Do GM plants create “super weeds”?

Weediness is a complex trait that refers to the capacity of weeds and of ‘volunteers’ (i.e. weedy individuals of cultivated species) to maintain their populations in the field  environment and to resist the crop management practices aiming at getting rid of them. This problem is essentially due to herbicide resistance and has been documented since the 70’s. A 1997 review listed 183 resistant weed biotypes in 42 countries (see Sandermann, 2006 for a recent scientific review)! This is before the large-scale field release of GM plants and obviously indicates that GM technology is not at the origin of the problem. The question remains as to whether GM plants may exacerbate the problem. This is an important question considering the overwhelming success of glyphosate tolerance, introduced in 80% of US cultivated soybean, in 60% of cotton and in 18% of maize (2004 data). Selection of herbicide-resistant biotypes is indeed enhanced by the use of a few molecules on large areas and over successive years. Where a few molecules are repeatedly applied on the same areas, multiple resistance may also occur, giving rise to ‘superweeds’ with stacked resistance genes. In fact glyphosate and glufosinate tolerance – the two most popular herbicides used by GM technology -  are so efficient that they also promote ‘conservation tillage’ in many areas, a sustainable cropping system reducing tillage and preserving agricultural soils, but requiring efficient weed killing techniques, like GM  herbicide tolerance. In order to benefit from the agroecological advantages of GM herbicide tolerant plants in the long term, stewardship plans need to be adopted, diminishing the risks of herbicide resistance in weed populations. Such management plans have been proposed by public and private actors, like the Glyphosate White Paper published by the  University of Wisconsin (Madison, WI, USA) at http://ipcm.wisc.edu/uw_weeds. As Europe has not yet adopted GM herbicide-resistant plants at a large scale, it should learn from the US experience in order to ensure long-term effectiveness of the technology on its agricultural land.

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