How is crop diversification like disease prevention?

Monoculture rules in the American midwest.  There are many reasons for this, both political and economic; its history has been documented by many, of course, including Michael Pollan in, e.g., The Omnivore's Dilemma.

Beginning in the fifties and sixties, the flood tide of cheap corn made it profitable to fatten cattle on feedlots instead of on grass, and to raise chickens in giant factories rather than in farmyards. Iowa livestock farmers couldn’t compete with the factory-farmed animals their own cheap corn had helped spawn, so the chickens and cattle disappeared from the farm and with them the pastures and hay fields and fences. In their place the farmers planted more of the one crop they could grow more of than anything else: corn. And whenever the price of corn slipped they planted a little more of it, to cover expenses and stay even. By the 1980s the diversified family farm was history in Iowa, and corn was king. 

And, inorganic fertilizer became widely available and affordable in the 1950's, which fed right into this system. 

When farmers rely on monocropping, that is, growing just a vast swathe of a single crop on their fields each year, they become more dependent on pesticides and herbicides and fertilizers, and eventually crop yields begin to fall, soil erosion and consequences of chemical usage begin to rise and at least as importantly, there is a loss of biodiversity, much of it subtle or unforeseen, and that negatively affects the stability of their local ecosystems (on loss of biodiversity: Cardinale et al, 2012, Nature).  This isn't sustainable agriculture.

And farming doesn't have to be done this way.  It has been known for a long time that crop rotation is better for the soil, requires less input in the form of fertilizer and toxic chemicals, and improves crop yield.  Why this works is not known, but what's important is that it works.

Ken and I were fortunate to learn of some eye-opening work on this issue by Matt Liebman, an agronomist who's doing important work on crop diversification at Iowa State. You might have read about his work last year in PLOS One, when he and his group published a paper on the multiple benefits of increasing crop diversity ("Increasing Cropping System Diversity Balances Productivity, Profitability and Environmental Health," PLOS One, Oct 10, 2012; Davis et al.).

King Corn and soybeans now dominate the Iowa landscape, replacing native prairie. Prairie ecosystems are different from corn and soy both above and below the ground.  Roots are much deeper, in some plants up to 10 feet deep, which enables plants to survive much harsher conditions than can corn and soy.  And, unlike row crops, native plants cover the soil, which inhibits soil erosion due to wind or rain.  And, prairies are better able to retain nutrients.

Liebman's group compared the effects of rotating crops in different rotations.

We compared a conventionally managed 2-yr rotation (maize-soybean) that received fertilizers and herbicides at rates comparable to those used on surrounding commercial farms with two more diverse cropping systems: a 3-yr rotation (maize-soybean-small grain + red clover) and a 4-yr rotation (maize-soybean-small grain + alfalfa-alfalfa) managed with reduced N fertilizer and herbicide inputs and periodic applications of composted cattle manure.

They found that lower crop diversity leads to lower yields, and although it's not entirely clear why that is,  most likely a number of interacting factors contribute.  In the study, maize grain yield was on average 4% higher in the 3 and 4-yr rotation fields than the 2-yr rotations, and soybean yield was on average 9% higher.  The 2-yr rotation fields required more herbicide, pesticide and fertilizer than the 3 and 4-yr rotation fields. Fossil fuel usage was lower in the 3 and 4-yr rotation fields, but labor requirements were higher. Weed management was effective in all 3 systems tested, though the longer rotation fields required 88% less herbicide. 

These results have been the basis of research into managing row crops differently in the mid-west.  Leibman and co. have developed a system they call STRIPs, Science-based Trials of Row-crops Integrated with Prairies.  It turns out that when as little as 10% of a field once planted entirely with corn or soy is covered with native prairie plants, pests and weeds decline, the need for chemical applications declines, soil erosion lessens, species diversity increases, nutrient run-off decreases (by more than 90% in the study), and the ecosystem generally functions more sustainably. STRIPs do a better job even than no-till farming.

Science but not technology
As Liebman says, there's a lot of science in his work but not a lot of technology.  His work is exciting for many reasons.  It is strong evidence that agriculture, even agribusiness, can be done profitably with less toxic input and less impact on the ecosystem.  But then, our grandparents and our great grandparents knew this.  We do not need big industry or technology to protect our soils and productivity, despite the fact that the industry-first approach is deeply embedded in our current culture.

Big Medicine is a lot like Big Ag.  Our culture takes the technology-first approach to research and to attending to disease.  Companies making the machines that look into our bodies to figure out what's wrong, and the chemicals we pour in once they have, are making a killing.  They have set up the array of vested interests and cultural thought-modes that make us think tech first, prevention last.

And, as does the analogous industry in agriculture, that creates our dependence on what they have to sell.  Indeed, the companies that make the machines that make possible high-tech research into disease are making a killing, and working hard to make us depend on what they've got to sell, just like big ag (see the piece by George Church in last week's Nature, for example, encouraging us all to have our genomes sequenced). We can't blame a company for wanting to sell its goods, but we can blame our society for falling for it to the extent that we do.  Will Liebman's work change how farming is done?  We'll see. 

We know to a great extent how to stay healthy.  Eat less, exercise more.  And it's been known for millennia that moderation is best.  Or as Michael Pollan puts it, "Eat food, not too much, mostly plants."

A crucial difference between us and farm fields, though, is that we all have to die some day.  Ecosystems, on the other hand, could long outlast us if we treat them well.