Oilseed rape is a globally widely grown crop of hybrid origin, descending from the parent species cabbage (B. oleracea) and turnip (B. rapa). It is unknown as a wild plant, but frequently occurs as a feral plant outside cultivation (Pessel et al., 2001). It is mainly pollinated by insects, more rarely by wind, facilitating hybridisation with related species (Chevré et al., 2004), potentially leading to transfer of transgenes into wild populations. For Austria, Pascher and Gollmann (1999) identified more than 20 species as potential hybridisation partners. Oilseed rape seeds are small (1.3–3.1 mm diameter: Rich, 1991) and remain germinable in the soil up to several years (Gruber et al., 2004). At present, GM OSR is widely cultivated in Canada (>90% of OSR cultivation estimated), USA, Chile, and Australia (James, 2015). In the European Union cultivation of GM OSR is currently not authorized, but several OSR lines (e.g., herbicide resistant GT73, MS8, RF3, MS8 × RF3) are admitted for import, processing and feed use.

Feral OSR plants may originate from seed banks at semi-natural sites due to local soil relocation, as a result of epizoochory—diaspores are dispersed via adhesion to animals (e.g., wild boars; Schmidt et al., 2004)—or due to seed spillage during transport and handling activities. Transport and handling have been identified as the main reasons for spillage of OSR (Crawley and Brown, 2004; Pivard et al., 2008) and are therefore the focus of the present work. In Central Europe, feral OSR plants are able to reproduce, and populations may persist for several years outside cultivation, especially along transportation routes such as railway lines or roads, as known for France (Pessel et al., 2001; Garnier et al., 2008; Pivard et al., 2008), Germany (Dietz-Pfeilstetter et al., 2006; Menzel, 2006; Elling et al., 2009; Middelhoff et al., 2009; Franzaring et al., 2016), the Netherlands (Tamis and de Jong, 2010), Great Britain (Crawley and Brown, 2004; Squire et al., 2010) and Austria (Pascher et al., 2000, 2006, 2010). In Switzerland, feral glyphosate resistant OSR (GT73) was identified on four of 79 sample sites (Schoenenberger and D'Andrea, 2012), although like in the European Union GT73 is not permitted for cultivation. These four sites were ports and railway stations on the borders to France and Italy. The feral plants probably originated from spillage of contaminated OSR seed from freight trains (Hecht et al., 2014; Schulze et al., 2014, 2015). Spillage of GM OSR seeds along transportation routes was also confirmed in Japan where cultivation of GM OSR is also prohibited (Saji et al., 2005; Kawata et al., 2009; Nishizawa et al., 2009, 2010; Aono et al., 2011; Mizuguti et al., 2011), the United States (in North Dakota 80% of feral OSR proved to be GM: Schafer et al., 2011; Sagers et al., 2012) and Canada (Yoshimura et al., 2006; Knispel et al., 2008; Beckie and Warwick, 2010; Knispel and McLachlan, 2010). Consequently, transportation and handling activities during import of GM OSR are considered the main cause of unintended occurrence and establishment of feral GM OSR in countries without GM OSR cultivation. In these countries, import restrictions have been put into place because introgression of herbicide resistant transgenes may cause problems with weed management and may negatively affect the integrity of genetic resources in wild relatives (Londo et al., 2010).

To illuminate this issue we assess the mid-term to long-term probability of spillage, establishment and distribution of imported viable OSR seeds, using field data collected along transportation routes and at loading and handling sites in Austria, where the import of GM OSR is banned because of ecological concerns (Pascher, 2011, 2012). Austria relies on import of OSR to meet the demands of the Austrian market. Most of the imported OSR seed originates from European countries (currently mainly Hungary, Serbia and Slovakia), smaller amounts are imported from Chile and New Zealand. Data concerning transportation activities of OSR seeds to and within Austria were, however, difficult to obtain or not made available to us. Seeds are imported as bulk mixtures complying with quality standards (e.g., oil content, low content of erucic acid and glucosinolates, absence of GM material) and thus usually without designation of defined OSR varieties (personal communication from managers of warehouses, ports, railway stations). Therefore, the identity of origin of OSR varieties imported to Austria is unknown. Furthermore, since 2012, OSR belongs to the goods category “other products of vegetable origin” and is no longer recorded separately, rendering traceability impossible. Oilseed rape seeds are transported on ship, trains, and trucks. Whereas, available data for transportation via ship are rather detailed, those concerning transport via railway and trucks are scanty. For instance, the route taken for transport on the road can be freely chosen by the truck-drivers. The biggest fraction of OSR seeds is transported on roads, also at the expense of the rail transport sector, which lost most of its market shares (information provided by the Austrian Federal Railways).

The establishment of feral crops such as OSR, especially of herbicide resistant lines, may contribute to the loss of biodiversity along transportation routes as well as at infrastructures in two ways. Firstly, feral OSR is able to form large populations, especially in disturbed and semi-natural habitats. Our study supports that some of these large populations establish in areas where no OSR is grown, which likely is due to import activities. Secondly, because of herbicide resistance of GM oilseed rape, these feral plants establish preferentially in habitats where herbicides are applied or that are affected by spraying in adjacent areas. Due to herbicide application selection pressure supports survival of feral glyphosate resistant plants, and this may lead to an introduction of transgenes into related species (Londo et al., 2010) growing in habitats that have to be kept free of vegetation. Consequently, sprayed railway tracks pose an appropriate habitat for the establishment of herbicide resistant OSR as it was shown in Switzerland (Schoenenberger and D'Andrea, 2012). Hence, in order to minimize and mitigate spillage of imported crops such as (GM) OSR, qualified measures need to be defined and implemented. First of all, the mode of seed packing during transportation plays a crucial role. In most cases OSR seeds are transported unpacked facilitating seed spillage. Spillage can be minimized by the use of sealed bags. The loading areas of ships and trucks should be checked regularly for perishing seals, in which case foam can be used to mend the holes. Moreover, at railway borders in Austria transportation facilities need to be checked for appropriate function, e.g., of unloading hatches. Likewise, applying intensified testing of grain cargos imported by ship might allow GM OSR contamination to be identified before unloading, handling and further transportation (Schulze et al., 2014). Cleaning of loading areas of trucks and ships, cargo boards of vehicles and train wagons, and storage areas at ports or oil mills should be intensified and performed carefully. Thereafter, these areas have to be checked for remaining OSR seeds. As seed spillage cannot be fully prevented, weed management has to be applied. Where possible, alternative weed management such as manual removal of plants (as currently already done in the surveyed Austrian ports), mowing, spraying of organic herbicides or grazing by animals is favored. In feral OSR populations spraying with herbicides may be considered to unmask GM OSR individuals, which then can be selectively removed. In contrast, in Switzerland the ports are sprayed and hence, long-term persistence of glyphosate resistant OSR seems to be very likely (Schulze et al., 2014). Weed control along railway lines is necessary to ensure worker safety as well as stability of railway gravel beds. In Austria, to maintain functionality of the beds, the gravel is regularly washed, reducing the amount of plant seeds in the material. Those measures, already performed for reasons of operational safety, would not be affected by the suggested additional weed management. The proposed safeguard measures that we consider necessary will, however, cause additional costs. The main responsibility to limit seed spillage involves the importers and traders, staff who handles and reloads the seeds as well as seed-processors.

To control appearance and persistence of feral crops along transport routes, a monitoring program for imported herbicide resistant crops such as OSR should be implemented in the future. In this context it needs to be considered that documentation of mode of transportation (ship, train, truck), transportation routes as well as loading and handling sites—so called commodity flows—is necessary to identify relevant sites for monitoring in each country and to implement a program taking local specificities into account. Relevant contact addresses (e.g., oil mills, ports) and sources (e.g., Statistik Austria, INVEKOS) for necessary information need to be provided in advance for an immediate update. For us, interviews with the staff of the contacted facilities have been particularly helpful to make domestic and foreign goods' transportation and handling more transparent. If monitoring of, for instance, OSR seed spillage during import activities were to be performed under financial and time constraints, monitoring should focus on reloading and handling sites of OSR in Austria, where the highest genetic diversity within populations of feral OSR plants, probably tracing back to repeated input of different OSR seeds also from abroad, has been found (Pascher et al., 2016). Besides the possibility to unmask herbicide resistant OSR, ecological concerns of the usage of herbicides exist concerning spraying of complementary herbicides, which could facilitate establishment of feral herbicide resistant OSR in case of GM OSR imports. Second in line for surveying are switchyards, transportation roads as well as railway lines leading to loading, handling and processing facilities. Analysing around 2,000 individual plants will provide comprehensive results for a genetic monitoring in small countries such as Austria (Pascher et al., 2016) or Switzerland (Hecht et al., 2014). This sample size is also financially feasible. Since single OSR varieties could not be characterized with eight SSR-markers (Pascher et al., 2016) a larger budget for genetic analyses must be provided to be able to enlarge the marker-set.

Safeguard mitigation efforts on long-term basis are in general needed to prevent establishment and spread of feral GM crops originating from seed spillage along transportation routes. Similar to OSR, the small seeds of cereals are also regularly spilled along roads (Von der Lippe and Kowarik, 2007) or along railway tracks (personal observation) resulting in numerous feral plants. As little documentation concerning cereal seed spillage exists, further detailed research is of utmost importance, in particular, because GM cereal lines have already been developed and could be in use in the near future. In order to succeed with weed management of feral crops along transportation routes, a broad collaboration both in research and practice for interdisciplinary exchange is necessary. Moreover, for a successful approach standards and international guidelines concerning transport of crops—as it would especially be necessary with OSR seeds—have to be harmonized in future.

KP conceived and designed the study and carried out field work. KP and CHR assembled agrarian data, information on transportation routes and import data. CHR carried out sampling and designed the figure. KP and GMS wrote the manuscript. GG commented it. All authors read and approved the final version of the manuscript before submission.