By Martin Webber
Editor business programmes, BBC World Service
GM crops are more accepted in the United States than anywhere else
US researchers have criticised claims that genetically modified (GM) crops can help feed a hungry world.
GM crops have been a "spectacular under-performer" in terms of yields, according to Doug Gurian-Sherman of the Union of Concerned Scientists.
Recent yield gains are just as likely to result from conventional breeding techniques as they are from genetic engineering, he says.
GM crops cover about 10% of all commercial farmland globally.
"The historic trend for corn is an average improvement in yield of about 1% per year, which doesn't sound like much but it adds up over time," Mr Guarian-Sherman told BBC's Business Daily.
"If you take the 13 years that Bt (genetically engineered) traits have been commercialised, they've probably only added a couple per cent to yield.
"That's only a small fraction of what we continue to do with means other than genetic engineering," he added.
Yet GM crops have many fans, especially in the US where farmers have widely embraced the technology.
Charles Wilfong farms in a wide valley in the central Appalachian Mountains in West Virginia and likes the fact that the chemicals he sprays on his crop to control weeds, do not affect the corn.
He says corn yields have grown in recent years from 100 bushels to 150 bushels per acre.
"The seed technology is already in the pipeline that can double that to 300 bushels per acre in the very near future," he says.
"More and more of our corn is going to be used for ethanol," he explains.
"If we don't have this technology to bump up the yield there's not going to be enough corn."
Although Mr Wilfong clearly likes the genetically engineered seed he uses, he has also seen its price rise.
"I was out at Des Moines, Iowa, at Pioneer's research facility, back in the summer and some folks were talking that within the near future we'd probably be paying $500 a bag for seed corn instead of the $200 we're paying now," he says.
Almost half of all corn grown in the US is genetically engineered and for soybeans it is 80%.
The yield for soybeans has gone down however, according to Bill Freese at the Centre for Food Safety.
"What we've seen with the herbicide tolerant soybeans overall, is a 5-10% lower yield with the Round-up Ready soybeans - that's the herbicide tolerant soybean sold by Monsanto," he says.
But Michael Fromm, director for the Centre of Biotechnology at the University of Nebraska, who helped develop GM crops for Monsanto, points out that farmers who buy genetically engineered seed decide whether the price is worth the yield they achieve."
"There's absolutely no one making him buy those seeds," he says.
"They're voting for the transgenic seeds at higher cost for higher benefits."
There is controversy over whether the regulators have been too lax in allowing the biotech giants to buy up their competitors and dominate the market.
There has been widespread opposition to GM foods especially in Europe
Mr Gurian-Sherman says that in the last decade or so, firms including Monsanto and DuPont have been buying up dozens of seed companies - small and large - and are controlling more of the seed market.
"The huge independent seed corn company Pioneer is now owned by DuPont," he says.
"And more recently concentration was increased when the [US] Justice Department allowed Monsanto to purchase Delta & Pine Lands, two of the biggest cotton seed producers, so now you have a mega-cotton-seed producer and in that market you're talking around 90% concentration of the market."
But Mr Fromm points out that market concentration is not unusual and allows the big investment needed in new technology.
"Little companies can't bring these products to market," he says. "It's simply too expensive for the size of their budget."
Adding to the lack of choice is the fact that many university seed breeding centres have been closed down, according to Mr Freese.
"The private companies have pretty much taken over the seed market and encouraged universities and other public sector breeders to focus on more basic research and not actually breeding seeds for sale to farmers because they've always traditionally seen university breeding programmes as competition," he says.
This is bad news for David Runyan, a farmer in America's vast Midwest.
"I'm down to three varieties of soybeans that I can plant and I have the last variety of soybean that Illinois Seed Association put out," he says.
"That was two years ago and they are not growing any more public varieties of soybeans."
Even after years of research and near market dominance in four major crops in the United States, most of the promises for genetically engineered varieties are still in the future.
"What we've seen is that there's a lot of hype of genetic engineering and biotech," says Mr Freese.
"We see biotech companies often claiming that they are going to introduce miracle seeds with all sorts of fabulous properties - drought-resistance, extra nutrition, salt-tolerance.
"What's interesting though is that despite all of this hype, in 20 years we only have two traits that have been successfully developed."
Biotech scientist Michael Fromm believes no one can dispute that genetically engineered crops have been successful in the United States in terms of sales or acres planted in these crops.
He also feels the best is yet to come.
"It's important to realize it's still the beginnings of the capabilities there," he says.
"As in all of biology, we're really learning more and more about how human health works and how plants work and that advances our understanding of how to engineer plants for better benefits accelerates," he concludes.