Transgenic Crops – An Introduction and Resource Guide


Pat Byrne
Sarah Ward
Judy Harrington
Lacy Fuller (Web Master)

The goal of this web site is to provide balanced information and links to other resources on the technology and issues surrounding transgenic crops (also known as genetically modified or GM crops). The site's authors are engaged in plant genetics research and teaching at Colorado State University. They receive no funds from companies involved in transgenic crop development, nor are they affiliated with groups campaigning against such crops. Funding for the web site currently comes from a three-year grant by the United States Department of Agriculture under the Initiative for Future Agriculture and Food Systems program.




Transgenic Crops Currently on the Market


Crops, Traits, and AcreageThe most important transgenic crop in terms of acreage planted is soybean, followed by corn, cotton, and canola.

The number of acres for each crop are given in the graph below (Source: James, 2001a, 2001b, 1997). This graph is also available in hectares instead of acres.

Adoption of transgenic crops in the United States has been far greater than in many other countries. The following graph shows the acreage of transgenic crops in the United States from 1996 to 2001.

In 1999, the area planted to transgenic varieties was approximately half of the U.S. soybean crop and about 25% of the U.S. corn crop. The estimated worldwide area planted to transgenic varieties in 2000 increased 11% over the 1999 area (James, 2000b). Most of the transgenic crop varieties currently grown by farmers are either herbicide tolerant or insect pest-resistant. In addition to the crops listed below, minor acreages were planted to transgenic potato, squash, and papaya.

Transgenic crop production area by country (source: James, 2000b)


Area planted in 2000
(millions of acres)

Crops grown



soybean, corn, cotton, canola



soybean, corn, cotton



soybean, corn, canola




South Africa


corn, cotton












soybean, potato













For information on transgenic crop acreage as a percentage of the total U.S. acreage in 2000, see the news update entitled Acreage for transgenic cotton and soybeans up, corn down.

Worldwide production area of transgenic crops and traits (source: Science 286:1663, 1999).


Area planted in 1999 (millions of acres)























Herbicide tolerance


Bt insect resistance


Bt + herbicide tolerance


Virus resistance


Herbicide Tolerance
Weed control is one of the farmer's biggest challenges in crop production, because poorly controlled weeds drastically reduce crop yield and quality. Many herbicides on the market control only certain types of weeds, and are approved for use only on certain crops at specific growth stages. Residues of some herbicides remain in the soil for a year or more, so that farmers must pay close attention to the herbicide history of a field when planning what to plant there.

Herbicide tolerant crops resolve many of those problems because they include transgenes providing tolerance to the herbicides Roundup® (chemical name: glyphosate) or Liberty® (glufosinate). These herbicides are broad-spectrum, meaning that they kill nearly all kinds of plants except those that have the tolerance gene. Thus, a farmer can apply a single herbicide to his fields of herbicide tolerant crops, and he can use Roundup and Liberty effectively at most crop growth stages as needed. Another important benefit is that this class of herbicides breaks down quickly in the soil, eliminating residue carry-over problems and reducing environmental impact. Herbicide tolerant varieties are popular with farmers because they enable less complicated, more flexible weed control. These varieties are commonly marketed as Roundup Ready® or Liberty Link® varieties.

Weed-infested soybean plot (left) and Roundup Ready® soybeans after Roundup treatment. Source: Monsanto

For more information on herbicide tolerant transgenic crops see the article "Herbicide Tolerant Soybeans: Why Growers Are Adopting Roundup Ready Varieties", J. Carpenter & L. Gianessi, AgBioForum online journal, Vol. 2 No. 2,

Bt Insect-Resistant Crops

European corn borer (left) and cotton bollworm (right) are two pests controlled by Bt corn and cotton, respectively.
Source: USDA.

"Bt" is short for Bacillus thuringiensis, a soil bacterium whose spores contain a crystalline (Cry) protein. In the insect gut, the protein breaks down to release a toxin, known as a delta-endotoxin. This toxin binds to and creates pores in the intestinal lining, resulting in ion imbalance, paralysis of the digestive system, and after a few days, insect death. Different versions of the Cry genes, also known as "Bt genes", have been identified. They are effective against different orders of insects, or affect the insect gut in slightly different ways. A few examples are shown in the table below.

Cry gene designation

Toxic to these insect orders

CryIA(a), CryIA(b), CryIA(c)


Cry1B, Cry1C, Cry1D



Lepidoptera, Diptera






Lepidoptera, Coleoptera

The use of Bt to control insect pests is not new. Insecticides containing Bt and its toxins (e.g., Dipel, Thuricide, Vectobac) have been sold for many years. Bt-based insecticides are considered safe for mammals and birds, and safer for non-target insects than conventional products. What is new in Bt crops is that a modified version of the bacterial Cry gene has been incorporated into the plant's own DNA, so that the plant's cellular machinery produces the toxin. When the insect chomps on a leaf or bores into a stem of a Bt-containing plant, it ingests the toxin and will die within a few days.

Bt insect-resistant crops currently on the market include

·         Corn: primarily for control of European corn borer, but also corn earworm and Southwestern corn borer. A list of approved Bt hybrids is available through the National Corn Growers Association web site ( Click on the "event" name to see the list of hybrids.

·         Cotton: for control of tobacco budworm and cotton bollworm

·         Potato: for control of Colorado potato beetle. Bt potato has been discontinued as a commercial product. See our Discontinued Products page for more information.

Corn hybrid with a Bt gene (left) and a hybrid susceptible to European corn borer (right). Source: Monsanto


Results of insect infestation on Bt (right) and non-Bt (left) cotton bolls. Source: USDA


Corn hybrids resistant to corn rootworm
Corn rootworm (Diabrotica spp.) is a serious pest of corn in many U.S. growing areas. It damages roots of young corn seedlings, resulting in reduced growth and poor standability of the plant. This insect is responsible for the application of the largest amount of insecticide to U.S. corn fields. What's more, to control this pest the insecticide must be applied directly to the soil, where it may leave residues or leach into the ground water. By replacing these chemical insecticides, corn rootworm resistant hybrids may provide major benefits to environmental quality.

Corn rootworm feeding on a young maize root. Source: USDA

Range of damage due to corn rootworm feeding, from severe (left) to no damge (right). Source: USDA

Although rootworm-protected hybrids apparently offer pest management and environmental benefits, there are serious concerns about development of resistance to Bt in this adaptable insect. More information about rootworm-resistant Bt hybrids is available in articles by Byrne (2001), Moellenbeck et al. (2001), and Ostlie (2001). Michigan State University has a discussion of rootworm-protected corn at Rootworm-resistant corn was approved in 2003.

Have Bt crops reduced the use of chemical pesticides?
The use of Bt varieties has dramatically reduced the amount of chemical pesticides applied to cotton. According to a story in Science (Ferber, 1999a), US farmers used 450,000 kg less pesticides on Bt-cotton than they would have used on conventional varieties in 1998. Yields and profits also improved in Bt-cotton fields. The benefits from Bt-corn, however, were not as clear-cut. Due to the difficulty of effectively controlling corn borers with insecticides, most farmers do not apply chemical controls to their conventional corn fields. Thus, Bt hybrids substituted for chemical pesticides on only about 20% of the total US Bt-corn area. Profitability of Bt-corn is not as certain as for cotton; it will vary over years and locations, depending on the intensity of the corn borer population. See our discussion of pesticide use on Bt crops on this site.

Will insect pests become resistant to Bt toxins?
Although Bt genes have proven to be quite effective in the short term for protecting against crop insect damage, as well as reducing fungal contamination of corn [Munkvold and Heimlich, 1999,], there are concerns that widespread use of Bt varieties will accelerate development of resistance to Bt in the target pests. This could mean the loss of Bt as an effective, environmentally friendly insecticide. In response to these concerns, the U.S. Environmental Protection Agency has mandated measures to reduce the risk of resistance development. These measures depend on a combination of high dose of the Bt toxin and a planting of refuges. A refuge refers to an area planted to a non-Bt variety that is physically close to a field planted with a Bt variety, as shown in the diagram below.

Diagram of the BT refuge strategy, in which at least 20% of a farm's corn acreage must be planted to non-BT corn. R = resistant European corn borer adult; S = susceptible adult.

Beginning in 2000, the EPA requires that farmers growing Bt corn must plant at least 20% of their total corn acreage to a non-Bt variety. The rationale is that the few Bt-resistant insects surviving in the Bt field would likely mate with susceptible individuals that have matured in the non-Bt refuge. Thus, the insect genes (alleles) for resistance to Bt would be swamped by the susceptible alleles. Whether this strategy will work or not remains to be seen. Some of the potential problems with the refuge strategy are:

·         The frequency of Bt-resistant alleles in insect populations may be greater than assumed in refuge models.

·         Resistance to Bt in European corn borer may be semi-dominant rather than recessive.

·         Resistant insects surviving in the Bt field may mature several days later than susceptible insects in the refuge, thus preventing their mating.

For information on compliance with the refuge requirements, see the news updates entitled 29% of Bt corn farmers in U.S. broke the rules last year, 13% of Bt corn farmers in U.S. still breaking the rules, compliance improves, and 14% of U.S. Bt corn farmers still breaking the rules.

Ferre and Van Rie 2002 discuss the biochemistry and genetics of insect resistance to Bt.

A discussion of designs for refuges is available from the University of Illinois Extension Office at Pioneer Hybrid explains the rules for planting a refuge at

Additional Information
Because there are a number of web sites with extensive information on Bt crops, we refer you to them for additional information on the topic.

Managing Corn Pests with Bt Corn: Some Questions and Answers. F.B. Peairs, Colorado State University.

Bt Corn: Health and the Environment. F.B. Peairs, Colorado State University.

Bt Corn & European Corn Borer: Long-Term Success Through Resistance Management. 1997. University of Minnesota Extension Service.

Genetically modified, insect resistant corn: Implications for disease management. G.P. Munkvold, Iowa State University, and R.L. Hellmich, USDA-ARS.

The Environmental Protection Agency's White Paper on Bt Plant-Pesticide Resistance Management. 1998.

Monarchs and Bt corn: questions and answers. 1999. Marlin Rice, Iowa State University.

Now or Never: Serious New Plans to Save a Natural Pest Control. Union of Concerned Scientists

100 Years of Bacillus thuringiensis: A Critical Scientific Assessment. American Academy of Microbiology

Papaya is a tropical fruit rich in Vitamins A and C, but susceptible to a number of serious pests and diseases. The transgenic variety UH Rainbow, resistant to the papaya ringspot virus, is currently in production in Hawaii.

Papaya is an important source of vitamins in tropical areas. Source: USDA

For more information, refer to:
Transgenic virus resistant papaya: New hope for controlling papaya ringspot virus in Hawaii.

Cornell University's page on virus resistant GM crops, including payapa.

Global Status of Approved Genetically Modified Plants
Agriculture and Biotechnology Strategies (Canada) Inc. maintains a database of trangenic plants that have been approved for environmental release, use in livestock feed, or use in human food. Information is organized by crop and by trait. The information can be accessed at

Transgenic Foods on the Supermarket Shelves
The cooperative extension office at Cornell University has assessed the likelihood that food products contain genetically engineered ingredients. Their assessment is available at

Greenpeace's List of GM and Non-GM Foods
Greenpeace, which campaigns against transgenic foods, maintains a list of food brands that they claim contain or do not contain transgenic ingredients. The list is available at

Discontinued Transgenic Products
Several transgenic products that received approval for marketing have been discontinued for a variety of reasons. Some, such as the FlavrSavr tomato and NewLeaf potato, were available for years before they were discontinued. We have assembled a list of these products with links to more information about their history and the reason for their disappearance.

Page last updated : March 31, 2003

© Copyright Center for Life Sciences and Department of Soil and Crop Sciences at Colorado State University, 1999-2003. All Rights Reserved.
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Risks And Concerns


The introduction of transgenic crops and foods into the existing food production system has generated a number of questions about possible negative consequences. People with concerns about this technology have reacted in many ways, from participating in letter-writing campaigns to demonstrating in the streets to vandalizing institutions where transgenic research is being conducted. What are the main concerns? What scientific support is there for these concerns?

The issues surrounding objections to transgenic crops can be broadly grouped into concerns about

These are complex issues and a thorough treatment of each one would occupy volumes. For each topic we provide a short discussion with a link to a longer discussion and outside resources.

Concerns about human health

The possibility that we might see an increase in the number of allergic reactions to food as a result of genetic engineering has a powerful emotional appeal because many of us experienced this problem before the advent of transgenic crops, or know of someone who did.

However, there is no evidence so far that genetically engineered foods are more likely to cause allergic reactions than are conventional foods. Tests of several dozen transgenic foods for allergenicity have uncovered only a soybean that was never marketed and the now-famous StarLink corn. Although the preliminary finding is that StarLink corn is probably not allergenic, the scientific debate continues. Every year some people discover that they have developed an allergy to a common food such as wheat or eggs, and some people may develop allergies to transgenic foods in the future, but there is no evidence that transgenic foods pose more of a risk than conventional foods do.
More on allergenicity

Common sites for allergic reactions. Source: FDA

Horizontal transfer and antibiotic resistance
The use of antibiotic resistance markers in the development of transgenic crops has raised concerns about whether transgenic foods will play a part in our loss of ability to treat illnesses with antibiotic drugs. At several stages of the laboratory process, developers of transgenic crops use DNA that codes for resistance to certain antibiotics, and this DNA becomes a permanent feature of the final product although it serves no purpose beyond the laboratory stage. Will transgenic foods contribute to the existing problems with antibiotic resistance?

Antibiotic pills.

One aspect of this topic is the risk of horizontal gene transfer, that is, transfer of DNA from one organism to another outside of the parent-to-offspring channel. Transfer of a resistance gene from transgenic food to micro-organisms that normally inhabit our stomach and intestines, or to bacteria that we ingest along with food, could help those micro-organisms to survive an oral dose of antibiotic medicine. Although horizontal transfer of DNA does occur under natural circumstances and under laboratory conditions, it is probably quite rare in the acid environment of the human stomach.

Another concern is that the enzyme product of the DNA might be produced at low levels in transgenic plant cells. While high processing temperatures would inactivate the enzyme in processed foods, ingestion of fresh or raw transgenic foods could result in the stomach containing a small amount of an enzyme that inactivates an orally administered dose of the antibiotic. This issue was raised during the approval processes for Calgene's FlavrSavr tomato and Ciba-Geigy's Bt corn 176. In both cases, tests showed that orally administered antibiotics would remain effective. While the risks from antibiotic resistance genes in transgenic plants appear to be low, steps are being taken to reduce the risk and to phase out their use.
More on antibiotic resistance

Eating foreign DNA
When scientists make a transgenic plant, they insert pieces of DNA that did not originally occur in that plant. Often these pieces of DNA come from entirely different species, such as viruses and bacteria. Is there any danger from eating this "foreign" DNA?

Diagram of DNA. Source: Foodfuture, Food and Drink Federation

We eat DNA every time we eat a meal. DNA is the blueprint for life and all living things contain DNA in many of their cells. What happens to this DNA? Most of it is broken down into more basic molecules when we digest a meal. A small amount is not broken down and is either absorbed into the blood stream or excreted in the feces. We suspect that the body's normal defense system eventually destroys this DNA. Further research in this area would help to determine exactly how humans have managed to eat DNA for thousands of years without noticing any effects from the tiny bits that sneak into the bloodstream.

So far there is no evidence that DNA from transgenic crops is more dangerous to us than DNA from the conventional crops, animals, and their attendant micro-organisms that we have been eating all our lives.
More on eating DNA

CaMV promoter
When scientists use transgenic technology to put a new gene into a plant, they put in additional pieces of DNA to direct the activity of that gene. One of these pieces is the "promoter" that turns the gene on.

Cauliflower mosaic virus infection in canola.
Source: Institute National de la Recherche
Agronomique, Versailles-Grignon

The most widely used promoter is the cauliflower mosaic virus 35S promoter, often abbreviated as the CaMV promoter or the 35S promoter. This promoter was obtained from the virus that causes cauliflower mosaic disease in several vegetables, such as cauliflower, broccoli, cabbage, and canola. There are concerns that the CaMV promoter might be harmful if it were to invade our cells and turn on our genes.

A multi-step chain of events would have to occur for the CaMV promoter to escape the normal digestive breakdown process, penetrate a cell of the body, and insert itself into a human chromosome. While there have been no tests to determine whether the CaMV promoter has invaded human tissues, experiments with mice indicate that normal body defenses eliminate stray fragments of foreign DNA that sneak into the blood stream from the digestive tract.

There is some evidence that the CaMV promoter poses little threat to human health. People have been eating it in small quantities for hundreds of years when we eat vegetables that are infected with the disease. Although vegetables heavily infected with CaMV are unappetizing, there have been no documented negative effects on health from eating the virus or its promoter.
More on the CaMV promoter


Changed nutrient levels
How do genetically engineered foods compare with conventional foods in nutritional quality? This is an important issue, and one for which there will probably be much research in the future, as crops that are engineered specifically for improved nutritional quality are marketed. However, there have been only a few studies to date comparing the nutritional quality of genetically modified foods to their unmodified counterparts.

The central question for GE crops that are currently available is whether plant breeders have accidentally changed the nutritional components that we associate with conventional cultivars of a crop. Because isoflavones are thought to play a role in preventing heart disease, breast cancer, and osteoporosis, the isoflavone content of RoundupReady soybeans has been investigated by several researchers.

The studies completed so far do not resolve the issue of whether RoundupReady soybeans have isoflavone levels comparable to conventional varieties, but the differences found in experiments appear to be small or moderate in comparison with natural variation in isoflavone levels. Additional evidence may clarify the arguments for and against Roundup applications as a risk factor in soybean cultivation.

Industry studies submitted in support of applications for permission to sell transgenic crops indicate that the nutritional components that are commonly tested are similar in transgenic foods and conventional foods.
More on nutrient levels

Source: Marck L. Tucker, USDA/ARS

Concerns about damage to the environment

Monarch butterfly


The suggestion that Bt corn pollen might kill Monarch butterfly larvae galvanized public interest in the effect of transgenic crops on the environment. We present a full discussion of this issue under Hot Topics: Monarch Butterfly.

Crop-to-weed gene flow
Hybridization of crops with nearby weeds may enable weeds to acquire traits we wish they didn't have, such as resistance to herbicides. Research results indicate that crop traits may escape from cultivation and persist for many years in wild populations. Genes that provide a competitive edge, such as resistance to viral disease, could benefit weed populations around a crop field.

Many cultivated crops have sexually compatible wild relatives with which they hybridize under favorable circumstances. The likelihood that transgenes will spread can be different for each crop in each area of the world.

For example, there are no wild relatives of corn in the United States or in Europe for transgenic corn to pollinate, but such wild relatives exist in Mexico.

Soybeans and wheat are self-pollinating crops, so the risk of transgenic pollen moving to nearby weeds is small. However, that small risk must balanced against the fact that there are wild relatives of wheat in the United States.

There are no wild relatives of soybean in the United States, but such wild relatives exist in China. Thus each crop must be evaluated individually for the risk of gene flow in the area where it will be grown.
More on crop-to-weed gene flow

Corn tassels shedding pollen. Source:

Antibiotic resistance
There is also concern that transgenic plants growing in the field will transfer their antibiotic resistance genes to soil micro-organisms, thus causing a general increase in the level of antibiotic resistance in the environment. However, many soil organisms have naturally occurring resistance as a defense against other organisms that generate antibiotics, so genes contributed occasionally by transgenic plants are unlikely to cause a change in the existing level of antibiotic resistance in the environment.
More on antibiotic resistance








Leakage of GM proteins into soil

Source: Rural Life Center, Kenyon College, Gambier, Ohio.

Many plants leak chemical compounds into the soil through their roots. There are concerns that transgenic plants may leak different compounds than conventional plants do, as an unintended consequence of their changed DNA.

Speculation that this may be happening leads to concern about whether the communities of micro-organisms living near transgenic plants may be affected. The interaction between plants and soil micro-organisms is very complex, with the micro-organisms that live around plant roots also leaking chemical compounds into the soil. Much more research must be done before we understand the relationships that occur between micro-organisms and conventional crops. Attempts to discover whether transgenic plants are changing the soil environment, and whether they are changing it in good ways or bad ways, are hindered by our lack of basic scientific knowledge.

More on leakage of proteins into soil

Reductions in pesticide spraying: are they real?

One of the most appealing arguments in favor of transgenic plants is the potential for reducing the damage we do to our environment with conventional methods of farming. Pest-resistant crops such as Bt corn and Bt cotton have been promoted as a means to reduce the spraying of pesticides, while herbicide-tolerant crops such as RoundupReady soybeans are said to reduce the application of herbicides. Large reductions in chemical spraying have been claimed to result from the introduction of these transgenic varieties. Are the claims true?

Bt cotton is the only crop for which claims of reduced spraying are clear. Analysts paint a mixed picture on the results of planting RoundupReady soybeans. Bt corn and herbicide-tolerant cotton and corn have not resulted in clear reductions in the spraying of chemicals.
More on pesticide spraying

Cotton boll. Source:

Concerns about damage to current farming practices

Crop-to-crop gene flow
Hybridization of transgenic crops with nearby conventional crops raises concerns about separation distances to ensure purity of crops and about who must pay if unwanted genes move into a neighbor's crop. As "Identity Preservation" and segregation of GM from non-GM crops become factors in marketing products, it will be important to ensure that hybridization is not occurring in the field.

Experimental plots of transgenic canola,
which has been shown to hybridize with
canola in neighboring fields.

Many factors influence the potential for gene flow from crop to crop. Some crops are highly outcrossing, with pollen carried to other fields by wind and by insects. Other species are highly self-pollinating, with little potential for pollen transfer to neighboring plants. Because of the differences among crops species, every case must be evaluated individually for potential to contribute to gene flow from transgenic to conventional crops.

If GM pollen pollinates plants in a neighboring field, then the issue of genetic trespass may arise. What level of GM presence, if any, should be allowed in products that are sold as organic or conventional? Should GM farmers and companies bear responsibility for preventing gene flow, or should conventional and organic farmers pay to protect their products from gene flow? Should GM versions of outcrossing plants be banned as too risky, while GM versions of self-pollinating plants are permitted? These issues have already prompted several lawsuits and they will continue to be a factor in the development and use of trangenic plants for years to come.
More on crop-to-crop gene flow

Page last updated : September 20, 2002

© Copyright Center for Life Sciences and Department of Soil and Crop Sciences
at Colorado State University, 1999-2002. All Rights Reserved.
View CSU's copyright policy