The new study, published by the US Department of Agriculture (USDoA), analysed commercial crop results for 1997 and 1998 in regions where traditional and GM varieties of cotton, maize and soya were being grown.
Yet despite covering millions of acres, the study generally did not find yields improved, while pesticide use was barely changed. "I would have a lot of trouble attributing any sort of `yield bump' to biotechnology," said Bill McBride, an economist at the USDoA who contributed to the study. "There is a lot of variation, depending on all sorts of factors including the weather."
On pesticides, he said there were no areas where use had risen with the introduction of GM crops. But even where farmers said they used less pesticide, the difference was not statistically significant. "Basically, you can't make any predictions before you plant about whether yields and pesticide use will change," Dr McBride said.
However Margaret Spike, of the American Crop Protection Association in Washington DC, told New Scientist magazine, which reports the research today, that "the analysis shows that crop biotechnology works".
British experts are unconvinced. Brian Johnson, of English Nature, the Government's official adviser on wildlife, said the figures suggested a reduction in pesticide use could disguise more significant changes. "The herbicides used with GM crops are broad-spectrum - they kill everything," he said. "If you change from using a lot of something with a mild effect to less of something with a dramatic effect, that is not necessarily good."
English Nature began pressing last year for an analysis of the ecological effects of using GM crops in the UK before commercial use is permitted, on the basis that they could endanger native species here by killing off insects and plants that birds and other animals rely on.
Pressure groups seized on the data as further backing for their demands for a five-year moratorium on commercial growing of GM crops. "It undermines the propaganda pumped out about the biotech industry," said Adrian Bebb, a campaigner for Friends of the Earth. "The DoA research also doesn't show whether these farmers are shifting to broad-spectrum herbicides when they plant GM crops. That would have a key impact on wildlife here."
The USDoA study found that more than 50 million acres were planted with GM crops in 1998, compared to 8 million in 1996. The DoA estimated that GM soybeans have been particularly rapidly adopted, making up 40 per cent of the total crop, while GM cotton and maize comprise about 20 per cent of the totals - though industry estimates suggest that GM cotton makes up a third of that planted, and soybeans about the same.