Eight years of planting genetically modified maize, cotton and soya beans in the US has significantly increased the amount of herbicides and pesticides used, according to a US report which could influence the British government over whether to let GM crops be grown.
The most comprehensive study yet made of chemical use on genetically modified crops draws on US government data collected since commercialization of the crops began.
It appears to undermine one of the central selling points of GM farming - that the crops benefit the environment because they need fewer manmade agrochemicals.
Charles Benbrook, the author of the report, who is also head of the Northwest Science and Environment Policy Center, at Sandpoint, Idaho, found that when first introduced most of the crops needed up to 25% fewer chemicals for the first three years, but afterwards significantly more.
In 2001, the report states, 5% more herbicides and insecticides were sprayed compared with crops only of non-GM varieties; in 2002 7.9% more was sprayed; and in 2003 the estimated rise was 11.5%. In total, £73m more agrochemicals were sprayed in the US during 2001-2003 because of GM crops, says the report, which was commissioned by Iowa State University, the Consumers' Union and others.
During 2002-2003, an average of 29% more herbicide was applied per acre on GM maize. But this trend was not sustained over the eight years. Overall, modest reductions in insecticide usage with maize and cotton were recorded, with no sign that the pests were starting to build up resistance.
UK farm trials found that two of the three GM crops grown experimentally in Britain, oil seed rape and sugar beet, were more harmful to the environment than conventional crops but that GM maize allowed the survival of more weeds and insects. The key to insects' and weeds' survival was the quantity of chemicals used on either conventional or GM crops.
Dr Benbrook said: "The proponents of biotechnology claim GM varieties substantially reduce pesticide use. While true in the first few years of widespread planting ... it is not the case now. There's now clear evidence that the average pounds of herbicides applied per acre planted to herbicide-tolerant varieties have increased compared to the first few years."
Last night, the Agriculture Biotechnology Council, a British GM industry trade group, criticized the findings, saying it was not possible to directly correlate pesticide use with GM crops. "There are lots of seasonal conditions that have effects [on how much pesticide is used]. Global warming is also important. We do not dispute that there was a 20% increase [of pesticides] in 2002 over 2001, but that  was the lowest figure in years."
Peter Melchett, policy director of the Soil Association, said yesterday: "This is compelling evidence that GM maize will lead to higher spray use and serious damage to wildlife if the crop is grown in this country.
"The biotechnology companies have been claiming that GM crops result in large reductions in the use of sprays, and GM maize is their frontrunner for commercial growing in the UK. Until now, there has been no clear evidence over the whole eight years of commercial growing in the US to show their claims are false - that's what the evidence in this report gives us.
"It would be inconceivable for the government to give the go-ahead to GM maize now this damning evidence is out."
However, one of the most important factors involved in the increase of herbicides is thought to have been the recent termination of the patent protection for glyphosate herbicide, made by the leading GM company Monsanto. This is the main chemical the plants are engineered to tolerate.
According to the report, new, competing products have halved prices and encouraged more spraying.
· Tony Blair yesterday said that public opinion would play a part in the development of GM products in the UK. Speaking at prime minister's question time about what impact recent government soundings would have on a final decision on GM in Britain, he said it was "vital that we proceed - by public consultation but also on the basis of the science of GM".
© Guardian Newspapers Limited 2004