By Richard Black
Environment Correspondent, BBC News website
A follow-up to the UK's major trial of genetically modified crops, the Farm Scale Evaluations, finds that impacts on wildlife can persist for two years.
The original trial found that spring GM rape and sugar beet were harsher than their conventional equivalents in the short term, while GM maize was better.
The new study shows the same pattern at two years for rape and maize.
The British government has welcomed the findings, which it says "provide important information" on GM crops.
The new information relates to three of the four crops studied in the Farm Scale Evaluations (FSEs): spring oilseed rape, sugar beet and maize.
Initial results on these crops were published in October 2003; data on the fourth crop, winter oilseed rape, was published separately in March 2005.
FSEs: QUESTIONS ASKED
Four crops - spring and winter oilseed rape, maize, sugar beet
GM varieties resistant to herbicides glyphosate or glufosinate
266 UK sites involved in trial
Half of each field planted with GM crop and sprayed with appropriate herbicide; other half planted with conventional equivalent and sprayed with comparator herbicide
Investigators monitored "biodiversity indicators" including weed seed numbers, insect and bird populations
"The new study confirms our impression of what would happen when we released the initial results," said Les Firbank, of the Centre for Ecology and Hydrology in Lancaster, the FSE project co-ordinator.
"We did expect the differences to persist, and I don't think it will affect any decision on approving GM crops," he told the BBC News website.
This follow-up, published in the Royal Society's journal Biology Letters, did not look at insects and birds as the initial study had done.
Instead, it confined itself to monitoring the weed seedbank - the number and diversity of weed seeds left in the soil, which will be food for insects and birds.
It found that the result seen at one year for maize, with the GM crop leaving a greater seedbank than conventional varieties, persisted through the second season after planting.
The converse result for spring rape - GM cultivation worse than conventional - also persisted.
"After the trial season ended, the land returned to normal management and farmers managed the two halves in the same way," said Dr Firbank.
Different herbicides are sprayed onto GM and conventional varieties
"So we would expect differences to persist because weeds are controlled by the farmers anyway; if you did see a big increase in weeds, you would expect the farmer to do something about it."
However, areas which had been sown with GM beet and had initially seen a fall in the seedbank compared with conventional cultivation appeared to mount a partial recovery.
The initial trial result on maize had proved controversial because of studies indicating that the herbicide used on the conventional varieties, atrazine, is associated with a range of toxicities; its use is now banned in most EU countries, though not yet in the UK.
The scientific team points out that any results seen in the studies are direct consequences of the herbicides used rather than the plants themselves, although these GM varieties are specifically created to be used with proprietary herbicides.
The results are unlikely to have an impact the broader question of whether genetically modified crops are grown in the UK.
Following the initial results from the FSEs, the government indicated that it would approve cultivation of the GM maize used in the trial, the Bayer product Chardon LL, and reject the others.
FSEs: THE RESULTS
At one year, biodiversity indicators worse for spring rape and beet; better for maize
Maize results compromised by concerns over comparator herbicide atrazine
Results for winter rape, published later, showed GM variety decreased seeds important for birds
New study shows initial impact can persist for two years
However, Bayer then decided not to press ahead with a UK introduction for Chardon.
The legislative situation regarding GM crops across Europe has since become more complex.
In principle, once one European Union member has approved a crop, it is automatically approved in all other member states.
However, some countries have vetoed certain crops; and the EU Council of Ministers decided in June not to remove that right, even though it may be illegal under European law.