Herbicide tolerant crops

Herbicide tolerant (HT) crops are the biggest selling GM crops. These crops have been made to be tolerant to either Monsanto's weedkiller RoundUp (glyphosate) or Bayer's weedkiller Liberty (glufosinate ammonium). These are broad spectrum herbicides which kill all green plants except those protected as a result of the genetic modification. Recently, the first crops tolerant to Dow's 2,4-D, a more toxic weedkiller, have been approved for commercial growing in South Africa.

Monsanto's RoundUp Ready soybeans are the leading product. Other herbicide-tolerant crops include maize, sugar beet, and oil seed rape (canola).

Herbicide-tolerant GM crops have been attractive to the biotechnology companies as they have been able to

  • sell both the herbicide and the GM seeds as a package
  • place the same gene sequences into each of these major crop types
  • gain intellectual property rights over the modified seeds and thus charge an annual technology fee.

Herbicide-tolerant GM crops have been attractive to a number of large-scale farmers in North and South America because in the early years after adoption they simplified the spraying regimes i.e. farmers only had to spray them once with with one type of herbicide, saving labour costs. However, repeated blanket applications of the same herbicide have led to herbicide-resistant superweeds evolving. These superweeds are now reducing crop yields and require repeated applications of multiple herbicides and sometimes even pulling up by hand. There are concerns about the impacts on of pesticide residues on human health, on wildlife, and on farmers' livelihoods.

The existence of superweeds is now being used by the companies involved to justify developing new GM crops tolerant to other herbicides, which may be even more damaging. "The weed resistance problems currently infesting U.S. farms are rapidly getting worse, and growers need new technology now to maintain productivity. Within the past year, the number of weed-resistant farm acres in the U.S. has increased by about 25 percent." Dow AgroSciences USA press release, April 2012.

In the USA, agro-chemical and GM seed companies now recommend that farmers use tank mixes of multiple herbides to try to tackle the superweed problem.

In 2012, South Africa approved Dow's GM maize (corn) engineered to be tolerant to the weedkiller 2,4-D (one of the ingredients in Agent Orange). The controversial GM maize, plus the same trait engineered into soybeans, are awaiting approval in the USA. The crops are intended to be planted in response to the spread of superweeds tolerant to Monsanto's weedkiller RoundUp (glyphosate), which have arisen due to use of the company's RoundUp Ready GM crops. In the short term, this will allow the superweeds to be killer by 2,4-D, but in the longer term resistance to this weedkiller will also develop. A significant increase in the use of 2,4-D is expected, leaving toxic residues in the food chain and damaging neighbouring crops due to drift during spraying.

In late 2012, Dow announced plans to introduce GM soybeans which are tolerant to three different weedkillers.

In South America, local people and some scientists have raised concerns about the health effects of being sprayed with the herbicides used on GM soya. Much of this soya is exported to Europe for use in animal feed.

In the UK, the Farm Scale Evaluations ran from 1999-2004. Herbicide-tolerant GM sugar beet, maize and oilseed rape (winter and spring) were grown along side conventional crops to monitor their ecological effects. The studies found that the changes in herbicide use associated with growing these GM crops had negative effects on wildlife.


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