Outcrossing frequencies and distribution of transgenic oilseed rape (Brassica napus L.) in the nearest neighbourhood

The outcrossing of transgenic oilseed rape in the neighbourhood is of major concern with regard to the actual EU labelling threshold of 0.9% for transgenic contamination in food and feed and the regulations for the co-existence of cultivation of genetically modified crops, conventional and organic farming. In a two-year field trial, the outcrossing frequencies and distribution from plots with different ratios of transgenic plants (100%, 1.0% and 0.1%) containing the pat-gene for resistance towards the broad-range herbicide glufosinate-ammonium were determined in surrounding acceptor plots within a distance of 3–11 m. Randomly distributed outcrossing with isolated pollination events became apparent, and an average gene flow of 0.28%, 0.01% and 0.0065% was detected for 100%, 1.0% and 0.1% transgenic donor plots, respectively. Significant effects on the distribution of outcrossing were found for distance, but not for the prevailing wind direction. The random distribution in combination with the behaviour of honey-bees and bumble-bees gives strong evidence that insects play an important role for short distance gene dispersal. A curve fit assuming an exponential decline was performed with the experimental outcrossing data as a function of distance and was applied to the actual EU labelling threshold. The contamination limit of 0.9% in food and feed could be kept without cultivation distances to the transgenic source. The impact of effective detection limits of analytical methods on the establishment of specific rules for the co-existence is discussed.