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Another link to a study done on increasing use of herbicides in GMO crops.

I have worked for many years on pesticide use, risks, and regulation, as well as the design, implementation, and benefits of Integrated Pest Management (IPM) systems. Given that all of the commercially significant, first-generation traits in genetically engineered (GE) crops are related to pest management, the real-world impacts of GE crops on pesticide use has always been on my radar screen.

I started tracking the development of herbicide-tolerant technology in the late 1980s during my tenure as the Executive Director of the NAS Board on Agriculture (1984-1990). Even back then, years before the technology’s commercial launch in 1996, weed management experts were expressing concern that glyphosate-tolerant, Roundup Ready (RR) crops could lead to the emergence of resistant weeds.

In the first years of use, RR crops triggered a shift in herbicide selection from several low-dose imidazolinone and sulfonylurea herbicides to a relatively high-dose herbicide, glyphosate. USDA pesticide use data released in the late 1990s clearly reflects such shifts. This is why I was skeptical from the start over claims that GE crops would and were reducing the pounds of herbicide applied on the three major RR crops.

Despite these uncertainties, the overall trends in pesticide use on GE crops are clear. Herbicide use is much greater on GE acres compared to conventionally managed acres planted to non-GE cultivars. The spread of resistant weeds has been driving herbicide use up for a decade, and will continue to do so for years.

In order to deal with resistant weeds, farmers are being forced to expand use of older, higher-risk herbicides. To stop corn and cotton insects from developing resistance to Bt, farmers planting Bt crops are being asked to spray the insecticides that Bt corn and cotton were designed to displace.
Without doubt, GE crop technology has profoundly changed corn, cotton, and soybean pest management, but the unintended impact on pesticide use is a harsh reminder that farmers should not put all their eggs in one pest control basket.

Results from the study.

Herbicide-resistant crop technology has led to a 239 million kilogram (527 million pound) increase in herbicide use in the United States between 1996 and 2011, while Bt crops have reduced insecticide applications by 56 million kilograms (123 million pounds). Overall, pesticide use increased by an estimated 183 million kgs (404 million pounds), or about 7%.

Contrary to often-repeated claims that today’s genetically-engineered crops have, and are reducing pesticide use, the spread of glyphosate-resistant weeds in herbicide-resistant weed management systems has brought about substantial increases in the number and volume of herbicides applied. If new genetically engineered forms of corn and soybeans tolerant of 2,4-D are approved, the volume of 2,4-D sprayed could drive herbicide usage upward by another approximate 50%. The magnitude of increases in herbicide use on herbicide-resistant hectares has dwarfed the reduction in insecticide use on Bt crops over the past 16 years, and will continue to do so for the foreseeable future.

Original article here.

Study here.