Genetically engineered crops do little to improve yields and instead promote the proliferation of herbicide-resistant weeds that actually curb production, according to a study by the Union of Concerned Scientists.

Corn and soybeans modified to resist insects and the herbicide glyphosate haven't been proven to boost yields, the Cambridge, Massachusetts-based group says in a 44-page report. The modified plants have increased the number of glyphosate-resistant weeds that compete for soil nutrients and moisture, reducing production, the group says.

"The two major types of traits now present in transgenic crops - insect resistance and herbicide tolerance - are often classic contributors to operational yield," said Doug Gurian- Sherman, a senior scientist in the group and the author of the report.

"Neither trait would be expected to enhance potential or intrinsic yield, and indeed, there is virtually no evidence that they have done so."

Operational yield is obtained under normal field conditions and includes factors such as pests and other stressors, the report says. Intrinsic yield is the highest that can be achieved with crops grown under ideal conditions.

Monsanto, the world's biggest seed producer, did not return calls seeking comment.

Sharon Bomer Lauritsen, an executive vice-president of food and agriculture at the Biotechnology Industry Organisation in Washington, said the report by the Union of Concerned Scientists was absurd.

"Biotech crops help to provide for more sustainable agricultural production," Bomer Lauritsen said.

"The benefits include a reduction in the environmental impacts of agriculture, increased production on the same amount of acreage, improved food quality and increased farmer incomes. It's absurd to deny biotechnology's contribution."

Shares of Monsanto, based in St Louis, fell US$1.31, or 1.6 per cent, to US$81.76. Before that dip they had dropped 31 per cent in the past 12 months.

Improvements in traditional breeding and other agricultural practices would be more effective in boosting production, Gurian-Sherman said in the report. Genetically engineered "soybeans have not increased yields, and GE corn has increased yield only marginally on a crop-wide basis", the union said.

"Overall, corn and soybean yields have risen substantially over the last 15 years, but largely not as a result of the GE traits. Most of the gains are due to traditional breeding or improvement of other agricultural practices."

The union is a "science-based non-profit" group started in 1969 at the Massachusetts Institute of Technology. The group's website says it has more than 250,000 members using scientific research to promote changes in Government policy, corporate practices and consumer choices.

The group looked at "the best peer-reviewed literature" to collect the information, Gurian-Sherman said. The union evaluated 20 years of research and details from 13 years of seed sales in the US.

Corn yields have increased to 9.7 tonnes per hectare this year from 7.5 tonnes per hectare in 1987, US Department of Agriculture data show.

Last week, Germany banned a strain of genetically modified corn made by Monsanto, citing "a danger to the environment". Austria and Hungary made similar moves last month.

The European Commission, the European Union's regulatory arm, has argued bans are unjustified because scientists have determined the products are safe for consumption and the environment.