tion affects the environment. It addresses impacts at the individual farm level and also at the landscape level, given that impacts from individual farms accumulate and affect other farms and their access to communal natural resources in the region. The use of GE crops has altered farmers’ agronomic practices, such as tillage, herbicides, and insecticides; these alterations have implications for environmental sustainability both on and off the farm, which are evaluated to the extent possible at this point in time (Box 2-1). In particular, we examine the effects of the adoption of GE crops on soil quality, biodiversity, and water quality.

ENVIRONMENTAL IMPACTS OF HERBICIDE-RESISTANT CROPS

The adoption of herbicide-resistant (HR) crops has affected the types and number of herbicides and the amount of active ingredient applied to soybean, corn, and cotton. This section first examines the substitution of glyphosate for other herbicides that has taken place and how the use of HR crops has interacted with tillage practices. It then assesses ecological effects of those changes on soil quality, water quality, arthropod biodiversity, and weed communities. Lastly, the implications for weed management in cropping systems with HR crops are considered, especially for systems in which glyphosate-resistant weeds evolve.

Herbicide Substitution

A higher proportion of herbicide-resistant GE soybean has been planted than of any other GE crop in the United States. Adoption has exceeded 90 percent of the acres planted to soybean by U.S. farmers (Figure 2-1). HR cotton acreage reached 71 percent in 2009 (Figure 2-2), while planted HR corn acres were 68 percent that year (Figure 2-3). The HR crops planted thus far have altered the mix of herbicides used in cropping systems and allowed the substitution of glyphosate for other herbicides.1 Figures 2-1 through 2-3 summarize the trends in the use of glyphosate and other herbicides on soybean, cotton, and corn (expressed in pounds of active ingredient per planted acre of these crops) and the adoption of HR soybean, cotton, and corn (Fernandez-Cornejo et al., 2009). It is important to recognize that, depending on the metrics used, the substitution of glyphosate for other herbicides has resulted in the use of fewer alternative herbicides by growers of HR crops. However, glyphosate is often applied in higher doses and with greater frequency than the herbicides it replaced. Thus, the actual amount of active ingredi-

1

A list of herbicides for which glyphosate is a common substitute can be found in Appendix A.



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