get conclusive data off of small plots that r beat to death.i be been planting in large rye for 4 yrs now and PSU trys to duplicate and they can’t so they publish it can’t work, right?
I get asked how come it works on your farm?I get off my tractor and make adjustments! I have a mortgage to pay!
I sit on the PA Notill Alliance and never even heard of this HUGE study done on covers . I would like to see this complete project.
On my farms my MS mixes are double the size of single spieces , but I’m just farmer. My data don’t matter.
Why in the world does farmer’s like GAbe and Gail and me get numerous E-mails on how to do these things?? Farmers don’t trust universities!
Hi Lucas, thanks for the comments. What I said was “agriculture is not like nature.” Regarding your “replenish land” comment, I wrote that cover crops “protect the soil, feed microbes, build soil structure, add root channels, and support beneficial insects.” I said it was a large study, not “huge”. I am only reporting what I read in published, reviewed, research papers. And the study at Penn State was just one that found similar results (see links at beginning of second paragraph). One study does not prove a point, but when the results of multiple studies done by different researchers in different locations under different conditions start to align, then I cannot dismiss it so easily.
So why is agriculture not like nature? Why can’t we mimic nature in AG?
Nature does not produce human food in the quantities that we need. This means we need agriculture (here I am mainly referring to intensive row crop production that gives the world most of its calories). And since agriculture must do something that nature does not do, it must necessarily be different than nature.
Nature also also has the advantage of not having to maintain itself AND annually export 50% of its production. If you really want to mimic nature in agriculture, only export 10% of what you produce, like nature does.
We can learn from nature, especially from the adaptations of individual species, but we should not assume that just because nature does it a certain way, that it will also work for agriculture. “Diversity may be there for a reason…but that does not mean that diversity is there for a purpose.” -RF Denison in Darwinian Agriculture.
I think the best way to add diversity in agriculture is through crop rotations (including cover crops) of monocultures.
A.J. (comment below), thanks for your thoughts. I am reporting on the published research because that is what Extension does. Perhaps you are right, and results take a longer time to appear, but that is something that farmers should know when trying cover crop mixes.
I think that, in the long-term, exporting 50% of the production and replacing those exported nutrients is more sustainable than exporting the same 50% and just relying on the soil to provide the exported nutrients. The latter turns farming into a soil mining operation with all the microbes as “little miners” (I’ve heard people call microbes this, and they thought it was a good thing). Taking out nutrients from the soil, however it is done, without replacing them – isn’t that depleting the soil?
Agreed. Taking nutrients from the soil without replacing them depletes the soil. However, I am of the opinion we can build these soil nutrients through the use of different management styles (no-till, cover crop mixes, crop rotations, livestock, decreased chemical and commercial fertilizer use…). Microbes as “little miners”…the resource they are mining is the inorganic nutrients that are not plant available without their “mining process.” By adding commercial nutrients to the system, a person is placing these hard working microbes on “welfare.” They are not required to work as hard and we see the compounding and cascading effects of our soils being more and more dependent on external nutrients.
“Nature does not produce human food in the quantities that we need.”
Does agriculture have a production problem in feeding the population, or do we have a distribution problem?
Regarding taking out nutrients and depleting the soil, there is this idea as expressed by soil microbiologist Elaine Ingham: “Every second of every day, the parent material underlying your soil is being broken down into rocks, pebbles, sand, silt and clay, such that until the bedrock of the planet ‘runs out,’ or you run out of organic matter, you should never have to worry about a lack of minerals in your soil.
“… Rather than applying nutrients and minerals, we must concentrate on repopulating the soil biology so that the organisms can break down the nutrients to make them available to plants.” — https://mosesorganic.org/soils-natural-process/
Part of the same idea expressed by Christine Jones: “Somewhere between 85 to 90 percent of the nutrients plants require for healthy growth are acquired via carbon exchange — that is, where plant root exudates provide energy to microbes in order to obtain minerals and trace elements otherwise unavailable.” — http://www.ecofarmingdaily.com/interview-sos-save-soils-dr-christine-jones-explains-life-giving-link-carbon-healthy-topsoil/
These articles are more casual, not providing the sources behind their statements, but sounds plausible and worth looking at their work more closely to see what can be learned. While also looking at farmers practicing methods in line with this view. May reveal something of value.
Dan, thank you for the comments. I am aware of the story that all these sources tell, but as you observed, the scientific evidence is scarce or non-existent. I have looked.
It is true that minerals can be weathered to release nutrients and that part of this weathering is done by microbes, but the difficult problem is the rate. All the studies that I have seen say that this rate of mineral weathering is slow or very slow, not near the rate needed to supply crops. Mineralization of soil organic matter is much faster, but organic matter is definitely a limited resource and generally we don’t want it to supply all our nutrients as we cannot build it up at that same rate, and so it is depleted.
In regards to mineral weathering, I wonder why there are many natural environments that are limited by phosphorus or nitrogen supply. If this natural weathering can do what they say it can do, why do these limitations exist?
For more on the the topic of mimicking nature see https://csanr.wsu.edu/dont-mimic-nature-improve-it/ and https://csanr.wsu.edu/ecosystems-are-not-smart/
I have a few questions for you. How do you so easily dismiss real world results by producers who are seeing significant advantages to multispecies cover crops? I think what you fail to realize is the benefits do not happen in 1,2,3 years. Just like we did not deplete our soils to the conditions they are in today in a few years.
How long do you assume we can continue to export “~50%” of crop production and have to add inputs to replace these exported nutrients? This is not a sustainable system.
I am afraid it appears that you are looking at ag with blinders and near-sighted vision. We cannot continue to deplete our soils and be reactive to the problems (add NPK from synthetic or mined sources). We have to change the way we look at agriculture.
I appreciate your time on looking at this research, but please try to step back and look at the larger picture, not just the bottom line of production.
Although an advocator of multi-specie cover crops, I appreciate Andrew’s concluding ststement:
“I think that any cover crop can do some good. If you like planting polycultures, do it. But don’t let the appeal of the silver bullet, of the secret solution, cloud your judgement. Novelty entices the most sober-minded of us into thinking “this is it.” I still sometimes grow cover crop mixes, but also monocultures. Both are good, but as I found, and as science is confirming, cover crop mixes are not the restore-everything-to-as-it-should-be final solution we hope for.”
Just as with any other production input, Cover Crops are not a miracle answer, but require proper use and probably additional management along with a different mindset compared to the addition of fertilizer or use of insectacide or fungacide. The current popular train of thought with diverse cover crops has arisen from the work of DJ Tilman et.al. from their work “The Influence of Functional Diversity and Composition on Ecosystem Processes” Published in Science magazine in 1997, and built apon by the experience of Gabe Brown, David Brandt, Gail Fuller and others who have become intimately involved in advocacy of more “Natural” systems and reduction in the need of , specifically, synthetic inputs and reduction in the need for chemical control of weeds, pest and disease. I believe that through their and others examples that this is proven to the point that it works in their production system. Even these advocates would tell individual producers that they have to find the system of nanagement that works for their farm and that there is not one size fits all solution. Specific concerns are best addressed with single species, like using radish or sorghum sudangrass to reduce compaction, or Mustard to control soil pest, or Cereal Rye for weed supression. As we move away from addressing single concerns this makes available the use of mixes where we have specie complimentarity.
Andrew’s meta analysis above does not specifically discredit the use of multi-specie cover crops, but points out the commonality found in the most recent published literature, and that I have found in published trials ever since the 1997 publication in Science magazine, that a cover crop mix may not address SPECIFIC functions in regards to weed supression (adequate biomass) or crop response (equal or increased yield). I do not recall if these have been multi-year studies or only based on single or short (3 yr?) trials, as they are normally connected to graduate student and grant funding timelines. Long term cover crop adopters will admit that there is not always a known crop production benefit, (as compared to other fields in the same production year) and that that is not necessarily the decision point being used for the cover crop practice. As I understand it, many producers using cover crop mixes have identified benefits of increased beneficial insect populations, reduced disease, increased crop resiliancy (less plant stress during significant weather event), reduced fuel consumption, all of which provides some financial return. Additionally, they like the way it looks compared to bare ground or brown crop residue. We can all use positive endorfins when we can get them, but let’s keep our eyes wide open all the same.
This is by no means the forum to hash out all the data that does or does not exist in regard to use of cover crops and multi-speci mixes. I do see much more work by land grant universities and private non-profits looking at these concepts to determine if there is a way that cover crop use can show trand data much like Andrew has pointed out here. The next step is to asy why? In this case we need to understand the end goal as determine if the mixtures prepared were adequate to meet the goal. Were the seeding rates appropriate? Were the specie complimentarity aspects considered? Are there other aspects that could be measured to quantify other benefit such as soil density, water permeation, aggregate stability or others?
I enjoyed reading the comments as much as reading the article. Thanks to everyone who replied in the comments. I have met Gabe Brown, Gail Fuller and Paul Brandt a few times over the last few years. They are all extraordinarily good and intelligent folks.
Personally I think that some cover crop is better than none, some diversity is better than none and grazing would be highly complimentary to all. This article makes me feel a little better for the times when I just don’t have the resources to spend $60/acre on high diversity cover crop seed though, haha.
Some Smart people in here. Need some advice on my rotation with intense grazing.
I no till corn in the spring. Harvest early in September as HMC and silage immediately start planting covers on 15 inch rows and spreading heavy bedded manure. Have various mixes of rye, tritacale, wheat, tillage and ground hog radishes, winter peas. Allow time for good establishment, then intense grazing by cattle. Repeat repeat etc.
I am very mindful of compaction when wet and these calves are well trained to go eat protein at the bunk each day. One of the tracts has water hemp issues that I monitor closely. I call it turbo hemp as the manure super charges it.
I want to set as much nitrogen as possible while also providing a fast growing forage for the yearlings to intensely graze. Like the Buffalo of the most recent ecosystem before agriculture I like to think it is mimicking. Encourage rooting, fertilizer, just looks really cool.
I kick 300 steers out onto each quarter section that have well established very wide waterways.
Ideally want to make all my own seed someday too, if feasible.
Throw ideas at me.
Issues:
1. Allelopathy in rye/corn
2. Plant green or terminate first
3. Allow regrowth before planting corn or immediately after final graze cycle
4. Bet way to set nitrogen
5.
Wow! What a delightful exchange. My hat is off to Andrew for sticking in there. You have defended the “externalist”or scientific point of view admirably and robustly. My hat also is off to the many who have chimed in with another perspective–that of the “internalist”or practitioner. Both perspectives are acceptable and based on reasonable choices. The externalist or scientist chooses to assume they are separate from nature (observer is independent of the observation) and seeks to extract consistencies from that external world and use them to their benefit. The internalist or practitioner chooses to assume they are part and parcel of nature and must work with it as a part of the whole. The externalist tends to be reductionist, separating systems into their parts and trying to reorganize those parts to gain more control. The internalist is always connecting their activities to the whole seeking harmony more than control. The externalist tends to work with relatively short time frames while the internalist is in it for the long haul. Thus both often have very different ideas and approaches to sustainability. These two perspectives are based in fundamentally different underlying assumptions, upon which reasoning follows; furthermore, we become emotionally attached to these assumptions (being unstated the emotional part is usually unrecognized) and no amount of reason causes someone with one perspective to adopt the other perspective. Such colossal changes in perspective usually take profound experiences. The diversity embodied in these two perspectives is wonderful, and the dialog contained in the above comments is amazing in its creativity and mutual respect–offering the truly open person the best of both worlds. Reading this paper with the follow-on comments has been a wonderful learning experience for me and I am grateful to everyone who participated. Thank you.
John, thanks for your comments. I am no philosopher so I will have to take your word on this. Also, as I understand it, I think it is my job to be an externalist. 🙂
Nice, detailed and thought-provoking article Andy! I also appreciate the comments/discussion this has generated.
I’ve been doing research with mixtures in California organic vegetable systems since 2001 and have found that their performance is extremely complex, and like most agricultural issues, and often site and year specific. Here’s a link to a 7 minute video that I made a few years ago: “Are legume-cereal mixtures a good fit for organic vegetable production?” https://www.youtube.com/watch?v=WREmHa-jFbc
Suppressing weeds in cover crops is critical issue in the systems I work with and this requires high seeding rates that are often several times more expensive per acre and per unit of biomass production than if a monoculture nonlegume was used.
We also have a serious problem with survival of the legume component in soils where there is excess N from previous crops.
Below is a list of a few of my publications on cover crop mixtures if anybody wants more details. These papers are available for free on the following site. http://www.researchgate.net/profile/Eric_Brennan/
-Brennan & Boyd, 2012. Winter cover crop seeding rate and variety affects during eight years of organic vegetables: I. Cover crop biomass production. Agronomy Journal 104, 684-698.
-Brennan & Boyd, 2012. Winter cover crop seeding rate and variety affects during eight years of organic vegetables: II. Cover crop nitrogen accumulation. Agronomy Journal 104, 799-806.
-Brennan et al, 2013. Winter cover crop seeding rate and variety affects during eight years of organic vegetables: III. Cover crop residue quality and nitrogen mineralization. Agronomy Journal 105, 171-182.
Brennan et al., 2009. Seeding rate and planting arrangement effects on growth and weed suppression of a legume-oat cover crop for organic vegetable systems. Agronomy Journal 101, 979-988.
Brennan et al. 2011. Comparison of rye and legume-rye cover crop mixtures for vegetable production in California. Agronomy Journal 103, 449-463.
Eric, thanks for the comments. I look forward to reviewing your papers.
Isn’t one of the key roles of a cover crop to feed the multitude of flora and fauna that live in a healthy soil, and just like us ( and grazing animal, as pointed out in an earilier comment) we do better on a diverse diet. Therefore if there is even a varietal affect of the same species on soil microbial populations (as outlined in the study by Mazzola) and the fact that we haven’t as yet identified everything that lives in the soil, we must have the best chance of supplying a suitable food source if we include a diverse range of plants in the cover crop mix.
Also mustn’t another benefit of diversity in the mix is the varied root structures different plants supply which allows them to explore more effectively the whole soil profile.
Craig, what you say about diverse plants feeding diverse soil biology is definitely the dominant hypothesis. And I am familiar and impressed with Mazzola’s work, but if the research that I am reporting on does not show the benefits of diverse mixtures, then maybe we need another hypothesis, or the effects of diverse soil biology are less than we have assumed (there are probably other possibilities too). The varied root structure is another hypothesis that gets a lot of attention, but again, if the research does not show the benefits, then perhaps we should reexamine our hypotheses. We should not continue to following hypotheses because they sound right, but because there is research that supports them.
Thank you Andrew for delivering all this research information.I am from New Zealand and a number of us are farming using the principles of regenerative agriculture. I like that your mindset is not out to prove the concept of regenerating agriculture wrong, but to look deeper into the science around it. Two questions.
1. Is the Jena study relevant to any of this?
2. If it is unsustainable not to replace nutrients that we have exported, is it sustainable to mine non-renewable nutrients instead?
The research of David C Johnson growing huge biomass in arid conditions seems fascinating, if not ground breaking but waiting to see if that translates into gains in non – arid farming conditions.
Hamish, can you please give me more information about the Jena study?
You are right about mining, wherever it is done; in the long-term, it is not sustainable. Recycling is the long-term solution with lots of problems.
Thanks for your reply.
Here’s the link to the Jena study in Germany in general with links to all papers…
https://www.sciencedirect.com/science/article/pii/S1439179116300913
This is probably one of the most relevant papers in the collection…
https://www.sciencedirect.com/science/article/pii/S1439179116300913 [https://www.sciencedirect.com/science/article/pii/S1439179116300913]
Hope that helps.
The Jena studies that you reference are for grasslands, and they do indeed find that increasing species richness (number of species) increases productivity. They even find transgressive overyielding. However, this is with perennial species, and as this and other studies have found, it takes several years for the the biodiversity effects to become significant. With cover crops, we are using annuals, and the time frame is months, not years. The Jena plots also include legumes with grasses, and some of the treatments are unfertilized. As I mention, this combination has often been found to outperform monoculture grasses.
I am reading you Andrew. This is where we see potential with pasture cropping in combination with BEAM Su/Johnstone compost so we can hopefully drill straight into a permanent perenial pasture with an annual without herbicides. Thanks for your great discussion and replies.