Science in Society Archive

GM Crops Increase Pesticide Use

Proponents claim that GM crops substantially reduce pesticide use, but new evidence shows otherwise. Lim Li Ching reports.

A new report from Dr. Charles Benbrook, director of the Northwest Science and Environmental Policy Center, Idaho, concludes that the 550 million acres of GM corn, soybeans and cotton planted in the US since 1996 has increased pesticide use (herbicides and insecticides) by about 50 million pounds. Benbrook is a respected agricultural economist and was Executive Director of the US National Academy of Sciences Board on Agriculture from 1984 to 1990.

The report is the first comprehensive study of the impact of all major commercial GM crops on pesticide use in the US over the first eight years of commercial use, 1996-2003. Most studies to date have only focused on the first three years of GM crop adoption (1996-1998), and no study has estimated impacts in 2002 and 2003.

Benbrook draws on official US Department of Agriculture (USDA) data on pesticide use by crop and state to calculate the overall impact of GM crops on the volume of pesticides applied on corn, soybean and cotton. These three crops account for nearly all the area planted to GM crops in the US. The analysis focuses on herbicide tolerant (HT) corn, soybeans and cotton; and corn and cotton genetically engineered to produce the natural insecticide Bacillus thuringiensis (Bt).

HT crops allow broad-spectrum herbicides to be sprayed over growing plants, controlling weeds while leaving crops unharmed, making them popular with farmers. Despite increased seed prices, HT systems have become less expensive, largely because the price of herbicides containing glyphosate (Roundup) has fallen by half since HT crops were first introduced. (Crops tolerant to glyphosate - known as Roundup Ready varieties - are the largest share of acreage planted to HT crops).

But the fall in price has meant farmers can spray more herbicides without feeling the economic pinch. In particular, farmers are spraying substantially more herbicides on HT soybean. Soybean accounts for about 75% of the 400 million acres of HT crops and 54% of all GM acres that have been planted since 1996. While total pounds of pesticides applied to Bt corn and cotton have fallen modestly (see later), the increase in herbicides applied on HT soybeans has been far greater. This, combined with the dominance of HT soybean, has led to dramatic change in overall impact of GM crops on pesticide use.

Benbrook calculates the difference between the average pounds of pesticides applied on acres planted to GM crops, compared to the pounds applied to otherwise similar conventional crops. In their first three years of commercial sale (1996-1998), GM crops reduced pesticide use by about 25.4 million pounds, but in the last three years (2001-2003), over 73 million more pounds of pesticides were applied on GM acres.

The increase in overall pounds of pesticides applied across the three crops is due mainly to the need to apply more herbicides per acre planted to HT soybeans. USDA data show a marked increase in the per acre rate of glyphosate applied to HT soybeans between 2001 and 2002 - about a 22% increase, from 0.85 pounds per acre to 1.04 pounds.

This 22% jump was caused by a major price reduction in glyphosate, the need to control more difficult weeds, and the emergence of resistance and/or lessened sensitivity in weed species that were once fully controlled by one glyphosate application. So for HT soybeans, the difference in average herbicide pounds applied per acre between GM and conventional crops shifted from a reduction of 0.36 pounds per acre in 1996 to an increase of 0.47 pounds per acre in 2003.

Pesticide use estimates for 2003 in the report are preliminary, since USDA will not release these data until May 2004. However, estimates for 2003 are based on 2002 levels and trends in recent years. Benbrook is of no doubt that average glyphosate application rates per acre of HT soybeans continued rising in 2003 due to:

  • Spread of glyphosate-tolerant marestail (horseweed);
  • Shifts in composition of weed communities toward species not as sensitive to glyphosate;
  • Early-stage resistance in some major weeds; and
  • Substantial price reductions and volume-based marketing incentives from competing manufacturers of glyphosate-based herbicides.

HT corn technology reduced herbicide use per acre from 1996 through 2001, but increased use thereafter. The difference in average herbicide pounds applied per acre between GM and conventional crops shifted from a reduction of 0.8 pounds per acre in 1996 to an increase of 0.58 pounds per acre in 2003, due to:

  • Increases in the rate of glyphosate applied per acre driven largely by shifts in weed communities, resistance, changes in tillage and planting systems, and significant reductions in the price of herbicides containing glyphosate;
  • Incremental increases in reliance of farmers on herbicides other than glyphosate to assure season-long control of grasses in HT corn; and
  • Downward trend in average rate of application of herbicides on non-HT acres.

The difference in herbicide application rates on HT and conventional cotton changed much like that of HT corn and soybeans, shifting from a reduction of 0.64 pounds per acre in 1996 to an increase of 0.17 pounds per acre in 2003.

The report acknowledges that the other major category of GM crops - Bt corn and cotton - continues to reduce insecticide use by 2 million to 2.5 million pounds annually. The reduction in insecticide pounds applied per acre planted to Bt corn and cotton ranges from 0.33 pounds in 1996 to 0.06 pounds in 2003, and from 0.38 pounds in 1996 to 0.2 pounds in 2001-2003, respectively.

However, the increase in herbicide use on HT crops far exceeds the modest reductions in insecticide use on Bt crops, especially since 2001. The calculations also don't take into account the volume of Bt toxin that is continuously expressed in the Bt crops' plant cells. This amount is significant compared to the rates of application in today's low-dose pesticides.

In short, over the last eight years, HT crops have increased pesticide use an estimated 70.2 million pounds, while Bt transgenic varieties have reduced pesticide use an estimated 19.6 million pounds. Thus, total pesticide use has risen some 50.6 million pounds over the eight-year period.

The increase in pesticide use, largely due to increased use in HT crops, especially HT soybean, is of no surprise, given that scientists had warned that heavy reliance on HT crops and a single herbicide (in this case, glyphosate) for weed management might lead to changes in weed communities and resistance. This triggers the need to apply additional herbicides and/or increase application rates to achieve the same level of weed control.

Many farmers have had to spray more herbicides on GM acres in order to keep up with shifts in weeds toward tougher-to-control species, coupled with the emergence of genetic resistance in certain weed populations.

"For years weed scientists have warned that heavy reliance on herbicide tolerant crops would trigger ecological changes in farm fields that would incrementally erode the technology's effectiveness. It now appears that this process began in 2001 in the United States in the case of herbicide tolerant crops," said Benbrook.

According to Prof. Bob Hartzler, an extension weed management specialist from Iowa State University, glyphosate-resistant marestail in Roundup Ready soybeans first appeared in Delaware in 2000, spreading since as far west as Indiana, and identified in the Southeastern US where Roundup Ready cotton is grown. Other records of glyphosate- resistant weeds (not necessarily linked to HT crops) are rigid ryegrass in an orchard in Australia and in wheat production systems in Australia and California, Italian ryegrass in Chile and goosegrass in Malaysia.

Furthermore, waterhemp populations with individuals capable of surviving 'normal' user rates were identified in Iowa and Missouri the first year Roundup Ready soybeans were marketed. While Hartzler doesn't think that waterhemp can as yet be considered glyphosate resistant, the potential exists and should be closely monitored.

Since the first report of glyphosate resistant rigid ryegrass in 1996, four additional resistant species with this trait have been identified. According to Harztler, this rate of development suggests that new resistant biotypes will continue to arise.

Prospects for GM crops leading to reduced pesticide use in the long-term don't bode well either. The pounds of herbicides required to achieve acceptable weed control is rising on most farms planting HT varieties, compared to the rates of application common between 1996-1998. In contrast, the amount of herbicides and insecticides applied per acre on conventional farms continue to trend downward as a result of incremental shifts toward newer low-dose pesticides and regulatory restrictions phasing out high-dose herbicides.

As a result, the difference in total pounds of herbicides applied on HT versus conventional acres has increased steadily since 2000. Given the emergence and spread of weeds resistant or less sensitive to glyphosate, this difference is likely to widen further if HT technology continues to be relied on as heavily as in recent years.

Article first published 11/12/03


Benbrook CM (2003) Impacts of Genetically Engineered Crops on Pesticide Use in the United States: The First Eight Years, BioTech InfoNet, Technical Paper No 6, Nov 2003,

Hartzler B 'Are Roundup Ready weeds in your future II', Submission to UK GM Science Review, 28 February 2003,

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