By Richard Black
BBC environment correspondent
GM crops are no more harmful to the environment than conventional plant varieties, a major UK study has found.
The Bright project looked at varieties of sugar beet and winter oil-seed rape which had been engineered to make them tolerant of specific herbicides.
The novel crops were compared with non-GM cereals grown in rotation.
The project concluded that the GM varieties, used in this way, did not deplete the soil of weed seeds needed by many birds and other wildlife.
The findings of the Botanical and Rotational Implications of Genetically Modified Herbicide Tolerance (Bright) Link project were released on Monday
Bright was designed to mimic normal agricultural practice, and measure how these GM crops would perform when used in a typical crop rotation pattern over four years.
Not only did the project find no evidence of seed depletion, it also pointed to potential benefits for farmers of growing the GM crops.
"What we have shown is that in the case of these two crops, there are ways of managing them which are quite practical, and farmers can deal with them quite readily," the study's scientific co-ordinator Dr Jeremy Sweet told BBC News.
"There appear to be some management advantages in the flexibility of the herbicide usage; there could well be cost-benefit advantages, depending on the price of the herbicides and seeds when the crops are commercialised.
"So there do appear to be a number of reasons why farmers might be quite interested in growing these crops."
However, there is little prospect of GM crops being introduced into the UK in the short-term.
Earlier this year another major trial, the Farm-Scale Evaluations or FSEs, found that two GM varieties, a sugar beet and a spring rape, were more damaging to biodiversity than conventional crops.
THE BRIGHT LINK PROJECT
Four-year study on relatively large plots (0.25-0.5 hectares)
Studied GM winter rape and beet grown in rotation patterns
Outcomes compared with plots of conventional plants
GM crops tolerant to glyphosate or glufosinate herbicides
One rape had cross-bred resistance to imidazolinone
Studied numbers and diversity of weed seeds left in soil
Looked for emergence of multi-herbicide-resistant plants
There were fewer insect groups, such as bees and butterflies, recorded among the novel plants.
A GM maize, on the other hand, appeared to do better than its conventional cousin.
Following the FSE results, Environment Secretary Margaret Beckett announced that companies wishing to bring GM crops into the UK would have to go through a long approval process.
Subsequently, Bayer CropScience, the only company with outstanding applications for government permission, withdrew those applications.
THE BRIGHT FINDINGS
GM crops showed increase weed seeds over 4 years
Weed seed diversity in soil similar in GM and non-GM
Advantages seen in GM in reduced weed control costs
GM crops offered greater flexibility in spray timing
Emerging multi-herbicide-resistant crops controllable
Nevertheless, Bright will help biotech companies and proponents of GM agriculture argue that the crops should not be banned on environmental grounds.
The European Union has indicated that member countries will in the future have to base decisions on whether or not to permit GM agriculture on science rather than public opinion.
However, Emily Diamand, senior farming researcher with the anti-GM Friends of the Earth (FoE), was sceptical that Bright really had mimicked normal farming practice.
She told BBC News: "It was done at agricultural research centres, and real farmers never do things in the same way as they are done on research stations.
"Its findings are only as useful as the questions it asks - there are so many other things to be considered with GM crops, such as the effect on the soil, gene transfer to other plants, and the social and health impact."
GM opponents have also pointed to results in the study which highlighted the emergence of multi-herbicide-resistance in "volunteer" plants.
Volunteers are plants that grow from spilt seed in the previous rotation and can breed with other herbicide tolerant crops in a new rotation to produce progeny with "combinations of herbicide tolerance".
These plants could lead to farmers having to use stronger, or combinations of, weed killers if they wanted to get rid of them, campaigners said.
"These experiments show that, practically, it will be very difficult to grow GM with non-GM - the issue of co-existence," FoE's Clare Oxborrow explained.
"With oilseed rape they found that up to 1,000 seeds on average per metre squared were actually surviving in the soil. This has huge implications for things like contamination of non-GM crops and for giving consumers choice."
English Nature, the UK government's independent wildlife advisor, said it found nothing to cheer in the Bright results.
It said both Bright and the FSEs showed weed control in these modified crops was more effective and reliable than conventional intensive agriculture.
English Nature argued that this risked further reducing already impoverished farmland wildlife by destroying even more of the weeds it depended on.
Dr Brian Johnson, English Nature's biotechnology advisor, said: "We will be asking the Advisory Committee for Releases to the Environment (Acre) to consider the validity of the scientific data presented in the Bright report and to assess the implications of these results for the conclusions of the FSEs."
And Dr Mark Avery, the director of conservation at the bird group RSPB, said: "This research tells us nothing about the impacts GM will have on wildlife.
"The government funded Farm-Scale Evaluations published last year demonstrate clearly that if GM herbicide-tolerant beet and oilseed rape were grown in the UK they would exacerbate the problems faced by our threatened farmland wildlife."
A UK government spokesman said of the Bright findings: "It's valuable research, and complements the Farm-Scale Evaluations.
"It provides some valuable results and we'll ask Acre to evaluate it, and we'll take it from there."
More than half of Britons who took part in the "GM Nation" survey last year said GM crops should never be introduced in the UK under any circumstances.