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  A Publication of UVM Extension's Vermont Vegetable and Berry Program

Summary of Concerns about Transgenic Crops

Compiled by Vern Grubinger
Vegetable and Berry Specialist
University of Vermont Extension

Introduction. Transgenic, or ‘genetically-modified’ crops are those created when genes, the hereditary units of living things, are moved into a plant from another species. In many cases the genes used to create transgenic crops come from bacteria or viruses.

In 1999, transgenic crops were grown on 63.1 million acres in the U.S., constituting 72% of the global acreage of these crops. The seven most widely-grown transgenic crops grown in 1999 were, in descending order of acreage: soybean, corn, cotton, canola, potato, squash and papaya. Transgenic soybean and corn accounted for 54% and 28% of global transgenic crop acreage, respectively; cotton and canola each occupied approximately 9% of this acreage; potato, squash and papaya each occupied less than 1% of this acreage in 1999. (James, C. 1999. Global Status of Commercialized Transgenic Crops: 1999. ISAAABriefs No.12: Preview. ISAAA: Ithaca, NY. www.isaaa.org/)

To date there are at least 38 commercially-available varieties of transgenic seeds, including the crops listed above as well as vegetables such as chicory, sweet corn and tomato (Union of Concerned Scientists, www.ucsusa.org/agriculture/gen.market.html). More transgenic crops are likely to be available soon, as field trials have been approved with transgenic barley, broccoli, carrot, cranberry, eggplant, gladiolus, grape, pea, pepper, raspberry, strawberry, sugarcane, sweetgum, sweet potato, watermelon and wheat. (www.aphis.usda.gov/bbep/bp/newsletter.html)

The majority of transgenic crops contain one of two genetically modified traits: an insecticidal toxin from the ‘B.t.’ bacterium, or resistance to an herbicide. However, resistance to viruses, altered oil content, and altered ripening are among other characteristics that have been bio-engineered into commercially-available crop plants.

The risks posed by transgenic crops, whether the risks outweigh the benefits, who bears the risk and who reaps the benefit are subjects of debate. There is factual evidence that can be used to support a wide range of viewpoints, so consensus among scientists in the forseeable future is unlikely. In part, that is because there are many possible risks and assessing them involves extrapolating from the evidence available and drawing conclusions that are inherently subjective.

Because transgenic crops in agriculture interface with the natural environment on such a large scale, many of the concerns about them are quite distinct from those relating to genetically engineered animals, medicines, and industrial products. Thus, concerns about transgenic crops warrant a separate discussion. Below is a summary of these concerns.

Questionable Pest Control Benefits. Transgenic crops offer agronomic benefits, or farmers would not be adopting them so rapidly. The benefits of herbicide-resistant crops include: improved control of problem weeds, use of fewer types of herbicides and the simplification of weed management strategies. The benefits of Bt-crops include: control of specific insect pests (such as European corn borer or Colorado potato beetle) without the need for other management or material inputs, and reduced levels of mycotoxins in crops because insect feeding and tissue damage is reduced. There are many other benefits that may result from the use of transgenic crops that do not relate directly to pest management, such as increased vitamin levels.

One concern about the use of herbicide-tolerant and Bt-crops for pest management is that these technologies may diminish the need for agro-ecosystem approaches to pest management such as crop rotation. The history of pest management shows that long-term success requires a diversified system that spreads the burden across differing mixes of chemical, biological, genetic, and cultural practices. Reliance on a single approach to pest management will fail because pests successfully evolve and thrive in response to single approaches. (C.M. Benbrook, 1999. World food system challenges and opportunities: GMOs, biodiversity, and lessons from America's heartland. www.pmac.net/IWFS.pdf)

Already, resistance problems are being reported with herbicide-tolerant crops. Volunteer canola resistant to three different herbicide-tolerant canola systems (RoundUp, Liberty and Pursuit) has been found in a field in northern Alberta. (M. MacArthur. Feb. 10, 2000. Triple-resistant canola weeds found in Alberta, Canada. February 10, 2000.Western Producer. www.producer.com/articles/20000210/news/20000210news01.html)

In some cases, Bt-containing crops and herbicide-resistant varieties (such as ‘RoundUp-Ready’) are being promoted and used in a way that tends to reduce diversity on the farm. In harsh terms, this could be called the ‘dumbing down’ of pest management. For example, Monsanto ads aimed at farmers using transgenic crops claim that RoundUp is “the only weed control you'll ever need". (Iowa State University, herbicide ad "hall of shame".  www.weeds.iastate.edu/weednews/roundupcottonad.htm)

Economic Impact on Farmers. The use of transgenic crops by farmers is usually accompanied by higher seed cost, and it may result in yield reductions. Thus there may be no additional profit, or there may be a net economic loss, despite short-term savings in pest management costs. In 1998, the Leopold Center at Iowa State University conducted a survey of  800 Iowa farmers, representing a random selection of 62 continuous corn fields, 315 rotated corn fields, and 365 soybean fields. For soybeans, non-GM varieties yielded more than GM varieties (an average 51.21 bushels per acre compared to 49.26). Herbicide costs were 30% less for farmers who grew GM soybeans, but much of the savings went toward higher seed costs. Returns to land and labor were almost identical. For corn, Bt varieties yielded an average 160.4 bushels per acre, compared to 147.7 for non-Bt varieties. Farmers still applied insecticides on 12% of their Bt corn fields, compared to 18% for farmers with non-Bt corn. The biggest cost differences were in seed: an average $39.62 per acre for Bt corn, compared to $29.96 per acre for non-Bt corn. After labor and other costs, Bt corn earned farmers only an additional $3.97 per acre, which was not considered to be a statistically significant difference. (Duffy, M. 1999. Does planting GMO seed boost farmer’s profits? www.leopold.iastate.edu/fall99leoletterindex.html)

A University of Wisconsin report analyzed soybean yield trials from 8 states in the northern U.S. It concluded: “The average yield of RoundUp Ready varieties ranged from 14% less to 13% more than conventional varieties in 40 performance tests conducted by universities in 1998. When averaged across all tests, RoundUp Ready varieties were 4% lower in yield than conventional varieties. It is anticipated that soybean growers will continue to increase acreage planted to RoundUp Ready varieties and will sacrifice yield for ease of weed control.” (Oplinger, E.S., M.J. Martinka and K.A. Schmitz. 1999. Performance of transgenic soybeans in the northern U.S.  www.biotech-info.net/soybean_performance.pdf)

A recent study by Purdue university suggests that in general the current premiums charged for Bt seed are higher than the expected value of the protection offered by the seed. (Hyde, J., M. A. Martin, P.V. Preckel and C. R. Edwards. The economics of Bt corn: adoption implications. www.agcom.purdue.edu/AgCom/Pubs/ID/ID-219/ID-219.html)

The rapid adoption of transgenic varieties has taken place without independent field testing prior their commercial release, perhaps due to the  rush to bring them to market in order to recoup the sizeable investment in their development. Historically, seed companies provided samples of new crop varieties to land grant university researchers to include in their variety trials, and new varieties were not available to farmers until their performance had been evaluated against other top varieties. Agronomists now say they are not even able to get transgenic varieties upon request from companies in the year they are first introduced (Magdoff, F., Director of the Northeast USDA Sustainable Agriculture Research and Education program, personal communication).

Unintended crop attributes. ‘Pleiotropic effects’ may occur when new genes are inserted into plants to give the plants desirable new traits (i.e. more than one change may occur in a plant as a result of the new gene). The US Food and Drug Administration (FDA) policy regulating transgenic crops assumes that pleiotropic effects will not occur, and that genetically modified crops are ‘substantially equivalent’ to conventional crops. This policy was implemented despite concerns raised by government scientists that it failed to adequately address risks to the environment or to animal and human health posed by pleiotropic effects.

Memos written by FDA scientific staff indicate that pleiotropic effects may indeed occur when new genes are inserted into food crops: “Until more of these experimental plants have a wider environmental distribution, it would be premature for FDA to summarily dismiss pleiotropy" and  "Pleiotropic effects occur in genetically engineered plants... at frequencies up to 30%. Most of these effects can be managed by the subsequent breeding and selection procedures. Nevertheless, some undesirable effects such as increased levels of known naturally occurring toxicants, appearance of new, not previously identified toxicants, increased capability of concentrating toxic substances from the environment (e.g., pesticides or heavy metals), and undesirable alterations in the levels of nutrients may escape breeders' attention unless genetically engineered plants are evaluated specifically for these changes. Such evaluations should be performed on a case-by-case basis, i.e., every transformant should be evaluated before it enters the marketplace." Instead of heeding theses concerns, FDA issued biotech food rules that assume no pleiotropic effects will occur, therefore no additional safety testing of transgenic crops is required.  (See internal FDA memos at: www.bio-integrity.org/list.html)

Pleiotropic effects may enhance the competitiveness of transgenic crops once they are released into the environment. Transgenic Arabidopsis lines were found in field studies to be 20 times more likely to outcross than the untransformed plants. Thus, the transgenic plants have a dramatically increased ability to donate pollen to nearby plants. The authors state that it is unlikely that this increase in outcrossing was due to genetic mutation, because two independently transformed lines both showed higher levels of outcrossing. This has implications regarding the risk of releasing transgenic crops into the environment. (Wichmann, G., C.B. Purrington, and J.Bergelson. 1998. Male promiscuity is increased in transgenic Arabidopsis. 9th International Conference on Arabidopsis Research University of Wisconsin, Madison June 24 - 29, 1998)

Transfer of modified genes to weeds. The movement of plant pollen could confer traits from transgenic crops to their weedy relatives. Most of the major crops in the US co-occur with at least one wild weedy relative with which they have some degree of sexual compatibility (Ellstrand, N.C. 1996. Potential for crop transgene escape and persistence in weedy populations. www.nbiap.vt.edu/brarg/cris/grant9403.html) While some crops lack wild relatives, such as corn, other crops, such as strawberry, canola, and cucurbits have the potential to transfer genes to weedy relatives in the northeast, over distances that depend on wind and insect pollination patterns. Little research has examined this risk in the field, but it has been confirmed that gene flow between cultivated and wild beet can occur. (Bartsch D, Lehnen M, Clegg J, Pohl-Orf M, Schuphan I, and Ellstrand NC. 1999. Impact of gene flow from cultivated beet on genetic diversity of wild sea beet populations. Molecular Ecology 8(11):1733-1741)

Even some proponents of plant biotechnology are calling for more responsible management of transgenic crops in order to prevent them from interbreeding with nearby weeds (Gressel J. 1999. Tandem constructs: preventing the rise of superweeds. Trends in Biotechnology 17(9):361-366.)

A recent British survey of the literature summarizes the nature of pollen dispersal by 5 crops and the risk of cross-pollination with wild or cultivated relatives. “Oil seed rape (canola) presents a high risk for cross-pollination between source and recipient fields. It is inter-fertile with a number of wild relatives found in the UK and introgression of transgenes seems likely. Pollen dispersal has been recorded at up to 4km by insects (some 20 fold higher than the recommended isolation distances), and to 3km by the air flow. Notable potential exists for cross-pollination with feral populations which are common in the UK, giving rise to well distributed further sources of possible contamination.”

“Sugar beet presents a medium to high risk for cross pollination both with other stands and with wild relatives. Sugar beet crops are biennial and do not normally flower within the harvesting regime, although some 'bolters' will flower giving the potential for limited gene flow from the beet crop. In areas producing sugar beet seed flowering is a necessity and here the risk of cross pollination increases accordingly. The pollen produced can be spread extensively on the airflow (significant quantities have been recorded at distances up to 800m) and by insects.”

“Maize presents a medium to high level of risk for cross pollination with other maize crops as the pollen can spread on the airflow. Pollen distribution, as determined by outcrossing between different maize varieties, has been recorded at up to 800m.  Maize also presents a medium to high risk for the inclusion of pollen into honey. However, the crop is not interfertile with any UK wild or crop relatives. The percentage of cross breeding with other maize crops in the vicinity will depend on factors such as separation distance, local barriers to pollen movement, such as woods and hedges, local climate and topography.”

“Wheat and potato can be described as low risk for contamination from genetically modified varieties. Wheat has limited potential for outcrossing even with proximal plants and is not inter-fertile with wild or crop relatives in the UK. Potato is a tuber crop, that is also planted as a seed tuber. There is little potential for the introgression of transgenes into the harvested potato tuber and the species is not inter-fertile with any UK wild or crop relatives.”

“Much published work relies on small field trials (including GM trials). Evidence indicates that the extent of gene flow between GM and non-GM fields, and between GM and feral populations depends mainly on the scale of pollen release and dispersal, and on the distances between source and recipient populations. The potential impact (including cross pollination and inclusion in honey) of pollen from GM crops increases notably with the size and number of fields planted.”  (Treu, R. and J. Emberlin. 2000. Pollen dispersal in the crops maize, oil seed rape, potatoes, sugarbeet and wheat - evidence from publications. Soil Association, 40-56 Victoria Street, Bristol BS1 6BY, UK.  www.soilassociation.org)

Contamination of organic farms and food. Organic farming standards do not allow the use of genetically modifed organisms, including transgenic crops. Organic farmers, processors and distributors stand to lose markets and income if their organic products are de-certified due to genetic contamination. Such contamination can be identified by a variety of commercially-available tests that range in price from $6 to $400, depending on what type of genetic material is tested for. Genetic contamination can occur in a variety of ways, on the farm, at the grain elevators, during transportation, and during food processing. Organically-grown crops such as corn or canola may become contaminated if they are cross-pollinated by wind-blown pollen from transgenic crops in nearby fields, although this is not much of a threat for plants like soybeans that self-pollinate. (Bett, K.S. 1999. Mounting evidence of genetic pollution from GE crops growing evidence of widespread GMO contamination. From: Environmental Science and Technology. www.purefood.org/ge/gepollution.cfm)
Terra Prima, a company that sells organic corn chips, used DNA testing to prove that corn grown by a certified organic farmer in Texas was contaminated by cross-pollination from a nearby field where Bt corn was grown. The company was forced to destroy $87,000 worth of its chips because the contamination did not come to light until after the corn was made into chips; it is a plaintiff in a lawsuit filed against EPA this February alleging that the agency registered genetically engineered crops without adequately considering their health and environmental impacts. (K.S. Bett. 1999. Mounting Evidence of Genetic Pollution from GE Crops Growing Evidence of Widespread GMO. www.purefood.org/ge/gepollution.cfm)

No system of field production can absolutely guarantee the purity of the seeds or plants produced. Low but acceptable levels of contamination of organic crops need to be identified and measures identified to achieve them. The best means of reducing the potential for contamination by GM pollen is for organic producers and/or transgenic crop producers to isolate their crops by an appropriate distance. Crop isolation distances have already been established for the production of seed of high genetic purity, and these (or specific multiples of these) could be used as a basis for the production of organic crops, for example, seed corn production requires an isolation distance of 666 feet.(Moyes, C.L. and P.J. Dale. 1999. Organic farming and gene transfer from genetically modified crops. MAFF Research Project OF0157, John Innes Centre, UK. www.gmissues.org/orgreport/gmissues%5B1%5D.htm)

Besides potential contamination from neighboring farms, transgenic crops are sometimes grown unintentionally on farms - organic and conventional. For example, a non-organic vegetable farmer in Vermont recently told me ‘he was horrified’ to learn he had been growing transgenic squash varieties without knowing it. (The varieties had been purchased because they were described in seed catalogs as virus-resistant but no mention was made of their being transgenic). Unintentional planting of transgenic seed might also occur if contamination happens during seed production. (Skinner, D. and R.N. Peaden. 1993. Risk of transgenic alfalfa disemination during seed production. www.nbiap.vt.edu/brarg/cris/grant9302.html)

To prevent contamination, handling practices throughout the food system must preserve the separated identity of organic and non-organic products. Because some businesses require products they purchase to be certified to contain less than 0.1% or even 0.01% of genetically modified organisms, carelessness such as not properly cleaning out a weighing bin can lead to contamination. People involved in moving organic products want to protect their interests so they have begun testing at different points along the way from farm to table.

Acceleration of insect pest resistance to B.t. is thought by many scientists to be likely since the selective pressure created by millions of acres of transgenic-Bt crops vastly exceeds that posed by traditional application of Bt pesticides. Bt is the most important bio-pesticide in the world, used to control caterpillar and beetle larvae such as European corn borer and Colorado potato beetle. It is an valuable tool for conventional growers practicing IPM, as well as for organic and transitional growers (Lipson, M. 1999. Transgenic Bt crops, pest resistance and the organic grower. Organic Farming Research Foundation, Information Bulletin No. 6). In February 1999, Greenpeace and a coalition of over 70 plaintiffs, including the Center for Food Safety and the International Federation of Organic Agricultural Movements, sued the EPA, charging the agency with the wanton destruction of the world's most important biological pesticide.

In response to concerns that transgenic-Bt crops may be accelerating the evolution of pesticide resistant insects, the EPA has announced new restrictions on GE corn and cotton cultivation. The new restrictions, which became effective January 14, 2000, require registrants to ensure that growers plant a minimum of 20% of their acreage in non-Bt corn refuges. For corn grown in cotton areas, the non-Bt refuge requirement is increased to at least 50 percent. (www.epa.gov/pesticides/biopesticides)

Escape and spread of transgenic crops.  Researchers at the University of North Carolina and the University of Georgia performed a field experiment in which Bt-transgenic and non-transgenic canola plants were planted in natural vegetation and semi-cultivated plots and subjected to various insect selection pressure in the form of herbivory. In the semi-cultivated plots, medium to high levels of defoliation decreased survivorship of non-transgenic plants relative to Bt-transgenic plants, and increased differential reproduction in favor of Bt plants. “Thus, where suitable habitat is available, such as roadsides, fallow fields, and field edges, there is a strong likelihood of enhanced ecological risk associated with the release of certain transgene/crop combinations such as insecticidal canola.”  (Stewart, C.N Jr.,  J.N. All, P.L. Raymer  and S. Ramachandran. 1996.  Inceased fitness of transgenic insecticidal canola under insect selection pressure. www.nbiap.vt.edu/brarg/brasym96/stewart96.htm)

Negative Effects on Non-Target organisms. In the case of crops engineered to contain the Bt toxin to kill foliar pests, below-ground insects may be affected as well, since all the cells of the plant contain the toxin. Biologically active Bt toxin has been found to be released from the roots of Bt corn into growth medium and into the soil environment. It has been suggested that the toxin may accumulate in soil under continuous cultivation of Bt corn and that this accumulation may lead to unanticipated impacts on non-target organisms in the soil. (Saxena, D., S. Flores and G. Stotzky. 1999. Insecticidal toxin in root exudates from Bt corn. Nature 402:480)

Another study examined the post-harvest effects of an insecticidal protein in transgenic tobacco residues. Compared to unmodified (parent plant) tobacco residues, transgenic residues that were buried in soil altered the species composition of the soil biota responsible for organic matter decomposition and nutrient cycling. The population of fungal feeding nematodes, which have been shown to influence N mineralization and plant growth, was altered by the transgenic residues. (Donegan, K.K., R.J. Seidler, V.J. Fieland, D.L. Schaller, C.J. Palm, L.M. Ganio, D.M. Cardwell, and Y.Steinberger. 1997. Decomposition of genetically engineered tobacco under field conditions: persistence of proteinase inhibitor I product and effects on soil microbial respiration and protozoa, nematode and microarthropod populations. J. Applied Ecology 34:767-777)

A Scottish study found that ladybugs which fed on aphids that had fed on transgenic potatoes produced up to 30% fewer progeny and lived only half as long as ladybugs feeding on aphids which had fed on conventional potatoes. (Birch, A.N.E., I.E. Geoghegan, M.E.N. Majerus, C. Hackette and J. Allen. 1997. Interactions between plant resistance genes, pest aphid populations and beneficial aphid predators. Scottish Crop Research Institute Annual Report. pp. 68-72). Similar evidence of ecological ramification was reported from the Swiss Federal Research Station for Agroecology and Agriculture. They found that the Bt endotoxin from transgenic corn killed most corn borers, but the green lacewings which fed on the corn borers were also killed. In subsequent studies, they found that 50% more lacewings died after consuming catepillers fed on Bt than consuming the purified Bt directly. (Hilbeck, A., M. Baumgartner, P.M. Fried and F.Bigler. 1998. Effects of transgenic Bacillus thuringiensis corn-fed prey on mortality and development time of immature Chrysoperla carnea. Environmental Entomology 27(2):480-487) Some ecologists suspect that the aphids in this study had sequestered the Bt endotoxin and succeeded in transferring it to predators.

Cauliflower mosaic viral (CaMV) promoter is used in practically all current transgenic crops released commercially or undergoing field trials. Concern has been raised about the safety of transgenic plants containing CaMV promoters. Although CaMV itself infects only dicotyledons, its promoter is ‘promiscuous’ and functions efficiently in monocotyledons, conifers, green algae, yeasts and E. coli. The transfer of CaMV promoter to other species could also give rise to unpredictable effects on gene expression, which may negatively impact the ecosystem as a whole. It is suggested that such transfer might occur if ‘naked’ or ‘free’ is DNA in released into the environment and taken up by the cells of another organism. Thus, some scientists recommend that all transgenic crops containing CaMV 35S or similar promoters should be immediately withdrawn from commercial production or open field trials, and that all products derived from such crops containing transgenic DNA should also be immediately withdrawn from sale and from use for human consumption or animal feed. (Ho, Mae-Wan, A.Ryan and J.Cummins. 1999. Cauliflower mosaic viral promoter — a recipe for disaster? Microbial Ecology in Health and Disease 1999; 11(4). www.scup.no/mehd/ho)

Food Safety. There are several concerns about how consumption of transgenic crops may negatively affect human health. These have to do with the potential for allergins to be accidentally introduced in new crop varieties, and with the potential to promote antibiotic and virus resistance in people and animals. (M.Hanson and J.Halloran. 1999. Jeopardizing the future? Genetic engineering, food and the environment. Consumer Policy Institute/Consumers Union). There is some evidence that a genetically engineered soybean containing a Brazil nut gene could cause allergic reactions to humans (Nordlee J.A., S.L.Taylor, J.A.Townsend, L.A.Thomas and R.K. Bush. 1996. Identification of a Brazil-nut allergen in transgenic soybeans. The New England Journal of Medicine, 334(11):726-728)

Market Implications. It is well know that markets for American crops in Europe have been reduced due to the use of trangenic crops. A 1999 survey indicates that many mainstream farmers now feel that the risks associated with planting GMOs may be too great. Farmers mention the loss of export and domestic markets, questions over cross-pollination, testing and certification, concern over grain elevator insistence on segregation and premiums being offered for GMO-free crops, as driving corn grower demand for alternatives to GMOs. In addition, grain elevators are expressing apprehension over the costs to segregate because their facilities are not equipped for separate dumping and drying. These concerns prompt the forecast of a dramatic reduction in GMO-corn acreage. (American Corn Growers Assn., www.acga.org/news/1999/092099.htm)

Consumer acceptance of transgenic crops in the domestic marketplace may also be a problem for farmers. A survey by the International Food Information Council, a Washington DC foundation, indicates that consumers are not well-informed about genetically engineered foods. However, even when asked questions biased toward favorable responses, such as how likely they would be to buy biotech foods that “required fewer pesticide sprays”, or “tasted better or fresher”, a significant portion (21 to 43%) of the 1,000 people interviewed still said they would not be likely to purchase such food. (U.S. Consumer Attitudes Toward Food Biotechnology. Wirthlin Group Quorum Surveys.  http://ificinfo.health.org/foodbiotech/survey.htm)

Conclusion. The rapid adoption of transgenic crops poses some significant risks to ecosystems, as well as some economic risks to farmers. In my opinion, it does not appear that these risks have been managed with sufficient caution, especially since the potential long-term benefits to farmers and consumers are not compelling. Organic farmers in particular are at risk, due to the lack of measures to protect them from losses due to genetic contamination. While transgenic crops may simplify management for conventional farmers, they also create economic risk because marketplace acceptance of trangenic crops is currently weak and may worsen. Transgenic crops do not support a sustainable approach to crop production that emphasizes 1) ago-ecosystem based management of pests and nutrients and 2) farm profitability derived from strong markets that are built on meeting the consumer’s desires.

Published: September 2000
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