Herbicide Tolerant Crops

Genetic engineering (GE) refers to techniques used to manipulate the genetic composition of an organism by adding specific genes. The enhancement of desired traits has traditionally been undertaken through conventional plant breeding. GE crops are often broken down into two categories, herbicide tolerant and Plant-incorporated protectants (PIPs).Crops are also engineered or “stacked” to express multiple traits like crops that are resistant to multiple herbicides or are resistant to herbicides and incorporates insecticides.

Herbicide tolerant crops are designed to tolerate specific broad-spectrum herbicides, which kill the surrounding weeds, but leave the cultivated crop intact. Currently, the only varieties Cultivated in the U.S. are engineered to be tolerant to glyphosate. However, the U.S. Department of Agriculture (USDA) is currently in the process of deregulating other new varieties of crops that are resistant to 2,4-D and other herbicides.

Glyphosate Tolerant Crops

Monsanto first introduced glyphosate-resistant soybean in 1996 and later introduced glyphosate-resistant corn in 1998. These crops, commonly called “Roundup Ready”, have become ubiquitous in American agriculture with 93% of soybeans, 82% of cotton, and 85% of corn planted engineered to be glyphosate resistant. This increase in glyphosate-resistant crops has led to an increase in herbicide use, herbicide-resistant weeds (also known as “superweeds”), and numerous other environmental and human health impacts.


Pesticide resistance, the ability of an organism to withstand a poison, is a predictable consequence of repeated pesticide use. How quickly pesticide resistance develops depends on: the frequency of use, the mechanisms of resistance, the genetics of the resistance mechanism, the size of the gene pool and how quickly the organisms reproduce.

  • A study published by Washington State University’s research professor Charles Benbrook, PhD, found that the use of herbicides in the production of three GE crops—cotton, soybeans and corn—had increased. This finding contradicts industry claims that the technology would reduce pesticide applications. According to the study, the emergence and spread of glyphosate-resistant weeds is by far the most important factor driving up herbicide use on land planted with GE crops. Glyphosate-tolerant weeds were practically unknown before the introduction of Roundup-tolerant crops in 1996. But heavy reliance on the herbicide Roundup, whose active ingredient is glyphosate, has placed weed populations under progressively intense and unprecedented selection pressure, triggering a perfect storm for the emergence of glyphosate-resistant weeds. “Resistant weeds have become a major problem for many farmers reliant on GE crops, and are now driving up the volume of herbicide needed each year by about 25 percent,” Dr. Benbrook said.
  • Glyphosate resistance issues have led farmers to request emergency exemptions to use untested herbicides on now glyphosate-resistant weeds. For example, in November 2012, the U.S. Environmental Protection Agency (EPA) granted an emergency exemption for the use of fluridone, an aquatic herbicide that has never undergone scrutiny for its effects on endangered species, on GE cotton crops in order to control resistant weeds.

Increased Herbicide Use

  • A recent USDA report found that herbicide use on GE corn increased from around 1.5 pounds per planted acre in 2001 to more than 2.0 pounds per planted acre in 2010. Herbicide use on non-GMO corn has remained relatively level during that same time frame.

Environmental Impacts

Loss of Habitat

  • The increased use of glyphosate-resistant crops has led to declines in pollinator habitat. Historically, for butterflies in the U.S., their key source of food, milkweed, was found in several key states where the butterfly feeds and breeds: Iowa, Minnesota, Wisconsin, Illinois, Indiana, parts of Ohio and the eastern Dakotas. Now fields have been planted with more than 120 million acres of corn and soybeans genetically engineered to be tolerant to glyphosate, as well as other herbicides, allowing farmers to use glyphosate to kill milkweed in the field. According to researchers, the utilization of these GE crops has all but eliminated milkweeds from these fields, thus eliminating the butterfly’s source of food.

Glyphosate in the Environment

  • A second 2011 study, Fate and transport of glyphosate and aminomethylphosphonic acid in surface waters of agricultural basins, conducted by USGS monitored water concentrations of glyphosate. The study found glyphosate persists in streams throughout the growing season of GE crops in Iowa and Mississippi, but is generally not observed during other times of the year. The degradation product of glyphosate, aminomethylphosphonic acid (AMPA), which has a longer environmental lifetime, is also frequently detected in streams and rain.

Genetic Drift/Contamination

Allowing GE crops to be grown close to organic and non-GE conventional produceincreases the riskof genetic cross-contamination, as pollen from GE crops has the potential to drift onto non-GE crops and produce offspring. Because GE crops are prohibited under organic standards organic farmers may suffer significant financial losses if certified organic crops become polluted with genetically-engineered pollen.

  • A recent Survey produced by Food & Water Watch, Organic Farmers Pay the Price for GMO Contamination, found that a third of U.S. organic farmers have experienced problems in their fields due to the nearby use of GE crops, and over half of those growers have had loads of grain rejected because of unwitting GE contamination. 
  • In May of 2013, USDA announced that unapproved GE wheat was found growing in an Oregon wheat field. After this discovery Japan cancelled its order to buy U.S. western white wheat. Monsanto has not conducted field trials in Oregon since 2001 when it reportedly withdrew from the state.
  • In September of 2013, the U.S. Department of Agriculture (USDA) refused to take action or investigate after it was confirmed that GE alfalfa contaminated non-GE alfalfa in Washington State. USDA claimed the contamination is a “commercial issue” and should be addressed by the marketplace and not the government.

Human Health Risks

  • The increased use of glyphosate on glyphosate resistant crops could lead to increases in human health problems. Glyphosate-formulated herbicides have been linked to numerous health problems including cancer, particularly non-Hodgkin’s lymphoma in three separate peer-reviewed studies (1,2,3), ADHD, rhinitis, and hormone disruption.  Short term health effects include lung congestion and increased breathing rates. Chronic exposures at levels above Maximum Contaminant Levels (MCL) are likely to produce kidney damage and reproductive effects. Click here to see Beyond Pesticides’ comments to the EPA concerning the reregistration of glyphosate.

2,4-D Tolerant Crops

Recently, USDA released for public input itsDraft Environmental Impact Statement(DEIS), which calls for the deregulation of GE corn and soybeans engineered to be tolerant to the herbicide 2,4-D. Much like glyphosate, these new varieties of GE corn and soybeans are set to usher in dramatic increases in 2,4-D. Dow AgroSciences produces these new GE crops under the brand name “Enlist” which will be stacked with glyphosate resistance.

Increased use of 2,4-D could have dramatic impacts on human and environmental health. Scientists around the world have reported increased cancer risks in association with its use, especially for non-Hodgkins Lymphoma (NHL). It is also neurotoxic, genotoxic, and an endocrine disruptor.

2,4-D has a high potential to leach from soils and can be a potential ground water contaminate. Environmental monitoring detected the herbicide in streams, groundwater and even drinking water.  Studies document 2,4-D’s negative impacts on a wide range of animals. In birds, 2,4-D exposure reduced hatching success and caused birth defects. Toxic to fish, 2,4-D can bio-accumulate inside the fish. 2,4-D also is toxic to honey bees and earthworms.

For more information please read Beyond Pesticides' comments to USDA on its recent DEIS.