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Perennial grains debate

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Thanks to Billy Kingery for calling my attention to a recent paper by Chris Smaje, titled "The Strong Perennial Vision: A Critical Review", published in Agroecology and Sustainable Food Systems. The same journal published responses by Steve Gliessman and by Timothy Crews et al., to which Chris has responded on his blog, Small Farm Future.

These authors have probably thought about these issues more deeply than I have. And I don't want to try the patience of a couple of coauthors who are waiting for revisions on papers. So, for now, I'll just make a few comments on Chris's paper.

Among his points are 1) "there is no reason to suppose that lower-yielding perennial cereal crops cannot play a significant role in the sustainable food systems of the future" but 2) it's unrealistic to expect that they won't be lower-yielding. I tend to agree with both points. In my book, arguing for a diverse agricultural-research portfolio, I wrote:

"As explained in Chapter 7, I have my doubts about perennial grain crops. But do we really want to focus 100% of our grain-breeding efforts on annuals?"

One argument against claims that perennial grains will probably have lower yield is that apples sometimes allocate as much of their photosynthate to fruit as annual grains do to seed, as noted in a paper by Van Tassel et al. Chris points out that the apple trees in question were "physically supported and on strongly dwarfing rootstocks-- [and] are rarely grown commercially for [more than 10 years]." Chris's point is that this apple system isn't reliable evidence that perennial grasses could have high allocation to grain. I agree, but how much could we push the protein and calorie yield of tree crops? For example, could almonds be bred and managed to match the yields of annual grains? Even dwarf trees are taller than most weeds, so weed control could rely more on mowing and/or cover crops than erosion-promoting tillage or herbicides.

Chris notes that:

"some perennials (often C-strategists) can undoubtedly outperform some annuals in yearly resource capture, and this has been shown empirically in some cases, for example, in comparisons of Miscanthus with maize, alfalfa with soybeans, and perennial with annual sorghum (Piper and Kulakow 1994; DeHaan et al. 2005; Dohleman and Long 2009)"

but questions how much "breeders can manipulate plant allocation" to put extra resources acquired by perennials into seeds. He discusses this issue in some detail, but this still seems like an open question. Key to his arguments is Grimes' classification of plant strategies as competitive, stress-tolerating, or disturbance-adapted. If these strategies are really discrete and reflected throughout a plant's genome, I guess it would be hard to change. But given the apparent ease of transitions between annual and perennial (Silvertown and Dodd, 1966, cited by Smaje) or unicellularity to simple multicellularity, I suspect that it may not be too hard to change these strategies through plant breeding.

This wouldn't eliminate tradeoffs between individual-plant competitiveness and whole-crop yield, however. As Chris's paper states:

"It may indeed prove possible to select for high seed allocation in short-lived, C-strategy perennials whose growth habit enables them to accumulate more photosynthate than their annual counterparts, thereby simultaneously funding seed production and perennation. But, as Van Tassel (2012) concedes, these plants will be less competitive than co-extensive wild or weedy species--suggesting that the margin for cultivating them without resort to irrigation, tillage, fertilization, pesticide application and other such practices that perennial cereal cultivation aims to circumvent may be thin at best."

There's much more, including comments on sociological issues, so I expect this paper (and responses to it) will stimulate a lot of discussion.

In Darwinian Agriculture, I argued that accepting tradeoffs rejected by past natural selection is key to past and near-future crop improvement, whereas novel phenotypes never tested by natural selection may eventually make major contributions. In this post, I will briefly discuss two recent papers relevant to this hypothesis.

Lee DeHaan and David Van Tassel published "Useful insights from evolutionary biology for developing perennial grain crops", in American Journal of Botany, while Lin et al. published "A faster Rubisco with potential to increase photosynthesis in crops" in Nature.

Perennial plants often develop more extensive root systems than annuals, reducing the risk of erosion. Well-managed perennial forages (pastures and hay fields) are arguably our most-sustainable agricultural system, supplying milk, meat, wool, and leather, and often getting most of their nitrogen from symbiotic rhizobia bacteria (the main focus of my own research) rather than external inputs.

The Land Institute, where DeHaan and Van Tassel work, has been attempting to develop perennial grain crops. I have argued that greater investment of photosynthate or nitrogen in roots will usually leave less of these limiting resources for grain (seeds). All else being equal, DeHaan and Van Tassel apparently agree:

"where annual crops can use a similar amount of water, light, and nutrients as the perennials... annuals will indeed have greater yield potential"
But they have argued (and I agree, on p. 97 of my book) that perennials may sometimes capture more of these resources than annuals can. The last chapter of my book, on diversity and bet hedging, therefore included perennial grains as an example of high-risk approaches deserving some funding.

The potential of perennials to photosynthesize more months per year than annuals also implies using water more months per year, but their superior root systems can sometimes help water soak into the soil rather than being lost to runoff.

Actual results so far are somewhat discouraging, however. DeHaan and Van Tassel cite a paper by Culman et al., which found greater above-ground biomass in a perennial grass, kernza, relative to wheat. So it might be a better forage than wheat, but its grain yield (with moderate fertilizer) was only 4% that of wheat in year 1 and 39% in year 2. So it would take about 5 acres of kernza to produce as much grain as 1 acre of wheat. Where are those extra 4 acres (and the water to irrigate them) going to come from? Or can we realistically expect significant yield increases without losing the benefits of perenniality?

I have argued that while some tradeoffs (e.g., root vs. grain) constrain crop improvement, other tradeoffs can represent opportunities. For example, the fastest versions of the key photosynthetic enzyme work best at CO2 concentrations greater than atmospheric. Lin et al. transferred genes for one of these enzymes from cyanobacteria into tobacco. The resulting plants grew more slowly than unmodified tobacco, even at 9000 ppm CO2 (atmospheric is now 400 ppm). So this looks like a step in the wrong direction, but it's only a first step. The cyanobacterial enzyme works well in cyanobacteria because they also have a CO2-concentrating mechanism. Some plants, including corn, have different CO2-concentrating mechanisms. See "The evolutionary ecology of C4 plants" for an interesting discussion of how these mechanisms evolved in plants. If someone could combine the faster cyanobacterial enzyme with a plant or cyanobacterial concentrating mechanism, they might achieve significantly greater photosynthesis.

That could take a decade or more, but it's worth noting that perennial grains have been a significant focus of the Land Institute for most of their 38-year history.

Field Crops Research, a leading agricultural journal, has just published a review of Darwinian Agriculture by Jeffrey White. He suggests that there is "ample scope for debate and further research on specific propositions of Darwinian Agriculture." I agree and look forward to those debates. For example, I argued that perennial grain crops may never approach the yields of annual grains, because perenniality requires investing resources in over-wintering that could otherwise by used for grain. White agrees that this may be true in temperate climates, but maybe not in the tropics, if the "off-season" is short. He gives "perennially cropped sugarcane fields" as an example.

I agree that perennials can potentially capture a larger fraction of annual solar radiation, as I noted in the book. For example, a time-lapse movie of rice growth shows that this "annual crop" (which may be grown 2 or 3 times in a year) completely covers the ground only a small fraction of the time. Most of the time, sunlight is mainly hitting soil, evaporating water rather than powering photosynthesis. Perennials can use some of last-season's photosynthate to power rapid leaf growth, capturing a larger fraction of solar radiation sooner.

This isn't only true in the tropics. For example, the book cites a study showing that perennial Miscanthus can produce more biomass than corn. The photo below shows the Miscanthus plots Steve Hamilton showed me at Michigan State's Kellogg Biological Station, where I gave a talk earlier this month. But this Miscanthus doesn't produce any seed and I don't think sugarcane produces much.
What about perennial grasses that do produce significant amounts of grain? Sieglinde Snapp and colleagues at Kellogg Biological Station and The Land Institute have shown that perennial intermediate wheatgrass (kernza) can reduce nitrate concentrations in soil water much more than annual wheat, presumably reducing pollution of wells and rivers. But its first-year grain yield was only 112-157 kg/ha, versus 2807-3761 kg/ha for annual wheat. (Interestingly, the higher yields were with organic management.) Second year yields were 1390-1662 kg/ha for the perennial versus 4248-5017 for the annual. Unless the perennial really improves in subsequent years, the perennial would take more than three times as much land to produce the same amount of grain. I worry that the additional land will come from clearing forests or draining wetlands.

Development of perennial grain crops has been a major focus at The Land Institute for more than 30 years. I question their chances of success -- see an excerpt at Page 99 Test -- but (sort of) endorse the attempt anyway, in the final chapter of my book:

"As explained in chapter 7, I have some doubts about perennial grain crops. But do we really want to focus 100 percent of our grain-breeding efforts on annuals? As long as they pay attention to tradeoffs-- a recent e-mail from David Van Tassel, working on perennial grains at the Land Institute, suggests that they are-- they might come up with something useful. This may sound like weak praise, but it's really all we can say about any long-term research project, including my own."
This triggered an email from Timothy Crews, also at the Land Institute, pointing out that they have discussed tradeoffs in peer-reviewed publications, not just in emails.

A recent example, titled "Missing domesticated plant forms: can artificial selection fill the gap?" was published in Evolutionary Applications. It's open access, so have a look!

There are two reasons that grain yields of perennials might differ from those of annuals. First, perennials may sometimes capture more resources than annuals can. For example, they may use some of last-year's photosynthate to grow a dense canopy of light-capturing leaves earlier than an annual could. This is because early leaf growth in annual seedlings depends on current photosynthesis of initially tiny leaves. Similarly, the deeper roots of annuals may access soil water that annuals would miss.

But capturing more early-spring sunlight for photosynthesis depends on the perennial storing enough photosynthate in the autumn to survive the winter, with enough left over to support early leaf growth. Similarly, capturing more soil water depends on having allocated resources to root growth that could otherwise have been used to make grain. It seems, therefore, that perennials would have to allocate a much larger fraction of their resources to below-ground storage and roots, relative to annuals.

The article by Van Tassel, DeHaan, and Cox, questions this assumption. For example, their Table 1 refers to published data showing that apple, oil palm, olive, and high-bush blueberry can all allocate more than 50% of annual photosynthate to fruits. This is comparable to high-yielding annual crops, like corn. I don't know how typical these values are -- my book cites almond yields of <2000 kg/ha in a region where corn yields >10,000 kg/ha -- but it suggests that woody perennials can have higher yields than I would have expected. These trees often have so little allocation to trunk that they have to be propped up on trellises, but that may be a more-acceptable tradeoff than "propping up" crop health with lots of pesticides. Still, what about herbaceous perennials?

I will focus my discussion on the section of the paper titled "Hypothesis 1", where the authors argue against the hypothesis that "[high-yielding] perennial grains would be impossible on logical or thermodynamic grounds." If that hypothesis is true, as I suspect, then the rest of their arguments miss the point.

Their first point in this section is that perennial plants can be larger, so they can make more seeds per plant, even if their percent allocation to seeds is low. True, but so what? What matters is grain yield per acre, not per plant, and you can fit more plants per acre if they're smaller.

Then they cite an interesting paper by Dohleman and Long (2009) showing that the perennial Miscanthus can produce more total biomass than corn. I cited this paper in my book as evidence for the hypothesis above, that perennials can sometimes capture more resources than annuals. Miscanthus can have green leaves capturing sunlight both earlier and later than corn does.

What about seed yield? Zero. The genotype used doesn't make any seeds. It's propagated from cuttings. So while corn is recycling nitrogen from leaves to seeds in the autumn, Miscanthus just keeps growing. Ok, but weren't we talking about the potential of herbaceous perennials to have high grain yields?

It's also worth noting that that it wasn't until the third year of growth that Miscanthus plants got big enough to capture that much sunlight and produce that much biomass. We worry that increasing the frequency of corn in a rotation to two years out of three risks disease, but wouldn't growing Miscanthus year after year have similar risks?

Next, Van Tassel et al. cite a paper by Aragon et al. (2009), noting that although "blossom removal
increased the survival rate of a short lived perennial, it did not affect reproductive effort." So I looked at Figure 2 in Aragon et al. Blossom removal in 2005 roughly doubled survival to 2006, and the difference was statistically significant. This seems like the key result, showing that producing seeds decreases the chances of surviving to the next year. It's true that, among the plants that did survive to 2006, there were no significant differences in 2006 seed production. But so what? Also, it's hard to get statistically significant differences when you only have one surviving plant in the control treatment (versus six in the deblossomed treatment)!

Van Tassel et al. also cite a paper by Ploschuk et al. (2005) finding no difference in allocation to seeds in two related species, an annual and perennial. Comparing two related species is a good start, but a sample size of one is too few to draw conclusions, as just noted. Silvertown and Dodd (1996) asked a related question:

When an annual evolves into a perennial, or vice versa, how does allocation to seeds change?
They looked at 13 such transitions and found a highly significant association: perennials allocate fewer resources to seeds than annuals do. Median harvest index values were 28% for annuals and 7% for perennials. They don't give as much detail on their data sources as I would have liked, though.

To summarize, the paper by Van Tassel et al. didn't convince me that we will ever be able to breed herbaceous perennials that allocate as large a fraction of their resources to grain as wheat or corn do, while also producing more roots than those crops do (to stabilize the soil) and still retaining enough resources to survive the winter in a dense enough stand to suppress weeds. Their paper has plenty of interesting information on trees and seed-free Miscanthus. But what I would most like to see is a graph of changes in allocation to seed resulting from their attempts to increase the yield of seed-producing perennials, together with associated changes in over-winter survival and root biomass. I assume there will be tradeoffs, but will they be tradeoffs we can accept?