Coexistence in Maize: Isolation Distance in Dependence on Conventional Maize Field Depth and Separate Edge Harvest

Abstract

The most reliable and practicable measure in assuring coexistence in respect to pollen‐mediated gene flow from genetically modified (GM) to conventional maize (Zea mays L.) is an isolation distance separating GM and non‐GM fields. Therefore, we tested distances between 24 and 102 m at three sites in northern Germany using a field orientation representing a worst case scenario concerning wind direction. During the 3 yr of field trials the highest levels of gene flow occurred at the site and year with the longest flowering synchrony and the strongest wind blowing constantly from the GM to the non‐GM field. It was shown that the GM content of a neighboring non‐GM maize field is mainly determined by wind speed and direction as well as by non‐GM maize field depth. Based on the maximum outcrossing data obtained it can be concluded that for non‐GM maize fields being 200 m deep or more an isolation distance of 50 m is sufficient to keep the GM content of the total fields grain harvest below the European Union (EU) labeling threshold of 0.9%. However, non‐GM grain maize fields with smaller field depth require larger isolation distances or additional coexistence measures. In most cases discarding 6 m of the GM maize facing non‐GM maize field edge has proven to be such a valuable measure. In silage maize production 50 m isolation distance is adequate even for non‐GM maize field depths down to 50 m. We recommend flexible separation distances in dependence on non‐GM maize field depth to comply with EU coexistence requirements.

Cross-fertilization between genetically modified and non-genetically modified maize crops in Uruguay

“…This finding is in line with previous studies in which crosspollination frequencies were usually found higher for the outer parts of the recipient field, and declined towards the inner parts due to a higher density of and competition by own pollen of the recipient crop (Devos et al, 2005). Langhof et al (2010), however, evaluated Bt adventitious presence at different depths into the recipient field, and found that cross-fertilization did not follow a regular pattern deep into the field. Therefore, discard of the rows facing Bt source may help to maintain cross-fertilization below a threshold level, but would not completely prevent it.…”

“…Field situations may differ in a range of conditions, such as acreage, topography, number and direction of pollen sources. Flowering time may also differ, and differences in wind speed and the occurrence of storms during flowering will vary from one case to another (Langhof et al, 2010). As a consequence, results from commercial fields may not be easily comparable, and this information may not be useful to analyze a single factor, for instance, the effect of the distance between GM and non-GM crops on crosspollination.…”

“…Forthcoming research should expand the investigated regions in the country, and increase both the number of sampled pools and plants within pools in order to diminish the detection thresholds and confidence of estimators for Bt adventitious presence (Montesinos López et al, 2010). The use of real-time PCR will help by providing quantitative results for a single composite sample (Langhof et al, 2010). As the GM maize acreage in Uruguay is tending to increase, updated information about transgene flow and adventitious presence of transgenes in non-GM crops will contribute to the evaluation of the feasibility of the regulated coexistence policy.…”

“…Besides the distance between crops and synchrony of flowering, gene-flow is also affected by size and orientation of fields (Langhof et al, 2010). Most gene-flow studies were founded on a single sources of pollen, often smaller or equal to the size of the receptor crop (Sanvido et al, 2008).…”

Coexistence in Oilseed Rape: Effect of Donor Variety Type and Discarding Field Edges

“…Under these circumstances, separate edge harvest has just a small effect on the GM content of the total field's harvest and is not as effective as for example in nearly exclusively wind pollinated crop species such as maize, where a considerable amount of outcrossing is found in the first few meters of the donor-facing field edge (e.g. Langhof et al, 2010). Although their study showed that a 5 m wide buffer zone surrounding the non-GM oilseed rape field can reduce outcrossing by about a third, Damgaard and Kjellsson (2005) concluded that isolation distances more effectively reduce pollen-mediated gene flow in oilseed rape than the use of a buffer zone, especially for small recipient fields.…”