Simple Method Found to Vastly Increase Crop Yields
By Carol Kaesuk Yoon
In a stunning new result from what has become one of the largest agricultural experiments ever, thousands of rice farmers in China have doubled the yields of their most valuable crop and nearly eliminated its most devastating disease -- without using chemical treatments or spending a single extra penny.
Under the direction of an international team of scientists, farmers in China's Yunnan Province implemented a simple change in their rice paddies. Instead of planting the large stands of a single type of rice, as they typically have done, the farmers planted a mixture of two different rices. With this one change, growers were able to radically restrict the incidence of rice blast -- the most important disease of this most important staple in the world. Within just two years, farmers were able to abandon the chemical fungicides previously widely used to fight the disease.
"I wasn't surprised that the system worked but I was surprised that it worked so well," said Dr. Christopher Mundt, population biologist at Oregon State University and the one American-based author on the study, which was published in the current edition of the journal Nature.
"I'm excited about the possibilities. There is a lot of potential even beyond rice."
Dr. Youyong Zhu, plant pathologist at Yunnan Agricultural University, heads the mostly China-based research team whose study now covers 100,000 acres and involves tens of thousands of farmers.
In fact, many researchers have long argued that planting a diversity of crops should lead to benefits like greater productivity and the suppression of disease, compared with single variety plantings known as monocultures. Yet the use of diversity and other ecologically based cures for agricultural ills have tended to be viewed as more politically correct than economically viable. Scientists say that this latest study shows that such environmentally friendly methods can be highly effective, even more effective, in this case, than standard chemical pesticides.
Calling the results "very significant," Dr. Alison Power, agricultural ecologist at Cornell University, said, "People have said that these kinds of ecological approaches wouldn't work on a commercial scale. This is a huge scale." She added, "We have more alternatives that are really viable than we often think about or include in our arsenal of possibilities."
Those studying natural ecosystems also welcomed the new work, saying it closely paralleled findings for the role of species diversity in reducing the incidence of disease in the wild.
"It's an important study," said Dr. David Tilman, ecologist at the University of Minnesota. "It's going to raise a great deal of interest."
The scientific hypothesis behind the study, the latest in a growing number examining the effects of biodiversity, is simple. If one variety of a crop is susceptible to a disease, the more concentrated those susceptible types are, the more easily disease can spread and the more victims it can claim. The disease should be less likely to spread, however, if susceptible plants are separated from one another by other kinds of plants that do not succumb to the disease and can act as a barrier.
Rice blast fungus, which destroys millions of tons of rice and costs farmers several billion dollars in losses each year, moves from plant to plant as an airborne spore -- a method of transport that should easily be blocked by a row of disease-resistant plants.
Scientists tested the hypothesis by asking farmers to plant their farms in experimental plots using two kinds of rice: a standard rice that does not usually succumb to rice blast disease and a much more valuable sticky rice known to be highly susceptible.
Farmers also planted control plots of monocultures, allowing scientists to rigorously test the importance of the mixtures in the health and productivity on these farms.
What scientists found was that farmers garnered even more benefit from the mixtures than expected. Resistant plants did block the airborne spores in a field, but as more and more farmers became involved in the study, these positive effects began to multiply across the region. Not only were disease spores not blowing in from the next row, they were no longer coming from the next farmer's field either or the next or the next, rapidly damping the spread of the disease on a grand scale.
In addition, scientists found that the sticky rice plants, which poked up above the shorter, standard rice plants with which they were grown enjoyed sunnier, warmer and drier conditions than they would have in a stand of tall, sticky rice plants. These conditions appeared to discourage the growth of the fungal rice blast in the sticky rice plants.
The fact that rice blast is the most devastating disease of rice, the staple crop of most people worldwide, would alone make the study important. Scientists interviewed said there was no reason, however, why mixtures could not decrease disease spread in other crops as well, though how powerful and useful a remedy it will be is likely to vary.
"There's already a lot of work with barley in Europe and coffee in Colombia," said Dr. Mundt. "I've seen beautiful disease reduction in mixtures of willows grown in England."
The study is of particular interest for organic farming as it involves the application of no chemicals.
"Biodiversity is an absolutely essential tool for organic production," said Dr. Martin Wolfe, director of research at Elm Farm Research Centre, an agricultural research institute. "It's an essential part of the armory." Yet, Dr. Wolfe said, even organic farmers underuse diversity, as they also have been indoctrinated in the simplicity of and seduced by the universality of monocultures.
Some of the obstacles to using different varieties include the extra work of keeping track of and maintaining a diversity of plants that can require different kinds of tending and harvesting. If kinds of crops require different harvesting methods or are harvested at different times, it can increase the effort a farmer must expend in a given field.
In Yunnan province, farmers harvested rice grains by hand, making it simple to gather and sell the two rice varieties separately. Some scientists argue such problems can also be overcome with mechanized harvesting. In the Pacific Northwest where an increasing number of farmers are using mixtures of wheat to increase yields and cut down disease, growers may choose varieties that differ in useful qualities, such as resistance to disease, but that can still be harvested and sold together.
Researchers say the study's implications extend to prairies, rainforests and other natural ecosystems.
Just as the new study examines the role of a diversity of crops in fighting disease and in productivity of a rice paddy, biodiversity studies have been examining whether increased species number can affect such things as the health of plants in natural settings.
So far the two kinds of studies appear to support one another well. For example, Dr. Tilman and Charles Mitchell, an ecologist at University of Minnesota, have found evidence in prairie ecosystems that an increased diversity of plant species decreases the incidence of disease. As in the rice paddies, in natural ecosystems, when there is a greater diversity of plants, it is simply more difficult for the disease to spread.
"There's been quite a push by the agrotech industry to market genetically engineered crops and genetically homogeneous crops that perform really well," said Dr. Shahid Naeem, ecologist at the University of Washington. "But what's really neat about this paper is that it shows how we've lost sight of the fact that there are some really simple things we can do in the field to manage crops."
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