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

Cotton boll. Photo: www.mahyco.com

One transgenic variety that has substantially reduced chemical applications is Bt cotton. Cotton bollworm, tobacco budworm and pink bollworm are major pests of cotton grown in the southern part of the United States. Because pest outbreaks can cause the loss of one-third of a crop, cotton farmers have been quick to adopt Bt varieties in areas where pest outbreaks are common.

A USDA report on reduction in pesticide use associated with genetically engineered crops indicates that several methods of analysis show a decline in pesticide use attributable to Bt cotton (http://www.ers.usda.gov/publications/agoutlook/aug2000/ao273f.pdf).

A report prepared by the USDA's Economic Research Service (http://www.ers.usda.gov/Emphases/Harmony/issues/genengcrops/
genengcrops.htm) also concludes that pesticide use has declined because of Bt corn.

Data for pesticide use in six states showed that 14 percent less pesticide of the type used on budworm and bollworm was used after the introduction of Bt varieties than was used before (Carpenter and Gianessi, 2001). The six states included some that have experienced serious pest outbreaks in the past and have planted high percentages of Bt cotton, and also some states where budworm and bollworm are not serious problems and the use of Bt cotton varieties is low.

An analysis by Charles Benbrook (2001) concludes that Bt cotton has resulted in substantial decreases in pesticide use.

Bt corn
Claims that the use of Bt corn has reduced chemical spraying are difficult to support.

The vast majority of the corn grown in the United States is field corn, which is processed into corn chips, corn flakes, and corn meal, or is fed to animals. Nearly all Bt corn varieties are field corn. Field corn is not usually sprayed with insecticides because there is some market tolerance for insect damage on this kind of corn. National figures indicate that about 8 percent of field corn acres are sprayed to kill the European corn borer, but local practices vary, so farmers in any particular region may spray more or less than this national average.

Sweet corn and popcorn make up a small percentage of the total corn acreage in the United States. There are no Bt varieties of popcorn. Sweet corn is sold as "corn on the cob" for fresh consumption at the table. There is no market tolerance for insect damage to sweet corn, because consumers will not buy ears with visible damage. Sweet corn is sprayed with insecticides frequently, sometimes every two or three days, to ensure that the ears will be attractive at harvest. There are few T varieties of sweet corn and very little Bt sweet corn is grown.

Since most Bt corn varieties are field corn and few acres of field corn were sprayed before the introduction of Bt corn varieties, the opportunity to have an impact on spraying was limited. The EPA's benefits assessment for Bt corn notes that "the potential benefits were anticipated to be yield increases rather than reduced pesticide costs or reduced pesticide use" (http://www.epa.gov/scipoly/sap/2000/october/brad5_benefits_corn.pdf).

Spraying of insecticides that target the European corn borer has declined from 8 percent to 5 percent of the total acreage since the introduction of Bt corn, according to the EPA's report (http://www.epa.gov/scipoly/sap/2000/october/brad5_benefits_corn.pdf). But analyists caution that Bt corn was not the only changing factor during those years. Some of the insecticides that kill the European corn borer also kill other insect pests, and may be sprayed against those pests whether or not the European corn borer is present. Thus, changes in the population levels of other insect pests influence the amount of spraying that occurs. Reduction in use of a particular chemical can occur for several reasons. As new insecticides are introduced, the use of older insecticides declines, and the development of insect resistance to a chemical may cause a farmer to switch to newer chemicals as they are made available. Because several factors affect the amount of pesticide that is sprayed, it is difficult to support a claim that the introduction of Bt corn varieties is responsible for all of the change in pesticide use.

Carpenter and Gianessi (2001) estimate that the introduction of Bt corn was responsible for a 1.5 percent reduction in chemical spraying. A USDA report on reduction in pesticide use associated with genetically engineered crops (http://www.ers.usda.gov/publications/agoutlook/aug2000/ao273f.pdf) indicates that the reduction in spraying associated with Bt corn is small according to some analysis methods and non-existent according to methods. The EPA reports that total insecticide use on corn has not declined, despite the reduction in use of certain insecticides that were often used against the European corn borer (http://www.epa.gov/scipoly/sap/2000/october/brad5_benefits_corn.pdf).

An analysis by Charles Benbrook (2001) concludes that the use of insecticides on corn has remained stable despite the introduction of Bt corn.

Herbicide-tolerant cotton
Farmers use a variety of herbicides to kill weeds in their fields both before planting and after the crop is growing in the field. Choosing a herbicide that will kill the weeds without injuring the crop is an important consideration. That's why herbicide-tolerant crops have generated so much interest and investment in agriculture. Because it is very difficult to spray only the weeds once the crop has started growing, farmers place a high value on herbicides that are gentle to the crop and on crops that are resistant to the herbicide. BXN cotton, which tolerates being sprayed with bromoxynil, and Roundup Ready cotton, which tolerates being sprayed with Roundup, compete in the marketplace with cotton-friendly herbicides such as pyrithiobac as farmers decide how to address their weed problems for the coming season.

When farmers plant herbicide-tolerant crops, they don't stop spraying herbicides. Instead they change the kind of herbicide that they spray. Thus, the introductions of BXN cotton and Roundup Ready cotton were accompanied by an increase in the use of bromoxynil and Roundup along with a decline in the use of other herbicides that had been used previously (Carpenter and Gianessi, 2001).

One USDA report shows reductions in herbicide use on cotton according to several measures (http://www.ers.usda.gov/publications/agoutlook/aug2000/ao273f.pdf), while another report concludes herbicide-tolerant cotton varieties have not played a role (http://www.ers.usda.gov/Emphases/Harmony/issues/genengcrops/
genengcrops.htm). Although herbicide use on cotton has declined in recent years, the result may not be attributable entirely to herbicide-tolerant cotton (Carpenter and Gianessi, 2001). Cotton-friendly pyrithiobac, which is effective in very small amounts, may be responsible for the reduction in pounds of herbicide applied because farmers use much smaller amounts of pyrithiobac that they would use of another herbicide. On the other hand, the reduction in number of applications and kinds of herbicides used is probably due to herbicide-tolerant cotton varieties, because farmers switch from using a combination of chemicals to using just one.

Advocates of Roundup Ready crops point out that Roundup is much less toxic to animals than many of the herbicides it replaces. For more information, see our discussion of Roundup, coming soon.

Herbicide-tolerant soybeans
Roundup Ready soybeans are used in the same way that herbicide-tolerant corn is used. The farmer can spray to kill weeds without injuring his crop. As with herbicide-tolerant corn, farmers who plant Roundup Ready soybeans reduce their use of their previous herbicide and increase their use of Roundup. The use of Roundup on soybean acreage has increased from 20 percent of the total acreage to 62 percent of the total acreage since the introduction of RoundupReady soybeans, while the percentage of soybean acres treated with most other herbicides has declined.

Experts paint a mixed picture on the effectiveness of Roundup Ready soybeans in reducing herbicide use. Two government reports say that herbicide use has been reduced significantly by the use of herbicide-tolerant varieties (http://www.ers.usda.gov/publications/agoutlook/aug2000/ao273f.pdf; http://www.ers.usda.gov/Emphases/Harmony/issues/genengcrops/
genengcrops.htm). Carpenter and Gianessi (2001), working from government statistics, say the total pounds of herbicide used on soybeans has not declined, but farmers make fewer trips through their fields and use fewer different kinds of chemicals, so the number of applications of herbicide has declined. An analysis by Charles Benbrook (2001) concluded that more total pounds of herbicide but fewer pounds of active ingredient are used on Roundup Ready soybeans than on conventional soybeans.

Herbicide-tolerant corn
As with herbicide-tolerant cotton and soybeans, herbicide-tolerant corn is not damaged when the farmer sprays to kill weeds. A USDA report on reduction in pesticide use associated with genetically engineered crops shows a decidedly mixed result from the introduction of herbicide-tolerant corn, with some measures indicating reductions and other measures indicating increases in herbicide use (http://www.ers.usda.gov/publications/agoutlook/aug2000/ao273f.pdf). A report prepared by the USDA's Environmental Research Service (http://www.ers.usda.gov/Emphases/Harmony/issues/genengcrops/
genengcrops.htm) concludes that no reduction in herbicide use has occurred due to herbicide-tolerant corn. An analysis by Charles Benbrook (2001) concluded that 30 percent more herbicides are used on Roundup Ready corn than on conventional corn.

Another discussion of pesticide use on genetically engineered plants, also by the USDA's Environmental Research Service, is available at (http://www.ers.usda.gov/publications/aer810/aer810h.pdf).

Summary
Although transgenic technology may result in the reduction of chemical spraying in some cases, it does not always do so. Bt cotton is the only crop for which claims of reduced spraying are clear.