GMO MYTHS AND TRUTHS REPORT

5.2 MYTH:

GM crops decrease pesticide use

TRUTH:

GM crops increase pesticide use

“GE crops have been responsible for an increase of 383 million pounds of herbicide use in the US over the first 13 years of commercial use of GE crops (1996–2008). This dramatic increase in the volume of herbicides applied swamps the decrease in insecticide use attributable to GE corn and cotton, making the overall chemical footprint of today’s GE crops decidedly negative… The primary cause of the increase [is] the emergence of herbicide-resistant weeds.”
– Dr Charles Benbrook, agronomist9

“The promise was that you could use less chemicals and produce a greater yield. But let me tell you none of this is true.”
– Bill Christison, president of the US National Family Farm Coalition10

GM crops are claimed by proponents to reduce pesticide use (the term “pesticide” includes herbicides, which technically are pesticides). But this is untrue. Herbicide-tolerant crops have been developed by agrochemical firms specifically to depend upon agrochemicals and have extended the market for these chemicals. Far from weaning agriculture away from environmentally damaging chemicals, GM technology has prolonged and extended the chemically-based agricultural model.

The adoption of GM Roundup Ready crops, especially soy, has caused massive increases in the use of glyphosate worldwide.9,11,12,13,14

A report by agronomist Dr Charles Benbrook using official US Department of Agriculture data looked at the effects on pesticide use of the first thirteen years of GM crop cultivation in the United States, from 1996 to 2008.9 Crops taken into account were GM herbicide-tolerant and GM Bt maize varieties, GM Roundup Ready soy, and GM herbicide-tolerant and GM Bt cotton varieties.

The report found that Bt maize and cotton delivered reductions in chemical insecticide use totalling 64.2 million pounds (29.2 million kg) over the thirteen years – though even the sustainability of this trend is questionable, given the emergence of Bt-resistant pests and the changes in insecticide use patterns (see 5.3, below).

But herbicide-tolerant maize, soy, and cotton caused farmers to spray 383 million more pounds (174 million kg) of herbicides than they would have done in the absence of herbicide-tolerant seeds. This massive increase in herbicide use swamped the modest 64.2 million pound reduction in chemical insecticide use attributed to Bt maize and cotton.

The report showed that recently, herbicide use on GM fields has veered sharply upward. Crop years 2007 and 2008 accounted for 46% of the increase in herbicide use over thirteen years across the three herbicide-tolerant crops. Herbicide use on GM herbicide-tolerant crops rose 31.4% from 2007 to 2008.

The report concluded that farmers applied 318 million more pounds of pesticides as a result of planting GM seeds over the first thirteen years of commercial use. In 2008, GM crop fields required over 26% more pounds of pesticides per acre (1 acre = 0.4 hectares) than fields planted to non-GM varieties.

The report identified the main cause of the increase in herbicide use as the spread of glyphosate-resistant weeds.

5.2.1. Glyphosate-resistant superweeds

The widespread use of Roundup Ready crops has led to over-reliance on a single herbicide – glyphosate, commonly sold as Roundup. This has resulted in the rapid spread of glyphosate-resistant weeds in countries where GM crops are planted.15 Resistant weeds include pigweed,16 ryegrass,17 and marestail.18

The Herbicide Resistance Action Committee (HRAC), financed by the pesticide industry, lists21 glyphosate-resistant weeds around the world. In the United States, glyphosate-resistant weeds have been identified in 22 states.19

When resistant weeds first appear, farmers often use more glyphosate herbicide to try to control them. But as time passes, no amount of glyphosate herbicide is effective and farmers are forced to resort to potentially even more toxic herbicides, such as 2,4-D, and mixtures of herbicides.15,16,17,18,20,21,22,23,24,25,26

US farmers are going back to more labour-intensive methods like ploughing – and even pulling weeds by hand.25 In Georgia, tens of thousands of acres of farmland have been abandoned after being overrun by glyphosate-resistant pigweed.27,28

An article in Monsanto’s hometown newspaper, the St Louis Post-Dispatch, said of the Roundup Ready system, “this silver bullet of American agriculture is beginning to miss its mark.”29 As glyphosate-resistant weeds undermine the Roundup Ready farming model, Monsanto has taken the extraordinary step of subsidizing farmers’ purchases of competing herbicides to supplement Roundup.25,30

5.2.2. How are superweeds created?

Many glyphosate-resistant weeds appear through what is known as selection pressure – only those weeds that survive being sprayed with glyphosate herbicides pass on their genes, leading to a steady increase in glyphosate-resistant plants in the weed population.

But there is a second route through which glyphosate-resistant weeds develop: GM crops can pass on their genes for herbicide tolerance to wild or cultivated non-GM relatives. GM canola has been found to pass on its glyphosate-tolerance genes to related wild plants such as wild mustard, turning them into difficult-to-control superweeds. The GM herbicide-tolerance gene was shown to persist in these weed populations over a period of six years.31

GM canola itself has also become a weed. Feral canola populations have acquired resistance to all of the main herbicides used in Canada,24 making it difficult and expensive to control “volunteer” canola in soy and maize fields. Feral herbicide-resistant canola has also become a problem in sugar beet fields in the US, where canola seeds are reported to be deposited by defecation from geese migrating from Canada.32

5.2.3. GM industry “solution” to superweeds: More herbicides

The industry’s solution to the glyphosate-tolerant superweeds crisis has been first, to aggressively market pre-mix herbicide products to farmers, and second, to develop “stacked trait” crop varieties resistant to multiple herbicides. These stacked trait crops enable farmers to spray mixtures of weedkillers freely, instead of having to apply them carefully in order to spare crops.26 Simple arithmetic indicates that this will double or triple the amount of herbicide applied to a given field.

Dow has applied to release a multi-herbicide-tolerant soybean, engineered to tolerate being sprayed with glyphosate, glufosinate, and 2,4-D34 – an ingredient of the defoliant Agent Orange. In 2012 Dow sparked public outrage when it applied to the US Department of Agriculture to commercialise its 2,4-D-tolerant corn.35

Weed scientists warn that such multi-herbicide-tolerant crops will cause an increase in 2,4-D use, trigger an outbreak of still more intractable weeds resistant to both glyphosate and 2,4-D, and undermine sustainable approaches to weed management.33

In fact, weed species that are resistant to dicamba,36 to 2,4-D,37 and to multiple herbicides38 already exist.

Most stacked-trait superweeds emerge through what is known as selection pressure, where only those weeds that can tolerate herbicide survive to pass on their genes.

But there is another route through which superweeds can emerge: cross-pollination of GM herbicide-tolerant crops within the crop species or with wild relatives. “Stacked trait” multi-herbicide-resistant oilseed rape (canola) plants have already appeared as a result of accidental cross-pollination between GM crops engineered to tolerate different herbicides. As early as 1998, oilseed rape plants were found that tolerated up to three different herbicides.39

A Canadian government study showed that after just 4–5 years of commercial growing, GM oilseed rape engineered to tolerate different single herbicides had cross-pollinated to create stacked trait plants resistant to up to three broad-spectrum herbicides, posing a serious problem for farmers.22,23,24

5.2.4. Conclusion

GM herbicide-tolerant crops have led to massive increases in herbicide use and a resulting spread of herbicide-resistant weeds. Farmers have to resort to spraying more herbicide, or mixtures of herbicides, to try to control weeds. This “chemical treadmill” model of farming is especially impractical for farmers in the Global South, who cannot afford to buy more or different herbicides in an effort to control resistant weeds.


  • For references, please click here.
  • Download a PDF of the full GMO Myths and Truths report
  • We work hard to bring you reports backed up by strong science.
    Please help us by supporting our work.
    Donate button

Add comment


Security code
Refresh