GM Crops Good for Streams?

April 29, 2008 -- New findings suggest that the use of genetically engineered corn and soybeans could contribute to a reduced load of herbicides in streams and rivers. But some say the story may not be so simple.

The most widespread use of genetic engineering in U.S. crops today is in corn and soybean plants that carry a gene that makes them resistant to the herbicides glyphosate (best known as Roundup, sold by Monsanto) or glyphosinate (present in Liberty, by Bayer CropScience). This allows farmers to apply these herbicides to their fields, killing the weeds but not the crops.

Demand for such "Roundup Ready" soybeans has grown so that they now make up more than 90 percent of the U.S. soybean crop. "Roundup Ready" and "Liberty Linked" corn represent a similar proportion of cornfields, according to Mark Loux of the Ohio State University in Columbus.

Glyphosate and glyphosinate herbicides are applied to leaves of plants after the crops come up. They stick strongly to soil, and degrade relatively quickly compared to other widely used pesticides like atrazine, metribuzin and alachlor, according to study lead author Martin Shipitalo of the U.S. Department of Agriculture's Agricultural Research Service, stationed in Coshocton, Oh.

"Since they're partially water soluble and persistent," he said of other herbicides, "they tend to show up where we don't want them to."

He and his team compared the amounts of herbicides in runoff from test plots growing corn and soybeans treated either with gluphosinate or glyphosate or with combinations of other herbicides including atrazine, metribuzin, linuron and alachlor, depending on the crop.

They found that for soybean crops, the average fraction of glyphosate lost was about one-seventh that of metribuzin and one-half that of alachlor. For corn, glufosinate loss was about one-fourth that of atrazine, and similar to the loss of alachlor and linuron.

The maximum levels of glyphosate found were just over 9 micrograms per liter, compared to the maximum level allowed in U.S. drinking water of 700 micrograms per liter, while alachlor and atrazine exceeded their drinking water standards by five to twenty times. The study's results appear in the current issue of the Journal of Environmental Quality.