As the growth in Roundup Ready crops continues, there are increasing questions about glyphosate-resistant weeds. Weeds resistant to Roundup or other glyphosates have now been confirmed in at least 10 states. Other states are seeing suspect situations.

Horseweed (marestail) is the primary resister. Weed scientists are also keeping an eye on waterhemp, giant ragweed and lambsquarters.

The Corn and Soybean Digest did an informal survey of weed specialists in six Midwestern states, asking about the glyphosate-resistant situation as of early 2004.

Although there seems to have been only a moderate spread in confirmed resistance during 2003, weed specialists are bracing for more. The most serious problems are on the East Coast, the Midsouth and the Eastern Corn Belt. Movement is from East to West.

“We found glyphosate-resistant horseweed in four southwestern Ohio counties in 2002, and preliminary results indicate it spread to an area three times that large in 2003,” reports Jeff Stachler, Ohio State University weed scientist. “That's a concern.”

The 2002 resistance was found primarily in continuous soybeans that had been treated 100% with glyphosate, Stachler says. In 2003, most of the suspected resistance was seen in soybean fields rotated with corn that had been treated 100% with glyphosate.

Weed specialists at Purdue University confirmed glyphosate-resistant horseweed in four southeastern Indiana counties in 2003. Suspected resistant populations were found in six additional southeastern counties, says Jeff Barnes, Purdue weed scientist.

Although no new confirmed cases of glyphosate-resistant weeds were found in Missouri last year, there was previously confirmed waterhemp resistance.

“We had two waterhemp biotypes with resistance in 2000,” says Reid Smeda, University of Missouri weed scientist. “We went back to those two sites in 2001 and to one site in 2002 and found some surviving plants. We've not had any indication that the problem spread.”

Smeda says some growers in the Missouri Bootheel were reported as having problems controlling horseweed last growing season.

Illinois, Iowa and Michigan have not yet confirmed glyphosate resistance despite problems in nearby states.

“Although we haven't confirmed glyphosate resistance in Illinois,” says University of Illinois weed scientist Aaron Hager, “our border states of Indiana and Kentucky have confirmed glyphosate-resistant horseweed. And in Illinois we know of waterhemp populations with decreased sensitivity or increased tolerance.”

Bryan Young, weed scientist at Southern Illinois University, agrees that, technically, there has not been resistance in the southern part of that state. “However, we're not getting as good control as in the past on waterhemp. The question is whether there is inherited resistance in the uncontrolled plants,”he says.

Iowa State University says there is nothing new in glyphosate resistance.

Michigan State University is screening horseweed populations in greenhouse studies, says MSU's Steve Gower.

The good news, say weed professionals, is that glyphosate resistance can be reduced without adding great cost or inconvenience to control programs. They offer several key steps.

Some experts recommend not using more than two applications of a glyphosate-based herbicide over a two-year period. Diversify with alternative herbicides/cultural practices.

They advise using the full label rate of glyphosate and tankmix partners.

Ohio State's Stachler recommends including 2,4-D ester preplant at the maximum rate that fits a grower's planting schedule to control glyphosate-resistant horseweed. “Also consider including a herbicide, such as Canopy XL or FirstRate, for added burndown and residual control,” he says. “Apply them by early May to nail horseweed while it's still small.”

Barnes also advocates a 2,4-D burndown with application one month to two weeks prior to planting. Otherwise, reduce the rate to 1 pt./acre, if applied seven days prior to planting.

“Producers who prefer not to use 2,4-D can mix glyphosate with FirstRate, Classic, Python or Sencor for burndown application,” he notes.

Barnes says FirstRate and Classic, when used as residual herbicides, have provided the most consistent control of glyphosate-resistant horseweed.

Independent crop consultant Joe Nester, Bryan, OH, recommends that clients rotate modes of action in their herbicide programs. He suggests herbicides with residual activity to manage the extended germination period of horseweed.

Nester says farmers, in wet springs, are inclined to drop 2,4-D preplant and rely entirely on glyphosate for burndown. That can lead to eventual resistance problems.

“Applying 2,4-D with a residual herbicide early, followed by glyphosate in-crop, gives growers the best chance for avoiding weed resistance,” he notes.

Missouri's Smeda points out that some growers consider it to be rotating modes of action when they use a base herbicide, such as glyphosate, and then begin tank mixing another product with it to pick up missed weeds.

“The grower risks other weeds becoming resistant to the base herbicide with long-term use,” he says. “It's better to truly rotate modes of action than to just add a product as a tankmix.”

Two Perspectives On Resistance

Just how much of a problem is glyphosate resistance? Here's what two crop protection company spokespersons have to say.

The resistance situation needs to be kept in perspective, says Greg Elmore, soybean technical manager for Monsanto.

“Worldwide, only six weeds have shown resistance to glyphosate after 29 years of use,” he says. “That's compared with approximately 80 weed species resistant to ALS-inhibitors and approximately 65 resistant to triazines.

In areas where resistant horseweed has been identified, Monsanto has provided a supplemental label recommending the use of 2,4-D with Roundup at burndown prior to planting soybeans and corn to control the pest. For cotton, it's a Roundup plus dicamba recommendation at burndown. In surrounding areas, as a preventive measure, the company includes a fact sheet suggesting a similar strategy.

“To obtain the best control, we emphasize applying these mixtures before horseweed gets more than 6 in. tall,” Elmore says. “Where growers have implemented these practices, we've had few complaints.”

Elmore recommends that growers base their weed-control programs on local needs and use the tools necessary to optimize weed control. These programs may include glyphosate alone or the use of other products where agronomically appropriate for difficult-to-control weeds. An example is an acetanilide applied pre-emergence to Roundup Ready corn followed by Roundup in-crop.

“The glyphosate resistance problem is real and growing,” says Chuck Foresman, technical business manager for Syngenta Crop Protection. “It's estimated that there are currently 2.3 million acres of glyphosate-resistant horseweed in the U.S. and the acreage is expanding every year. There also is glyphosate-resistant ryegrass in California. Worldwide, six weed species are resistant.”

Resistance will grow if we continue multiple applications of glyphosate within the same crop year, Foresman cautions.

“However, if growers follow guidelines for prudent use of glyphosate (see accompanying story), it will prolong its effectiveness and reduce unexpected, yield-robbing resistant weed escapes,” Foresman says. “We are very limited in tools for controlling those escapes, postemergence in the crop once they appear.”