Biotechnology's Greatest Challenge
Can the great potentials of biotechnology be directed
towards ensuring food security
Dr. Nigel J. Taylor and Claude M. Fauquet
(appeared in Forum for Applied Research and Public Policy; Fall 2000 issue)
The human race recently passed two milestones which caught brief international press coverage drawing public attention to an issue of growing concern. Late in 1999, the world's population passed the 6 billion mark, having doubled in only 40 years. A few months later the billionth Indian citizen was born.
Presently, 80% of the world's population live in what are considered the Lesser Developed Countries (LDCs). Despite declining birth rates, world population will continue rising, reaching between 8 and 10 billion persons by the year 2050. Almost all this increase will occur within the developing countries, adding an extra 2 to 4 billion people to the nations of the LDCs. Described in another manner, population density in the developing countries will increase from approximately 55 persons/km2 at present to 90-100 people/km2, or nearly one person per hectare, by 20501. These statistics highlight a reality that has far reaching consequences, and in the opinion of many, constitutes the single most important challenge facing mankind for the coming decades. How can food supplies, health and economic well-being be secured for all the world's citizens and how can this be sustained without destruction of the remaining forest and wilderness regions?
Article 25 (1) of the Declaration of Human Rights states "Everyone has the right to a standard of living adequate for health and wellbeing for himself and of his family, adequate food, clothing, housing and medical care....". Despite these brave words, approximately 800 million people in the developing world do not have enough to eat. A population equivalent to North America and Europe combined do not have access to sufficient food to maintain their body weights and perform light activity. Their children are overly susceptible to disease and have inadequate daily nutritional intake to reach full physical or mental development. Somewhat surprisingly, the recently announced figure of 800 million seriously undernourished people is viewed as a partial success. It actually represents a drop in real numbers, and a significant reduction in the percentage of the LDC population suffering from malnutrition as compared to the situation in the early 1970s. Nevertheless, the rate of progress in addressing food insecurity in the developing countries is below that set at the World Food Summit in 1996, which demands that 20 million people per year be removed from the trap of persistent hunger. Regional inconsistencies such as that for in sub-Saharan Africa where the actual number of people suffering from insufficient nutritional intake in that region has increased since 1992, are also cause for concern.2
Access to sufficient food is known to depend not just on crop yields, the so-called "Malthusian optimism", but on a complex interaction of factors, the most important of which are the price and availability of agricultural products, access to employment and the income or purchasing power of any given individual. These in turn are determined by macro and microeconomics factors, international trade policies and also uncontrollable parameters such as weather patterns. The present attitude held by some commentators in the industrialised North, that there is enough food in the world and that it only needs to be redistributed better, is in our opinion dangerously misleading. It is a delusion to seriously consider that the surpluses of the North can, or will be, sustained on an indefinite timescale in order to feed present and future populations in the South. Agriculture is both the foundation of human nutrition and health and the major economic activity within most LDCs. Reliance on subsidised food imports from the North would undermine the stability and integrity of one of the most important wealth generating systems in the tropical and subtropical regions. Furthermore, it distracts from the central issue, which is how and where investment must be made within the LDCs to ensure that they are able to support their own populations. Even in regions where access to food may not be a problem, increased yields from staple food crops, frees land, time and resources for small farmers to invest in cash crops or other income generating activities. Although increasing crop production in the LDCs will not by itself mean an end to poverty or malnutrition, it will be an essential contributing factor for ensuring the future well-being of the vast majority of the world's population.
It is widely accepted that agricultural systems within the developing countries will have to meet most of the growing food and industrial needs of LDC populations over the coming decades. It is estimated that for rice alone, a 70% increase in productivity is required by the year 2025 to keep pace with growing demand. The scale and urgency of the situation is compounded by several addition factors. Increased crop production in the LDCs has traditionally been achieved by bringing more land under cultivation. For example, the area committed to cultivation of the tropical root crop cassava has increased 43% since 1970, while production per hectare has risen by only 20% over the same time. Such activities are unsustainable and undesirable as they will result in severe depletion of the world's remaining natural ecosystems. The tropical and sub-tropical regions contain approximately 80% of the world's biodiversity, the loss of which would have disastrous consequences for future crop production and pharmaceutical developments. In fact it is now considered that most of the world's high quality farmland is already under cultivation, especially in Asia, where land and population pressure is greatest. In some regions the amount of available farmland is actually decreasing as prime agricultural areas are lost to urban sprawl, soil erosion and desertification.
Demographic transitions within the developing countries add another twist to the overal picture. Throughout the LDCs, migration to the urban areas is increasing dramatically. In the coming decades, it is predicted that rural populations will remain roughly at present levels and that greater than 90% of the population growth will take place in the burgeoning cities of the developing countries3. Thus, not only is increased production required but significant changes are occurring in the types of demand being placed on LDC agricultural systems. The major market for agricultural products will clearly be in the cities. Supplying this growing demand in a consistent manner requires transport infrastructure, storage facilities and post harvest technologies which are underdeveloped in many tropical countries.
It is clear that significantly increased production from the agricultural systems of the LDCs must be generated and sustained over the coming decades and that this must be obtained largely from the land already under cultivation. Achieving these aims on the scales required is a daunting prospect.
Improving Tropical Crop Yields
Over the last thirty years the practices of the Green Revolution have been instrumental in achieving increased crop yields. In this strategy a combination of plant breeding, agrochemical applications and irrigation is utilised to maximise yields in the cereal crops; most especially rice and wheat. By many assessments this has been a successful approach, leading to a 130% increase in wheat yields in the LDCs since 1970 (Table 1)3. Food prices have fallen on international markets and the proportion of chronically undernourished people has significantly diminished over the last thirty years. Due to these agricultural practices, India, the world's second largest and fastest growing country with respect to population, has been able to greatly increase its food self sufficiency, to reduce its financial commitment for food imports and to limit destruction of its natural habitats.
There are also, however, a number of acknowledged negative aspects to the Green Revolution. Reliance on agrochemicals is environmentally damaging and overuse of irrigation has resulted in loss of soil fertility and falling yields in some regions. In addition, the majority of small, resource-poor farmers, who still constitute 75% of the land users in the LDCs, cannot afford to purchase the required chemical inputs and so have not benefited from the Green Revolution. The major beneficiaries have been the larger land owners whose increased affluence has resulted in greater divisions between the rich and poor in the LDCs. The Green Revolution was directed primarily at rice and wheat and failed to address many of the most important food crops of the tropical and sub tropical regions. These non-cereal staple food crops have received relatively insignificant research inputs over the last 50 years and as a consequence have not attained the resulting yield improvements. Known as "orphan crops" they include cassava, the plantain and cooking bananas, sweet potato, taro, sorghum and millet. Figure 1 illustrates how yield increases for these crops are lagged significantly behind rice, wheat and maize. For plantain, the fourth most important source of calories in the tropics, yields have improved by a total of only 3% over the last thirty years. Hundreds of millions of small farmers cultivate these orphan crops, relying on them as their primary source of calories and as a source of income when traded in local markets. Billions more will rely on them in the coming decades. Lastly, and most importantly, since the mid-1990s evidence has accumulated which indicates that annual increases in rice and wheat yields are dropping meaning that the strategies of the Green Revolution are nearing their limits and will not by themselves be capable of providing the crop production increases required to supply future demands.
Biotechnology - a "doubly green revolution"?
Scientists, agronomists and policy makers have been looking for the next revolution in agriculture. This has optimistically been termed the "doubly green revolution", one which will provide the required increases in crop yields with minimum impact on the environment and one which can address small farmer needs as effectively as the larger commercial producer. For many, it is biotechnology which holds this promise. Here we refer to biotechnology as the application of DNA or gene technologies for the agronomic improvement of crop plants. Genetic engineering is the best known, and the most powerful of these techniques, holding great promise for improving both crop yields, quality and value of agriculture products. Biotechnology allows the genetic code imparting a specific trait, for example resistance to a disease infection or drought resistance to be identified and isolated from a given organism. Once reduced to a few microlitres of sticky fluid, this genetic material can be adjusted as required and introduced into the cells of a given plant to become an integral component of the crop's native genetic makeup.
The great power of this technology lies in its ability to take genes from one organism and insert them into crop plants to impart novel characteristics. This capability is rooted in the biological reality that the genetic codes (genes) for all living organisms are organised in a similar manner and can, with minimal changes, be made to operate in a non-native genetic background. It is possible, therefore, to transfer genetic information from algae, bacteria, viruses or animals to plants or to move genes between sexually incompatible plants species. For example, crop plants can be engineered to produce their own pesticides, to have resistance to previously toxic chemicals or to have elevated nutritional qualities. Technical advances over the last five years have also demonstrated the ability to simultaneously transfer as many as 12 genes into a plant genome4. This greatly enhances the potential to engineer complex disease and pest resistance pathways to produce more robust crop plants. Biosynthetic pathways can also be manipulated to produce high value pharmaceuticals and other polymers within the plant tissues. These are then available for direct consumption or for subsequent extraction on commercial scales. The ability to transfer beneficial agronomic traits cross species boundaries, within and out with the plant kingdom, opens a multitude of possibilities which are limited at this time only by our imaginations and by ethical and biosafetly considerations. It is now widely considered, however, that if handled in a responsible manner biotechnology represents a revolution with immense potential impact for the well-being of mankind5.
New Products from Biotechnology
As described above the greatest requirement for improved crop production lies in the developing countries. A major challenge is to ensure that the huge potential of biotechnology is directed to where it is needed most, that is to benefit small farmers and the populations of the developing countries. Recent advances in scientific research and proven performance of genetically modified crop plants in the field, provide indications as to how biotechnology could be applied to impact food production in the LDCs. However, harnessing biotechnology to address issues of food security and economic development in the LDCs is proving to be problematic. Working with poorly understood tropical and subtropical crop species certainly provides challenges, but the major obstacles to applying biotechnology to developing country requirements are less biological in nature and more a consequence of economics and politics.
First generation transgenic crops and the LDCs
The production of transgenic crop plants with improved resistance to pests and diseases and elevated nutritional qualities is making rapid progress in the industrialised North. If assessed by rate of adoption, introduction of the first generation of transgenic crop plants has been the most successful application of a new technology in the history of agriculture. Plantings have risen from zero in 1995 to 39.9 million hectares (almost 100 million acres) in 1999; increasing by 44% between 1998 and 1999 alone. Slightly more than 50% of this area consists of soybeans genetically engineered with a bacterial gene which imparts resistance to the herbicide glyphosate. Nineteen percent is maize engineered to be resistant to European stem borer, an insect which is difficult to control by conventional methods, and which can cause widespread yield losses in this crop. The remainder is composed of cotton and oil seed rape, potato, squash and papaya transgenic for the above genes or for resistance to viral diseases. The present market for transenic crops is estimated at $2.3 billion per year but is projected to reach $25 billion by 20106.
At this time genetically engineered crops are cultivated mostly in North America, with the USA and Canada harvesting 72 % of the planted acreage. Yield improvements have not been dramatic, but the transgenic crops described above were designed primarily to improve pest and weed control and to reduce requirements for agrochemical applications. To this end, their success has been dramatic. Monsanto Company claim that 2 million gallons pesticides applications have been saved since the introduction of Bt corn and cotton. Within the developing world the first generation of transgenic crops have had less impact. These products were conceived, developed and marketed specifically for release within the economic realities of the industrialised countries. They were not designed to address developing country requirements. Nevertheless, enthusiastic adoption of transgenic maize and soybean by farmers in countries such as Argentina, China, Mexico and South Africa show that they can be of relevance in at least some developing country scenarios. Eighteen percent of all transgenic crops were planted in LDCs in 1999, with Argentina embracing this technology on a remarkable scale and committing 90% of its soybean crop to genetically transformed plants last year.
Directing transgenic technologies at LDC needs
The LDCs will most likely continue to benefit from crop biotechnologies developed in the North. For example, India will commence cultivation of transgenic cotton in the near future. However, they do not by themselves provide the answer to developing country requirements. Application of biotechnology as a contribution to food security in the LDCs requires that the specific needs of small farmers in the tropical and subtropical regions are targeted. As stated above small scale and subsistence farmers still constitute the majority of the land users in the developing countries7. If biotechnology is to have a real impact on world health we must direct resources at producing improved varieties of the relevant local corn and rice varieties. Genetic engineering technologies must be developed for the orphan crops such as cassava and plantain on which a large proportion of the population depend, for which so little yield improvement has been previously been achieved and for which conventional breeding is both difficult and lengthy. If the technical hurdles can be overcome, success is likely as past neglect means that the latter crops retain a large potential for yield improvement. For example, in Africa cassava produces on average 7-8 tons/hectare fresh weight harvested product. Field trails performed under optimised conditions, which eliminated pressure from weeds, insects and virus infections have demonstrated that yields upwards of 80 tons/hectare are possible. Achieving and sustaining production improvements of only a fraction of this will have significant impact on food supplies in many parts of Africa.
A number of interrelated reasons have combined to make progress towards the application of biotechnology to the world's subsistence crops frustrating slow. DNA technologies and the gene transfer protocols which are required to find, analyise and then insert transgenes with potential agronomic interest into crop plants were first developed in research laboratories in North America and Europe. They are relatively expensive and capital intensive to develop and require specialist training to utilise fully. The required investments and high tech nature of these activities have hindered easy transfer to the LDCs. More importantly, lack of public investment in agricultural research from the late 1980s to the present time, has ensured that the majority of research and development has been, and continues to be, directed at temperate crops or at tropical cash crops such as cotton, rubber, coffee, papaya and pineapple, from which a financial return is expected. As a result, the majority of biotechnology research and expertise resides within the private sector in the industrialised countries, most especially the USA. By definition, developing world subsistence crops including cassava, sweet potato, plantain sorghum and millet have little or no place in these market driven activities. Commercial enterprises have protected their significant investments through the application of patents and intellectual property rights, restricting access of emerging technologies to developing country applications. Either the genes, technological tools and expertise are not made available for application to tropical crops, or release of the genetically engineered products to the farmers in LDCs is blocked or delayed by unresolved property right issues.
Cause for optimism
Despite the problems highlighted above, recent reports have provided encouraging indications as to what can be achieved when resources are focused on applying biotechnology for the improvement of developing country food crops. These advances include among others:
1. demonstration that a bacterial gene, which, when genetically engineered into a plant increases its ability to take up phosphorous. Phosphorous is an essential plant nutrient which can significantly reduce yields when not available in sufficient amounts. Phosphorous is often chemically fixed within tropical soils, making it a limiting factor in crop production.
2. the much publicised "golden rice", in which rice plants have been genetically engineered to synthesis and accumulate vitamin A in the grains. This product could not have been achieved without the application of biotechnology and is an excellent example of how the new technologies can contribute to health improvement in the LDCs.
3. identification and isolation of genes responsible for dwarfing characteristics in plants. If transferred to millet and sorghum such traits could have a significant impact on yield enhancement in these neglected tropical cereals.
4. transfer of a gene from the photosynthetic system of maize into rice, where it is able to boost productivity in the genetically engineered plants by up to 30%.
5. a product from our own laboratory, The International Laboratory for Tropical Agricultural Biotechnology (ILTAB), produced in collaboration with the University of California, Davis. A gene (Xa21) from an African wild relative of rice which imparts resistance to a severe bacterial blight disease, was isolated and transferred by genetic engineering directly into breeding lines of Chinese rice which are highly susceptible to this disease9 Transfer of this single gene has resulted in plants crossed with local Chinese varieties which are highly resistant to the pathogen seven sexual generations after gene transgene insertion.
6. demonstration at ILTAB of cassava plants genetically engineered to have increased resistance to African cassava mosaic diseases, the most important disease of a crop plant in Africa and responsible for the loss of up to 50 million tons of food each year.
The above examples represent only the tip of the iceberg. Agricultural biotechnology is a relatively young discipline and has been applied to address developing world problems for little over a decade with few resources being directed to this end. With increasing time and investment much greater and far reaching discoveries are assured. The full genetic sequences of rice and the model plant Arabidopsis thaliana will be completed and published of within the coming year. Coupled with vastly improved, high throughput analysis of how genes control development, metabolism, defence and all aspects of biosynthesis in plants, new information will greatly empower the development of biotechnology applications for improved crop production.
ILTAB, a small player in the bigger picture ILTAB is one of a relatively small number of research organisations dedicated to applying biotechnology for the improvement of tropical subsistence crops and is based at the Donald Danforth Plant Science Center, St. Louis, MO. Location within a center of research excellence provides ILTAB with immediate access to cutting edge science, allowing new technologies to be applied to tropical crops in a manner otherwise difficult to achieve. Activities at ILTAB are directed towards three major areas which illustrate what in our opinion is required at scientific level to advance the application of agricultural biotechnology for tropical crop improvement. These include; 1. basic research to discover genes with potential benefits in tropical agriculture, in ILTAB's specific case this involves research into the causes and controls of the plant viral diseases which severely reduce crops yields in the LDCs, 2. development of the genetic engineering technologies required to insert genes into tropical crop plants and 3. technology transfer from industrialised plant species to the orphan crops and from developed countries to the LDCs.
Technology transfer is central to the ILTAB's mission and refers to the training of scientists from the South and the transfer to the tools, equipment and expertise required to generate indigenous capacity for biotechnology research and development in the developing countries. A number of research institute located within the LDCs, most notably the larger research centres of the CGIAR (Consultative Group on International Agricultural Research) are well equipped to carry out biotechnology research and development on tropical crops. However, increasing this capacity is considered to be essential for the successful application of biotechnology in the developing countries. For the near future it is likely that most genetically engineered plants will be developed and produced in the advanced laboratories of the North and transported to the LDCs for field testing and evaluation6. However, it is important that the LDCs do not purely become recipients of finished products, but instead are full participants in application of these technologies to their food and cash crops. Each developing country, and even regions within countries, has its own combination of agricultural constraints to address. They must be empowered to address the issues of yields, improved post-harvest qualities or nutrient deficiencies specific to their particular needs and therefore use these as they see fit. An indigenous capacity will empower the developing countries to establish their own biotechnology industries and to negotiate on equal terms with companies and research entities in the North. In addition, the vast majority of the worlds biodiversity resides in the forests of the developing countries. As full partners in biotechnology the LDCs will have increased motivation to protect these regions in order to reap the benefits of this immensely valuable resource8.
One of ILTAB's successful products, the Xa 21 gene was described briefly above. Other contributions have included: - the development of highly efficient genetic engineering protocols for rice, -recovery of the first genetically transformed cassava plants, -production of rice plants genetically engineered to be resistance to one of the most severe viral disease of that crop (rice tungro disease) which are now being tested in Malaysia, production of rice plants genetically engineered with 12 transgenes -development of promoters for diving the controlled expression of transgenes in engineered crop plants and - progress towards the production of cassava plants resistant to African cassava mosaic disease, a virus infection responsible for the loss of millions of tons of food each year on that continent. With support from the Rockefeller Foundation, the IRD (Institut de Recherche et de Dévelopment, Paris), the Donald Danforth Plant Science Center and a number of other national and international funding agencies, ILTAB has trained 135 scientists and technicians from 19 developing countries in the technologies required to produce genetically transformed rice, cassava and tomato plants7.
Significant effort is required to direct and develop the above technologies to produce products with potential benefit to farmers and processors in the LDCs. In addition to development of the technology itself, they must be adapted to the relevant crop varieties and then trailed in the field in the LDCs. One attraction of biotechnology is that it relies on the genetic improvement of a crop plant to a particular constraint or set of constraints. Increased technological input is not required by the farmer who receives a self supporting improved cultivar. However, investments are required at the research and development levels in order to produce the improved germplasm and the generate the infrastructures required for field testing and distribution to the farmers. Unlike the case for North America and Europe the infrastructure to enable this is not in place in the majority of LDCs. Indeed, many developing countries do not even have the necessary regulations in place to allow field testing of genetically engineered crops to take place. Establishing such structures is a prerequisite to gaining from the technology.
Time for change
Although most encouraging, the successes of ILTAB and other laboratories involved in similar activities around the world, represent only a small fraction of the resources required to ensure that biotechnology is fully employed to meet the needs of the world's growing populations. For example, ILTAB is one of only five laboratories actively engaged in developing and applying genetic transformation technologies for the improvement of cassava, a food crop which is consumed by approximately 600 million people, twice the population of the USA, every day. There are more than 1700 cultivars of cassava grown in Africa, South America and Asia and the crop suffers from severe yield reductions due to virus and bacterial diseases and insect pests. The story is the same for plantain, the fourth most important source of calories in the tropics, and indeed all the other "poor mans" crops within the LDCs. The resources presently being committed to the orphan crops are clearly out of scale with the task at hand.
If we consider not just biotechnology but investment to research and development of LDC food crops as a whole, the picture is even more dramatic. The CGIAR system which is charged with crop improvement for the developing countries receives an annual budget around $400 per annum These resources must support 16 research centres scattered over the world's continents and is expected to address the whole spectrum of conservation, research, development and delivery of improved products and techniques for upwards of a dozen essential food and industrial crops in the LDCs across a range of socioeconomic and agronomic requirements. Further funding is available, especially from the aid agencies and NGOs from the industrialised countries. However, these are applied in a piecemeal manner and generally lack the level of support and the coordination, required to make major impacts on developing country agriculture as a whole. Quite clearly the resources currently being committed to the new century's most pressing problem are insufficient. If we are at all serious about meeting the basic rights of 4.6 billion people in the developing countries the situation must be addressed in a more realistic manner.
We have attempted to outline the urgent need for increased agricultural output in the LDCs and how biotechnology could be applied to contribute to this effort. Despite the recent negative public reaction to biotechnology, we remain convinced that the genetic engineering of crop plants has a vital role to play in addressing the world's present and future agricultural requirements. In our opinion, the scale of the risks involved if the LDCs are not provided with the opportunity to secure their own food supplies and economic development, far outweighs the inconclusive evidence of any environmental damage attributable to genetically modified organisms. That is not to say that all possible applications of this new technology are inherently safe. Each new product must be viewed as a separate case and assessed in context. For example, release of transgenic cassava in Africa or Asia where there are no naturally occurring related plant species, carries different risks than in South America which is the centre of origin for this crop, and where possible release of the transgenes into the wild by cross pollination with its wild relatives must be considered.
As for most complex issues there is no single simple remedy. Biotechnology is not a panacea for world hunger. However, when combined with traditional breeding, good agricultural practice and sound economic policies it can be an important factor in achieving improved standards of health and economic security for all the world's people. Whether the new technologies can be effectively directed as those who need it most, or will only benefit the already affluent North remains a significant question. It essential that we do not proceed to a situation comparable to that for the treatment of AIDS where the costs of high technology cures make them applicable only to the those infected in the North and has completely failed to address the epidemic as a whole, providing no hope to victims in the LDCs where the disease is spiraling out of control.
Indeed, we consider that the impetus for successful application of both traditional methods and biotechnology to address world crop production must come from the North. The industrialised countries possess the vast majority of the world's financial and technological resources. Only the North has the financial capability to implement the research and development programmes required. We believe that mobilisation of these substantial resources to address developing world needs is fundamental to the future well-being of the world, its natural resources and its people. The challenge is of considerable magnitude and must be sustained over several decades. There are no easy answers, no quick fix; instead serious commitments are required from all entities which have the ability to contribute. Much greater public funding is required from all Western governments, most especially from the USA, which currently contributes less per capita to developing world issues than any of the other industrialised countries. Private corporations can have a significant role by releasing products and expertise with application to small farmer needs in the LDCs, but which will have limited impact on their profits within indistrialised agriculture. The LDCs must also come forward, as many are now doing, and make commitments to training their scientist and providing them with the incentives and support they require to sustain effective research programmes. They should seek to enter into collaborations with the public and private research sectors and development organisations in the North to enable biotechnology to be directed and applied to their needs. They must ensure than the proper legislative regulations are put in place to allow genetically engineered crops to be trailed and adopted within their respective countries.
One final point which is apparent to us, is a conspicuous lack of global or regional coordination between the entities dedicating resources for improving developing country crop production. We consider that there is need for the creation of a coordinating organisations and structures which would act to facilitate communication and collaboration with all the organisations capable of making a contribution to this effort, and to raise public awareness in the industrialised countries as to the issues involved. In this way the limited resources presently available can be better focused and maximised to ensure that improved products reach farmers in the tropical and subtropical regions.
A massive challenge faces mankind for the first half of the new century. There is no doubt that the resources, knowledge and the tools are available to address this issues. The question is whether those of us in the North, those of us who have never faced a day with insufficient food, are prepared to divert a portion of our wealth towards the benefit of the majority of the world's people.
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2. FAO. The state of food insecurity in the world 1999. Available on the Web at http://www.fao.org/FOCUS/E/SOFI/home-e.htm.
3. FAO. FAOSTAT Statistical Database, Agriculture Data. Internet address: http://apps.fao.org.Contains an enormous amount of regularly updated statistical information concerning crop yields, agricultural commodities, population demographics etc.
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This document is Donald Danforth Plant Science Center Manuscript No. 00-17.