Outcrossing and coexistence of genetically modified with (genetically) unmodified crops: a case study of the situation in the Netherlands

With the introduction of genetically modified (GM) crops the EU has demanded that individual member states enact measures to prevent inadvertent admixture-through outcrossing-of genetically modified organisms (GMOs) with products from conventional and organic farming. A literature review on outcrossing was prepared for the Coexistence Committee installed in the Netherlands in 2004. For sugar beet and potato, isolation distances do not appear to be of overriding importance, as true seeds are not part of the harvested product. The only route for admixture is through persistence of GM hybrid volunteers, and these should already be subject to strict control in good agricultural practice. Data on maize indicate that a distance larger than 25 in is needed to keep admixture below the EU labelling threshold of 0.9%, and larger than 250 in to remain below the 0.1% threshold as favoured by organic farming organizations. Oilseed rape is more complex because apart from pollen flow also persistence of volunteers in and outside arable fields, and hybridization with wild relatives play a role. At the present state of knowledge, isolation distances of 100-200 in and rotation intervals of 6-8 years might be warranted for the 0.9% threshold. It is as yet not clear whether a threshold of 0.1% is achievable in practice. The conclusions are compared with the measures recommended by the Dutch Coexistence Committee.