A US study has concluded that genetically modified cotton can offset some of the environmental impacts of intensive agriculture by reducing pesticide use.
It also showed that cotton carrying a bacterial gene for a toxin that kills the crop's main insect pest had no significant effect on non-target species.
More than half of all GM cotton is grown in the United States. Among developing nations, China, India, Mexico and South Africa all grow it on a large scale.
Last year, researchers in India warned that Bt cotton grown there was not effective at killing bollworms, and that farmers had to spray more insecticide as a result (see Indian GM cotton is 'inadequate'; enquiry demanded).
The researchers, led by Yves Carrière of the University of Arizona, compared the amounts of insecticides and herbicide farmers used, and the amount of cotton produced in 81 fields growing the different types of cotton.
Overall the three varieties had similar yields, but farmers growing conventional cotton had to use extra insecticides to control the bollworm and other insect pests. Farmers used similar levels of herbicide for all three types of cotton.
Insecticides and herbicides can harm non-target species in the fields they are sprayed and in the wider environment.
"Our findings indicate that Bt crops could be useful to reduce the environmental impacts of agricultural intensification," say the researchers. But they point out that this depends on whether farmers need to use insecticides to control insects unaffected by the Bt toxin.
They emphasise that farmers should consider all key crop pests and means of controlling them when they are deciding whether to use Bt crops to reduce the impacts of agricultural intensification.