As U.S. farmers struggle to control the rise of "superweeds" choking key cropland, a leading herbicide that’s shown good weed control for decades appears to be losing its effectiveness, a report from a science journal said Wednesday.

Chemical makers have been racing to find an answer to resistance that has built up against the broadly-used glyphosate Roundup, and they had hoped 2,4-D was at least a partial answer to the problem of how to stop weeds that can reach over six feet tall and decimate crop production.

Dow AgroSciences is seeking federal regulatory approval to roll out corn, soybeans and cotton that are genetically altered to tolerate treatments of glyphosate and a 2,4-D-based herbicide, Enlist.

The aim is to wipe out weeds that have become resistant to glyphosate alone. Many critics have protested Dow’s plans, citing fears of increasing weed resistance along with other environmental concerns.

But Dow has said its new herbicide and 2,4-D cropping system is needed to fight back the weeds that have taken over millions of acres of key U.S. farmland.

Dow officials did not immediately respond to questions about the journal report.

But the journal Weed Science, issued by scientists at the Weed Science Society of America, said 2,4-D-resistant waterhemp has been discovered in Nebraska.

Although scarce 30 years ago, waterhemp is now a major problem for crop production in the U.S. Midwest.

In Canada, USDA’s Natural Resources Conservation Service classifies the non-resistant biotype as "present" in Ontario, Quebec and Manitoba.

The NRCS also lists the non-resistant waterhemp as present in all U.S. states sharing borders with Canada, except Montana and Alaska.

"Selection pressure"

After 10 years of treatment with 2,4-D, waterhemp was no longer effectively controlled in a Nebraska native-grass seed production field, the Weed Science report said.

The highest doses of 2,4-D that were used in an on-site field study were insufficient to control 50 per cent of the waterhemp population.

Researchers gathered waterhemp seeds from this field and performed greenhouse testing against a susceptible waterhemp variety.

Twenty-eight days after treatment with the herbicide, visual observation and dry weight values showed a 10-fold resistance in the affected sample.

Farmers routinely use 2,4-D to control weeds in grassland and crop production. Its effectiveness has largely held despite worldwide use since the 1940s, with only 17 weeds previously known to be resistant to it, the report said.

Some weed scientists, farmers and others fear that development of crops that tolerate direct spraying of 2,4-D will increase the use of the herbicide markedly, which in turn will increase weed resistance as was the case with Roundup.

Where Roundup once killed weeds easily, experts say that now, even heavy use of the herbicide often fails.

"The fact that resistance to 2,4-D has evolved in at least one waterhemp population should be emphasized to corn, soybean, and cotton producers to show that proper stewardship of these new technologies is critical for maintaining their effectiveness," the Weed Science report’s authors wrote.

Commercialization of 2,4-D-tolerant soybean, cotton and corn crops should be accompanied by "mandatory stewardship practices that will minimize the selection pressure imposed on other waterhemp populations," wrote the authors, who work for the University of Nebraska-Lincoln and Western Illinois University.

— Carey Gillam is a Reuters correspondent in the Kansas City area. Includes files from AGCanada.com Network staff.

Related stories:
Dow to take on Monsanto with new biotech soybean, Aug. 22, 2011
2,4-D-tolerant Canadian corn, soy a few years away, March 4, 2011
2,4-D passes PMRA re-evaluation, May 20, 2008