Science in Society Archive

Ending 10 000 Years of Conflict between Agriculture and Nature

Organic agriculture is not enough; we must replace annual with perennial crops. Dr. Stan Cox

Humans now directly manage 27 percent of the Earth's surface area, harvesting more than 40 percent of the planet's biological productivity for our own uses. Yet food production per person is on the decline, and agriculture worldwide is doing more than ever to worsen the global ecological crisis. Like the Hindu god Shiva, today's agriculture is both a creator and a destroyer, partly as the consequence of conscious decisions taken by farmers, agribusiness executives, government officials, and food buyers. But the productivity and ecological impact of agriculture are also inherent in the crops and cropping methods that humans have relied upon for 10 000 years.

The problem of agriculture

Since its inception, agriculture has relied on annual plants that are grown from seed every year and harvested for their seed. That requires tilling of the soil, which can be done on a small scale without causing great harm, as in small, intensively hand-managed plots or on annually flooded land along a river. But every civilization that has practised tillage on a large scale has suffered the often catastrophic consequences of soil erosion [1, 2]. Industrialization has compounded the problem through burning fossil fuels and chemical contamination.

The world's natural landscapes are covered mostly by perennial plants growing in mixed stands [3], w hereas more than two-thirds of global cropland is sown to monocultures of annual crops . Conversion from natural to agricultural landscapes dramatically alters ecological conditions. Across the planet, more land has been converted from perennial to annual cover since 1950 than in the previous 150 years. This recent expansion of cropland has made it more and more necessary to apply chemical fertilizers and pesticides, which disrupt natural nutrient cycles and erode biodiversity [4, 5].

Perennial plants are highly efficient and responsive micromanagers of soil, nutrients, and water. Annual crops are not; they require churning of the soil, precisely timed inputs and management, and favourable weather at just the right time. With shorter growing seasons and ephemeral, often small root systems, annual crops provide less protection against soil erosion, wasting water and nutrients, storing less carbon below ground, and are less tolerant of pests than are perennial plant communities [6].

Today, vast swaths of entire continents have been scoured of their perennial vegetation, leaving the soil uncovered for a good part of the year. Even when the soil is covered during the growing season and even under organic management, lightly rooted annual crops fail to manage water and nutrients the way their deeply- and densely-rooted, persistent perennial antecedents did. Agriculture's destruction of perennial root systems has wrecked entire underground ecosystems, subtracting from the soil much of what makes it soil.

Agriculture is a problem older than history. It has always depended largely on annual grass and legume species that humans domesticated between 5000 and 10 000 years ago. That domestication of annuals set in motion a somewhat ironic series of events. First, annual grain crops made civilization both possible and necessary. Much later, civilization - largely through exploitation of fossil fuels and synthetic chemicals - created conditions under which agriculture could become both extraordinarily productive and ecologically destructive. But today, it is the fruits of the very civilization made possible by agriculture - scientific knowledge, data, and techniques - that have clearly revealed to us both the necessity and the possibility of correcting the well-intentioned wrong turn our species made 10 000 years ago [7](Jackson, 1980).

No-till, organic, and perennial

By far the most important factor that determines the degree of soil erosion in a field is the type of vegetation , annual or perennial , that covers the land. In one field experiment encompassing 100 years of data collection, perennial crops were more than 50 times more effective than annual crops in maintaining topsoil [8]. So-called “no-till” methods (in which annual crops are farmed without tillage) reduce soil loss but require heavy chemical inputs. No-till also performs as poorly as conventional farming in controlling percolation of nutrients and water out of the soil profile [9].

Global data for maize, rice, and wheat indicate that only 18 to 49 percent of nitrogen applied as fertilizer is taken up by crops; the remainder is lost to runoff, leaching, or volatilisation [10]. That occurs with or without tillage. Nitrogen losses from annual crops may be 30 to 50 times higher than those from perennial crops [9]. Organic farming with annual crops solves the problem of chemical contamination, but except in rare circumstances, requires as much or more tillage than conventional agriculture. And the inadequate root systems of annual species handle water and nutrients inefficiently even when crops are grown organically.

Modern societies, stuck as we are with annual crops, have little alternative but to treat grain cropping not as a source of life but as a dangerous activity against which humans and nature must be protected. Environmentally conscious researchers and farmers are making the most of the only perennial plants available to them, by growing more hay and pasture; growing perennial biofuel crops; planting more trees and grass along rivers and streams to soak up the contaminants that haemorrhage from cropland; and taking more erodable lands out of grain production altogether [11]. It's a monumental and discouraging task made necessary because we are still dependent on annual crop plants.

We cannot go back to the crossroads where our ancestors took that wrong turn, and a return to ancient farming methods would not address the problem of annual cropping. But by taking the successes of organic farming through another stage of evolution, it may be possible to produce food while simultaneously allowing the Earth itself to manage the soil, water, and air as it did before the dawn of agriculture.

To do that, we need perennial grain crops. Humans obtain three-fourths of our total calories from grains and oilseeds. By developing perennial grains, plant breeders could help dramatically enlarge that portion of the agricultural landscape that is kept intact by diverse, dense, and deep root systems. With a few very small-scale exceptions, no perennial cereal, pulse, or oilseed crops currently exist. But through a massive, long-term plant breeding effort, that situation can be resolved [12, 13].

The missing link in crop domestication

Neolithic people gathered and ate foods from a huge range of plant species, but once they began domesticating, it was annual plants like wheat, barley and rice that they transformed. Among the world's top 20 staple food crops, the banana is the lone non-woody perennial. Perennial grain species are not to be found anywhere among the world's crop plants [13].

Ancient grain-gatherers apparently did eat the seed of some perennials. Anthropologists have observed traditional methods of harvesting seed from perennial grasses in Poland , Mongolia and North America [14]. People living south of the Sahara harvested the seed of a wide range of perennial grasses [15]. The Vikings probably cultivated perennial lymegrass before barley reached Scandinavia [16]. Archaeologists have found charred seeds of three perennial and twelve annual species of small-grained grasses that people were consuming 23 000 years ago at a site in what is now Israel [17]. Yet no domesticated perennial grain species were handed down to us by plant domesticators.

Perennials were unlikely to follow neolithic people back to the fertile, churned soil around their dwellings in the way annual plants did. In any such situation, they would have been overwhelmed by the weedy annuals that specialize in colonizing such unfriendly territory by growing quickly and scattering their seed before dying. Meanwhile, in undisturbed natural stands, perennial plants re-growing from well-established roots and rhizomes would have been much more vigorous than new seedlings of the same species emerging from dropped seeds. People would have felt little incentive to sow a new generation of perennials as long as plants returned from previous seasons continued to produce well.

Changes in plant traits during domestication and breeding of annual crops occur through cycles of sexual hybridisation and selection. With annuals, the mechanics of food production and plant breeding are virtually identical. Through that process, Neolithic people domesticated annual plants without, at least initially, even realizing that they were doing so [18]. But collectors of perennial seeds, h arvesting year after year from the same perennial plants with no incentive to re-sow, would have had little or no genetic effect on the population.

Woody perennials became a part of the human diet, but h erbaceous perennials were left behind. But today, a small number of plant breeders are beginning to domesticate perennial species and hybridise them with annual relatives, in an effort to open the door to the kinds of dramatic changes in seed production and other traits that plant breeders have achieved in annuals.

The high yields of modern crops are the result of long-term, intense selection for improved harvest index (allocation of biomass to seed rather than vegetation) and decreased intraspecific competition. On the other hand, the relatively low seed yields of wild perennial species result from natural selection in highly competitive environments [19]. The evolutionary fitness of a wild annual plant is heavily dependent on seed production and dispersal, but the fitness of a wild perennial depends more on the survival of vegetative structures than on seed traits. Therefore, it's not surprising that many (but not all) wild perennials have low seed production.

It is predicted [19] that artificial selection in a properly managed agricultural environment could increase seed yield while maintaining the perennial trait. Artificial selection has the potential to generate perennial grain crops with acceptable yields, if it is applied to agronomic traits and perennial growth habit simultaneously. This is suggested by four characteristics of perennial plants that differentiate them from annual plants and provide them with extra resources that can be re-allocated to grain production [13]:

Current perennial-grain research

Through our research at The Land Institute, we have concluded that, unlike a field of maize or soybean, a field of perennial grain-bearing crops can provide food while at the same time protecting soils, water, and biodiversity. The genetic raw material is available, ready to be put to use. The Land Institute and a few universities and other agencies have created the foundations of breeding programmes that will, over the next decades, develop a wide array of perennial grain crops. Most current genetic and breeding effort is going into the following species and species hybrids.

Intermediate wheatgrass ( Thinopyrum intermedium ) is a perennial relative of wheat ( Triticum aestivum ). We are domesticating this species by breeding for increased seed size, seed yield, and ability to thresh freely. The main approach is to evaluate thousands of individual plants over two years, followed by a third year crossing the 5 percent of best-performing plants. We have completed one cycle and are now in a second. Experiments have shown that the first round of selection increased mean yield by about 18 percent and mean seed size by about 10 percent. Some individual families perform much better. In a separate population, four fast cycles have increased the fraction of free-threshing seed from about 8 to around 30 percent.

Wheat and triticale (X triticosecale ) can be hybridised with several different perennial species. We have crossed these annual species with perennial relatives, primarily intermediate wheatgrass, and backcrossed to the annual to produce thousands of relatively fertile, large-seeded plants. In the greenhouse, a large proportion of these plants continue to live after their mature seed has been harvested, but in the field we have so far only identified about 10 plants out of thousands that were able to re -grow after harvest.

To obtain larger numbers of perennial plants, we have crossed hundreds of interspecific hybrid plants to the perennial parent, wheatgrass. We produced hundreds of hybrids, but many of them were sterile, as expected. We have since crossed the few plants that produced viable pollen with pollen-sterile plants in an effort to restore fertility. A few resulting plants have exhibited good fertility, large seed, and vigorous regrowth.

Grain sorghum , a drought-hardy feed grain in the US and a staple food in the Eastern Hemisphere , can be hybridised with the tetraploid (4 sets of chromosomes, 4 x 10 = 40) perennial species Sorghum halepense . We have produced large plant populations from hundreds of such diploid (usual 2 sets of chromosomes) x tetraploid hybrids. The better strains currently produce about 40 percent of the grain yield of their annual grain sorghum parents, at half their seed size; this is a fourfold improvement over S. halepense and represents some of the largest seed of any perennial grain in development. Being a tropical plant, sorghum survives the winter in temperate regions through the survival of rhizomes, or underground stems. Rhizome development is a complex genetic trait, dependent on 9 of sorghum's 10 chromosome pairs [20]. Winter survival is even more complex, and harder to achieve through breeding. However, we have found no strong negative correlations between these components of the perennial trait on the one hand and grain productivity on the other.

Illinois bundleflower ( Desmanthus illinoiensis ) is a native prairie legume that produces relatively large harvests of protein-rich seed [21]. It is a strong candidate for domestication as a crop. The Land Institute has assembled a large collection of seed from a wide geographical area and initiated a breeding program. Making controlled hybrids is extremely difficult technically, but methods have been developed to foster natural hybridisation and identify hybrids using morphological or molecular markers. Non-shattering families – crucial to domestication – have been selected and used as initial parents. The species is a strong, widely adapted perennial, and selection criteria will include shattering resistance, synchronous maturity, seed yield, and seed size and quality.

Maximilian sunflower ( H. maximiliani ) and Kansas rosinseed ( Silphium integrifolium ) are native perennials related to sunflower. The Land Institute is in the process of domesticating these species as perennial oilseed crops, via methods similar to that described above for intermediate wheatgrass. There is a parallel programme for inbreeding to expose rare, valuable recessive genes. Phenotypic variation is extensive in these species. In addition to the usual traits, selection pressure is being applied to fuse the numerous small seed-heads into larger, more compact heads and eliminate seed dormancy. In the case of the large-seeded Kansas rosinseed, selection to increase seed fertility of the head is in progress.

Sunflower ( Helianthus annuus ), the highly productive annual oilseed crop, can be hybridized with several perennial species in its genus, including the diploid (2 x 17 = 34) Maximilian sunflower and two hexaploid (6 x 17 = 10 2 ) ( ? )species: rigid-leaf sunflower ( H. rigidus ) and Jerusalem artichoke ( H. tuberosus ). Hybrids between annual and Maximilian sunflower are highly sterile, unless their chromosome numbers are doubled to produce tetraploids. The best strategy to produce perennial, partially fertile plants is to cross both annual and Maximilian sunflower to the hexaploid species to produce tetraploid plants and then inter-cross the different tetraploids. Large perennial populations have been produced in this way, and they are being subjected to selection for greater seed fertility.

There are many more groups of species that could be used to develop perennial grains [12].

A new organic agriculture

The perennial grain crops listed above and others are intended to be grown in a food-producing system that replicates as closely as possible the ecological functioning of the natural landscape it replaces; in The Land Institute's case, the prairie grassland. In other regions, different sets of perennial crops can be used with similar effect, to be as hardy and resilient as a forest or a savannah, or whatever ecosystem preceded human habitation in a given region. What all such future systems have in common is that they will be based on perennial plant communities with ample genetic diversity both among and within species, and t he energy that supports them will be the sunlight that falls directly on them. They must function independently of fossil energy and synthetic chemicals.

Researchers in organic and sustainable agriculture research have, out of necessity, attempted to mitigate the impact of agriculture by improving what I'll call the “software”: the application of experience, knowledge, techniques, and natural materials to existing annual crop species. But those annual species represent a kind of dysfunctional “hardware” that limits what can be achieved with even the best agricultural “software.” To open up possibilities for truly new agriculture that is sustainable in the long term, new hardware – a wide range of perennial grain crops - is needed.

Plant breeders have long had difficulty finding their role in organic and sustainable agriculture. Now we have a clear-cut, difficult, but achievable mission laid out for us: to develop perennial crops that can make agriculture truly sustainable for the first time ever [22]. It will require a huge effort that must extend far beyond The Land Institute to agronomy, horticulture, and plant breeding departments in institutions across the globe. Plant breeders live in the future. Even in well-established crops, the pollinations we made this spring or summer won't lead to cultivars until we are well into the next decade. Breeding varieties and hybrids only for currently available farming systems becomes a self-fulfilling prophecy. Plant breeders in organic agriculture, indeed all plant breeders, must instead work toward goals that will be important in the longer term. And that “longer term” will by necessity include perennial grain crops.

Dr. Stan Cox is Senior Research Scientist at The Land Institute, Salina, Kansas, USA .

Article first published 26/06/08


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