Dicamba-resistant crops could give soya farmers more options to beat weeds.Brett Hampton / University of Nebraska Institute of Agriculture and Natural ResourcesResearchers have created what could be the next generation of transgenic crops by inserting a gene for herbicide resistance from a bacterium into plants. The new crops could help to combat the spread of resistance to other commonly used herbicides.
The approach is not a new one — many farmers already grow crops that have been engineered to resist the herbicide glyphosate. But the new plants are resistant to a compound called dicamba, and could offer farmers an alternative in areas where glyphosate-resistant weeds have become a problem.
Dicamba, which kills broadleaf weeds but spares grasses, has been used for decades to protect fields planted with corn, a member of the grass family. The researchers have now created transgenic soya beans, tomatoes and other broad-leaved crops that are resistant to this herbicide — a development that will expand the range of dicamba's uses.
Dicamba lasts only a few months in soil, and rarely contaminates water. The chemical itself is stable, but it is quickly devoured by hungry hordes of microbes living in the soil.
Shuffling genes
Don Weeks and his collaborators at the University of Nebraska in Lincoln isolated a gene from Pseudomonas maltophilia that is responsible for the breakdown of dicamba. They then transferred this gene into tobacco, soya beans, tomatoes and the model plant Arabidopsis thaliana. In every case, the plants became resistant to dicamba, the researchers report in this week's Science.1
Monsanto, the makers of the 'Roundup Ready' line of glyphosate-resistant crops in St Louis, Missouri, has already licensed the dicamba technology. The company says it hopes to make dicamba-resistant soya beans available commercially in three to seven years, with cotton to follow after that. Monsanto does not market dicamba itself.
Farmers will probably embrace the crops, says Robert Hartzler, a weed specialist at Iowa State University in Ames. "It's definitely going to help some of the problems that are developing with our heavy reliance on glyphosate," he says.
Some 90% of soya-bean crops in the United States and 60% of its cotton are genetically engineered to resist glyphosate. Although critics argue that transgenic technology encourages use of herbicides, advocates say that the crops have helped many farms to stop tilling the earth before they plant — a practice that causes added soil erosion and water pollution. Instead, farmers can plant glyphosate-resistant crops directly after 'burning down' existing weeds with glyphosate.
The problem is that glyphosate-resistant weeds are on the rise, particularly on farms that do not till. And this issue threatens to drive up the cost of no-till farming, sending farmers back to their old tilling ways, says William Johnson, a weed specialist at Purdue University in West Lafayette, Indiana.
But dicamba comes with problems of its own, says Hartzler. The compound's volatility means that it can kill off broad-leaved plants on fields and houses up to half a kilometre away, meaning that farms close to sensitive areas such as vineyards or private gardens may not be able to use dicamba-resistant crops. "It's one of the most common consumer complaints that I deal with," notes Johnson.
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Hartzler says that this 'dicamba drift' is more a cosmetic nuisance than an ecological disaster. Weeks, who along with his collaborators stands to receive royalties from Monsanto for products developed from their discovery, adds that farmers have decades of experience with dicamba, and management techniques are already well established.
What's more, over the 40 years of dicamba use, only a few isolated dicamba-resistant weeds have sprung up, and they have not posed a threat to agriculture. But that's not to say that resistance won't become a problem with more widespread future use. "I don't think we can say resistance won't develop," says Hartzler, "but it is a much lower likelihood than with other herbicide classes. But then, that's what they originally said about glyphosate."
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