"A hard-nosed assessment of this expensive technology's achievements to date gives little confidence that it will play a major role in helping the world feed itself in the forseeable future," said the report by the Union of Concerned Scientists.

The study evaluated the effect on corn and soybean crop yields of genetically engineered varieties commercialized in the United States over the past 13 years, examining peer-reviewed academic studies that date back to the early 1990s.

"Based on that record, we conclude that GE (genetic engineering) has done little to increase overall crop yields," it said.

The report said genetically engineered soybeans account for 90 percent of soybeans grown in the United States, while genetically engineered corn accounts for 63 percent of the US corn crop.

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

It found that corn and soybeans that were genetically modified to increase their tolerance to herbicides "have not increased operational yields, whether on a per acre or national basis, compared to conventional methods that rely on other available herbicides."

Corn modified with genes from Bt, or Bacillus thuringienisis, bacteria for resistance to several kinds of insects did provide higher yields, but the study estimated the increase at between 0.2 and 0.3 percent a year on average over the past 13 years.

Overall corn yields in the United States have increased an average of about one percent a year, it said.

"More specifically, US Department of Agriculture data indicate that the average corn production per acre nationwide over the past five years (2004-2008) was about 28 percent higher than for the five-year period 1991-1995," it said.

"But our analysis of specific yield studies concludes that only 4-5 percent of that increase is attributable to Bt, meaning an increase of about 24-25 percent must be due to other factors such as conventional breeding," it said.

The report contrasted the record of genetic engineering with that of more environmentally friendly organic or so-called "low external input" farming methods, which it said produce corn and soybean yields comparable to those of conventional methods.

Recent experiments in "low external input" have produced yields 13 percent higher than for genetically engineered soybeans, it said.

"It is also important to keep in mind where increased food production is most needed -- in developing countries, especially in Africa, rather than in the developed world."

"Several recent studies have shown that low external input methods such as organic can improve yield by other 100 percent in these countries, along with other benefits," it said.

"To summarize, the only transgenic food/feed crops that have been showing signficantly improved yield are varieties of Bt corn, and they have contributed gains in operational yield that were considerably less over their 13 years than other means of increasing yield," it said.

"In other words, of several thousand field trials, many of which have been intended to raise operational and intrinsic yield, only Bt has succeeded," it said.

"This modest record of success should suggest caution concerning the prospects of future yield increases from GE," it said.

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