Apr 7, 2008

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Swedish study:

Rape seeds in the soil still viable after ten years

Ten years after the end of a release trial with genetically modified oilseed rape, Swedish scientists found oilseed rape plants on the former release site that still carried the inserted gene. In the study, presented in Biology Letters, the scientists led by Tina d’Hertefeldt of the University of Lund reported a total of 15 transgenic oilseed rape plants that had almost certainly germinated from ten-year-old seeds from the release trial.

“Volunteer rape is the factor with the greatest significance. If it is not properly controlled on a farm, coexistence cannot be guaranteed in the long term. Soil cultivation after the rape harvest, systematic control of volunteer rape plants and suitable crop rotation are the most important levers for keeping volunteer rape under control.” Antje Dietz-Pfeilstetter of the Julius Kühn Insitute, who has studied the dispersal behaviour of oilseed rape for many years, in an interview with GMO Safety.

According to the authors, until now there has not been much data about the persistence of oilseed rape over long periods of time. Comparable studies have been able to show that oilseed rape can persist for up to eight years on fields. The scientists believe that the fact that they have now found transgenic oilseed rape plants after ten years underscores the ability of oilseed rape to survive over long periods and is in line with theoretical forecasts.

The scientists found the equivalent of 0.01 plants per square metre. Since genetically modified oilseed rape was sown on only a quarter of the trial field, higher volunteer rates would be expected in the case of commercial cultivation, especially since volunteer plants are not usually controlled as strictly as they were in this trial. The authors stress that great attention needs to be paid to controlling rape volunteers when genetically modified oilseed rape is grown, especially if conventional rape is to be grown on the same field afterwards.

The release trial

The deliberate release took place in Sweden in 1995 on a trial field measuring thirty by forty metres. The genetically modified rape contained a gene (bar gene) that made it resistant to the herbicide glufosinate.

The plants were harvested in the autumn of 1995. In order to minimise the seed potential of the GM rape in the soil, specific measures were taken to promote germination of the seeds shed during harvest – a procedure also usual in Germany during such trials.

In the subsequent years until 2005 wheat, barley and sugar beet were grown on the field in rotation. The field was ploughed and harrowed every year before sowing. No further GM oilseed rape was grown in the vicinity of the field. Volunteer rape plants were controlled with herbicides – ones containing active substances other than glufosinate – and visual checks were also carried out.

Transgenic rape volunteers ten years on

In the spring of 2005 the field was checked again for rape volunteers. 38 rape plants were found and were planted in pots and examined. They were sprayed with glufosinate twice, three weeks apart. 15 of the 38 plants survived the glufosinate treatment, i.e. still carried the herbicide-resistance gene. Molecular tests (PCR) also showed that the surviving plants were descendants of the genetically modified rape.

In order to examine the soil for possible seed banks, in January 2006 forty soil samples (2.5 cm diameter, up to depth of 25 cm) were taken and the seed banks germinated in the greenhouse. Seven different types of weed germinated from the samples, but no oilseed rape.

Safety precautions for deliberate releases in Germany

Since 1994 the authorising bodies have required a specific procedure for cultivating the soil on trial fields with GM rape. In this procedure, there is a stipulated rest period after the harvest to enable any seeds shed during the harvest to germinate. In dry spells, germination must be encouraged by artificial sprinkler irrigation. The seedlings that emerge must be destroyed and the plant material is then incorporated by means of surface tillage. This procedure is designed to enable a very high germination rate among the rape seeds shed during harvesting.

The current approval process provides for a three-year follow-up monitoring period, which is extended should volunteer plants emerge. Generally, the follow-up monitoring period is extended by a year each time if volunteer plants were found in the last monitoring year.

Volunteer rape: a problem for coexistence?

The fact that oilseed rape seeds can survive for a long time in the soil and emerge as volunteer plants in subsequent crops is known from numerous studies, including some carried out as part of biosafety research. When the aim is to enable GM and non-GM oilseed rape crops to be grown at the same time, volunteer rape becomes a significant factor that needs to be considered. However, most of the scientists and farmers who have studied the problem are of the opinion that there are adequate possibilities for controlling volunteer rape, e.g. through soil cultivation after the rape harvest or through systematic control of volunteer rape in the following crop. If an interval of at least three years is respected before rape is grown again on the same field, volunteer numbers are reduced significantly, according to Bernd Hommel of the Julius Kühn Institute, who was interviewed on the issue by GMO Safety: “So complying with the labelling threshold of 0.9 per cent shouldn’t be a problem.”

Whether individual persistent oilseed rape plants represent a risk to the environment is another issue, which depends on the new trait that has been transferred.