The next generation of Touchdown(R) herbicide from Syngenta Crop Protection is the first glyphosate-based herbicide to identify the mode of action on the label using a herbicide classification system from the Weed Science Society of America (WSSA). The numbering system was designed to help growers prevent the development of resistant weeds.

The new Touchdown Total(TM) herbicide label includes a 'Group 9' symbol to let growers know the formulation contains glyphosate. Once glyphosate is inside the plant it inhibits EPSP synthase, an enzyme essential to the production of key amino acids needed for plant growth. 'Group 9' is one of 18 mode-of-action classifications in the WSSA system. [Artwork of 'Group 9 Herbicide' symbol available upon request]

Syngenta's move is important because glyphosate-resistant horseweed (marestail) (please call for photo) now infests more than 2.3 million acres in the U.S. according to recent researcher estimates. Ten states now document glyphosate-resistant weeds, with five confirming glyphosate-resistant horseweed since the fall of 2002: Ohio, Indiana, Arkansas, Mississippi and North Carolina. Other states reporting glyphosate-resistant horseweed infestations are: Tennessee, Delaware, Maryland and New Jersey. California was the first state to confirm glyphosate-resistant weeds in the U.S. on rigid ryegrass in 1998.

Scientists recognize that excessive use of any herbicidal mode of action increases selection pressure for resistant weeds. It is extremely important that growers are aware of the mode of action of the herbicides they use on each field -- and plan to rotate herbicide modes of action to manage against resistance.

Too Important to Lose

Glyphosate-based herbicides are of special importance to U.S. agriculture because of their broad-spectrum weed control, favorable environmental profile, and convenient, over-the-top use with Roundup
Ready(R) [RR(TM)] crops -- particularly RR soybeans.

"This technology is too important to lose. But if we overuse glyphosate, we start to lose the full value of the technology," says Chuck Foresman, technical business manager of Syngenta. "So it is important that growers are aware of the mode of action of the herbicides they're using -- and that they rotate modes of action," he says.

Last year, in hard hit Tennessee, glyphosate-resistant horseweed acreage jumped fourfold, after doubling the previous year, according to Tom Mueller, professor of Plant Science at the University of Tennessee. Researchers in Ohio also observed an increase in glyphosate-resistant horseweed infestations, and estimate at least 250,000 acres are now infested in the state.

The potential for overuse grows with the success of Roundup Ready crops. The U.S. Department of Agriculture reports 81 percent of soybean acres were planted to herbicide-tolerant varieties in 2003 -- up six percent from 2002. And RR corn acreage rose to 11 percent, up two percent from 2002.

Planting RR crops continuously or in successive years, rather than rotating to conventional crops, can lead to the overuse of glyphosate-based herbicides and intensify selection pressure for glyphosate resistant weeds, experts say.

"Glyphosate suppliers, university extension services, distributors, dealers and growers must adopt use guidelines that can serve to prolong the effectiveness of glyphosate for the long term," Foresman says.

"Glyphosate-resistant weeds are already complicating control programs and driving up costs. And researchers are telling us that it won't likely end with just these two weed species," he adds.

Touchdown Total -- registered for over-the-top use on RR soybeans, RR cotton and RR corn -- controls more than 170 weeds and is backed by a comprehensive crop assurance warranty, the Touchdown Assurance Plan. With IQ Technology(TM), a corn-based adjuvant system, Touchdown Total is absorbed through the weed leaf and efficiently moves glyphosate down to the roots.

The Touchdown Total label also includes guidance on glyphosate-resistance management (see sidebar). Syngenta drafted the 'Best Weed Management Practices' with input from university weed scientists, including those from the Weed Science Society of America and the Herbicide Resistance Action Committee, which monitors weed resistance globally.

Syngenta's forthcoming Touchdown herbicides will also carry the 'Best Practices' and 'Group 9' symbol on their labels. These include Touchdown HiTech(TM), pending registration approval from the U.S. Environmental Protection Agency for the first quarter of 2004.

Why Glyphosate Deserves Greater Care
The concern for resistance is greater for glyphosate than for other classes of herbicides because glyphosate-resistant weeds threaten the value of what has been called the most successful cropping system in the history of U.S. agriculture.

Syngenta is an industry leader in addressing concerns of RR growers, says Foresman, publishing regional guidelines for resistance management and speaking out on the issue ever since glyphosate-resistant horseweed first started to appear.

In the rolling hills of western Tennessee, where growers have depended heavily on glyphosate for weed control and applied it both pre-season and over-the-top of RR crops, resistant horseweed remains a serious challenge.

"Glyphosate herbicides make it easy to control weeds in RR crops and they are very important for us, because we'd really like to preserve no-till farming," Mueller of the University of Tennessee says. Yet he points out that input and environmental costs are rising.

"A lot of growers tilled their acres in 2003 because of glyphosate-resistant horseweed. Tillage helps take care of resistant horseweed, but causes other problems. Heavy rains can easily wash out erodable soils, especially on our rolling hills," he explains.

Ounce of Prevention
Dale Shaner, Ph.D., co-author of "Herbicide Resistance and World Grains" and USDA plant physiologist, believes U.S. growers are keenly interested in the cost and efficiency of their weed control programs -- and advocates an ounce of prevention.

"If you're doing the right things -- if you're using an alternative for a burndown, plus a residual, then coming back with the glyphosate over the top, and not using it three and four times a year -- you should be able to keep the problem off your farm.

"But don't bury your head in the sand. Develop a pro-active program and a heightened sense of, 'Am I losing control? Are there patches appearing here and there that should have been controlled?'" If that happens, control those weeds even if it means pulling them out by hand or destroying the crop in the immediate area and taking a loss, he says.

Touchdown(R), Touchdown Total(TM), Touchdown HiTech(TM) and IQ Technology(TM) are trademarks of a Syngenta Group Company.
Roundup(R) and Roundup Ready(R) are registered trademarks of Monsanto Technology LLC. 
RR(TM) is a trademark of Monsanto Technology LLC.