The first economic analysis of growing genetically modified crops on a wide scale has found that the biggest winners were the farmers who decided not to grow them.
The study, which looked at maize yields in the corn belt of the United States, found that farmers who continued to grow conventional crops actually earned more money over a 14-year period than those who cultivated GM varieties.
All farmers benefited from the significantly lower level of pests that came about after the introduction of GM maize to the US in 1996, but the conventional farmers who continued to cultivate non-GM varieties also benefited financially from not having to pay the extra costs of purchasing GM seeds.
Previous studies into the economics of growing GM crops have concentrated on the farmers who have taken up the technology, but the latest research looked at a wider area, including non-GM fields that may have benefited from being near fields planted with GM varieties.
GM maize, which is called corn in the US, has a bacterial gene called "Bt" added to it so that the plant excretes a protein which has a toxic effect on the European corn borer, a serious insect pest introduced accidentally into America in 1917.
Nearly two-thirds of the US corn belt is now cultivated with Bt maize, and it has had a dramatic impact on the decline of the corn borer moth, which cannot distinguish between the GM and conventional varieties. When female moths lay their eggs on Bt corn, the larvae die within two days of hatching.
Paul Mitchell, an agricultural economist at the University of Wisconsin-Madison where the work was carried out, said the main corn-growing states of Wisconsin, Minnesota, Illinois, Iowa and Nebraska experienced a total economic benefit of $6.9bn (£4.6bn) over the period from 1996 to 2009 as a result of less maize being lost to the corn-borer pest.
But the non-GM corn areas accounted for 62 per cent of this total economic benefit because, in addition to preventing crop losses resulting from lower levels of pests, these farmers did not have to spend any extra money on the technology fees associated with the purchase of GM maize.
"Previous cost-benefit analyses focused directly on transgenic crop acres. This study is the first to include the value of area-wide pest suppression and the subsequent benefits to growers of non-transgenic crops," Dr Mitchell said. "In this case, the value of the indirect yield benefits for non-Bt crops exceeded the net value of direct benefits to the BT corn acres."
The study, published in the journal Science, found that Wisconsin farmers benefited to the tune of $325m (£217m) as a result of the overall suppression of the corn borer pest between 1996 and 2009. About 75 per cent of this cumulative economic benefit went to the farmers who cultivated non-GM maize.
William Hutchinson of the University of Minnesota, the study's lead author, said an analysis showed that the European corn borer moth has declined in the fields neighbouring those of GM crops by between 28 and 73 per cent, depending on the initial level of infection. The scientists were able to make these estimates because of good records of pest populations going back 45 years.
"Additionally, environmental benefits from corn borer suppression are likely occurring, such as less insecticide use, but these benefits have yet to be documented," Dr Hutchinson said.
Farmers in the US who cultivate GM maize with the Bt gene are encouraged to set aside plots of land adjacent to the GM fields where they grow conventional varieties of maize. This tactic helps to prevent the evolution of Bt-resistance in the corn borer moth, which is more likely to occur when the entire crop is genetically modified.
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