Even tiny amounts can devastate nearby flower, seed, and fruit production
CORVALLIS, Ore. -- A new group of herbicides that had been hailed as a major improvement over some other agricultural chemicals may also have ecological impacts that had not been anticipated, some scientists believe.
New studies and a growing body of evidence suggest that the new weed killers do their assigned job very well -- but they can also devastate the flower, seed and fruit production of some nearby crops if even tiny amounts of the chemicals drift onto them at certain stages of development.
This emerging controversy could have significant repercussions for United States agriculture.
It may also end up pitting one group of farmers, who endorse the new herbicides, against others who feel they are suffering crop losses, researchers say.
The concerns that are becoming more apparent with this family of "sulfonylurea" herbicides illustrate the inability of laboratory procedures to predict all the effects of chemicals in the real world, say scientists from Oregon State University and the Environmental Protection Agency.
As a weed killer they are 100 times more toxic than some other herbicides
studies show detrimental effects from the herbicides and other evidence raises further questions, said Thomas Pfleeger, a doctoral candidate at OSU and plant physiologist with the EPA.
The newest study, just published in the professional journal Physiologia Plantarum, outlines how chlorsulfuron, one of the sulfonylurea herbicides, can reduce garden pea yield by up to 99 percent if just trace amounts of the herbicide contact the plants at their most vulnerable stage of development.
Even though a pea crop could be virtually destroyed, the study found, there would be little other physical evidence that the plants were exposed to the chemical. Their height and appearance would seem nearly normal.
"I think at this point the evidence is fairly clear that these herbicides can affect plant reproduction, even at very low application rates," Pfleeger said. "The chemicals just need to come into contact at the right time of year and stage of plant development."
The sulfonylurea herbicides have extremely low toxicity to animals or humans. Yet as a weed killer they are 100 times more toxic than some other herbicides and can be effective at very low rates of application.
They could reduce the need for other crop chemicals, such as atrazine, that is commonly used but has been linked to groundwater contamination.
With their reputation for a comparatively benign environmental impact, Pfleeger said, the use of sulfonylureas might be expected to increase. Each year, about 400 million pounds of herbicides are used in the U.S. on 270 million acres. Plants genetically engineered for herbicide resistance and the move towards "no-till" agriculture could also expand herbicide uses.
But in recent years, Pfleeger said, evidence has mounted that the sulfonylurea chemicals may affect the reproduction and yield of crops quite unrelated to the ones that were being treated.
Two studies, in laboratory environments and at the OSU Lewis-Brown Horticulture Farm, have been done by Pfleeger, John S. Fletcher and Hilman C. Ratsch at the EPA. They have shown serious concerns with garden peas and sweet cherry trees. One more paper is to be published on effects of low levels of sulfonylureas on soybean, smartweed, canola and sunflower reproduction.
Other allegations -- which have not been scientifically confirmed -- have alluded to problems with rice in Louisiana, flowers and peas in Washington, brussels sprouts in California and fruit trees in Oregon, Pfleeger said.
Drifting chemicals affect native plants as well as commercial crops
of the most significant controversies has erupted in south central Washington state, where some growers believe drifting herbicides have contributed to low crop yields and caused millions of dollars in losses.
"If reproductive development of other crop species is as sensitive to chlorsulfuron as cherry trees," the researchers said in the new study, "then small amounts of drifting chlorsulfuron onto non-target crops could severely reduce crop yields without causing major, easily recognized damage."
Such drifting chemicals might affect not only commercial crop yields, but also fruit development on native plants, an important component of the habitat and foodweb for wildlife, the scientists say.
At comparatively low rates of exposure, the researchers found, other common herbicides such as 2,4-D, atrazine and glyphosate had no such impacts on plant reproduction. The sulfonylurea herbicides even appear to be up to 1,000 times more potent as a reproductive inhibitor than some other chemicals that have been studied and used expressly for that purpose, the study found.
No further research is planned due to funding cutbacks, Pfleeger said.
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