Aerial photograph showing a test field for determining minimum spatial separation. Up to what distance can cross-pollination occur in conventional or organic maize crops？Field trials have been carried out in several EU countries and there is now a great deal of information available on maize，rape and other crops. Photo：www.biosicherheit.de/H.Pienz

Cultivation of genetically modified（GM）crops is increasing steadily all over the world. More than half of the countries in which these crops are grown are developing countries. Maize，soya and cotton are the main GM crops grown here（see 1.13 “Genetic engineering in agriculture：how does it impact on biodiversity？”）. Many consumers，farmers and conservationists are sceptical of GM plants and the products derived from them. In many European countries，there are calls for GM-free zones. While consumers are primarily worried about health issues，the concerns of farmers and conservationists centre on the consequences for the existing，unmodified crop cultivars and their relatives in the wild-consequences that are not yet fully understood. They also fear economic damage and increasing commercial dependence on GM seed producers. Wind and insects disperse the pollen of genetically modified cotton，maize and soya plants. The transgenic genetic material is thus intermixed with unmodified material.

Legal provisions on coexistence aim to prevent this intermixing as far as possible. This is also true for admixtures of GM products in organic or other non-GM produce. Significant organisational and financial efforts are required to prevent inadvertent admixture，from field to supermarket shelf. In the event of damage occurring，an insurance system would need to be established in law，but as yet no country has an adequate system in place.

The clear separation of GM and non-GM products is a challenge that even the industrialised countries are sometimes unable to meet. This challenge is likely to be a good deal greater for developing countries. As complete separation appears to be unrealistic，many countries have already established threshold values. Strict separation of GM and non-GM products during production，harvesting，storage，transport and processing in conjunction with the necessary monitoring systems can limit the admixture of genetically modified products to non-modified products to a GM content of 0.5-0.9 percent. This is in keeping with the threshold values adopted in many countries. The additional costs of staying within these limits is estimated to be in the region of 10 percent of the producer price.