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Posted 13 December 2002. PMN Crop News.

Glyphosate-Resistant Waterhemp Moves into
the Corn Belt

Syngenta Crop Protection.

(December 12, 2002) First it was horseweed (marestail) in Delaware, confirmed resistant two years ago to glyphosate-based herbicides such as Touchdown® and Roundup®. Then the same weed proved resistant in Tennessee, spreading to about 500,000 acres this year, and Ohio researchers say as many acres are likely infested between southwest Ohio and south central Indiana.

Now, at least one weed scientist is ready to add waterhemp to the list of glyphosate-resistant weeds.

Iowa State University researchers have tested waterhemp plants from a number of locations around the state. Individual selected plants survived an average of 2.6 times the labeled rate of glyphosate, with some plants able to shrug off even higher rates.

Mike Owen, extension weed management specialist with Iowa State University, says there’s enough evidence for him to label these individual weeds ‘resistant.’

“Our work demonstrates that the glyphosate resistance in these waterhemp plants is heritable,” he says. “It’s passed on from one weed generation to the next, although the exact mechanism and frequency is still unknown. We do not feel that there is a major concern for glyphosate resistance in waterhemp at this time.”

In similar waterhemp studies conducted by Reid Smeda, weed scientist with University of Missouri, waterhemp plants from Missouri and Illinois survived up to eight times the labeled rate of glyphosate. For now, Smeda is calling these weeds ‘insensitive’ to glyphosate. “It’s more than a natural tolerance, but we don’t have enough evidence yet to call it ‘resistance,’” he says.

“Even if we’re not saying resistance, we’re not controlling waterhemp as well as we did six years ago,” adds Bryan Young, assistant professor of weed science at Southern Illinois University. “We’re using higher rates of glyphosate and more applications.”

Regardless of semantics, says Owen, the problem in a grower’s field is the same: “You can’t kill the weed with the labeled amount of glyphosate.”

Roundup Ready System at Risk

For most growers in the 2003 season, the waterhemp and marestail problems may not amount to anything more than a chink in the armor of glyphosate, the largest selling herbicide in the U.S. and the world.

But weed scientists in the Corn Belt say glyphosate resistance will spread, and growers should take steps now to manage it. Resistance could spoil the effectiveness of Roundup ReadyÒ (RRÔ) technology, which is now used on about 75% of the soybeans in the U.S.

It was only a few years ago that waterhemp developed resistance to the most popular soybean herbicides at that time, a family of chemistry known as ALS-inhibitors. The timing coincided with the introduction of RR soybeans, which quickly eclipsed ALS-inhibitors and solved the problem - at least for a while.

This time around, however, there’s no major new technology waiting if a problem develops.

“There’s no new silver bullet around the corner,” says Owen. If glyphosate fails, the only alternatives will be older herbicides already on the market, different management tactics, and mechanical control strategies.

“Without glyphosate in the arsenal or waterhemp, we would go back to narrower windows of application, intensive scouting, and tank-mixing,” says Tom Hoverstad, weed scientist at the University of Minnesota. “We would be evaluating crop injury versus control, and if our conventional herbicide didn’t work, we wouldn’t always be able to apply it again.”

One piece of good news: weed experts say that because of the way the weed reproduces, resistant waterhemp isn’t likely to spread as rapidly as horseweed has. Also, most growers in the Midwest grow RR soybeans but not RR corn, so they’re rotating herbicide modes of action annually. That’s one real key to preventing or slowing the spread of resistance.

Horseweed (marestail) spread from a few fields in Tennessee in 2000 to an estimated 500,000 acres in Tennessee and Kentucky in 2002. On the East Coast, glyphosate-resistant horseweed spread from six Delaware fields to over 50,000 acres in Delaware, Maryland, Virginia, and New Jersey - all in the same three-season span.

In Ohio, extension weed specialist Jeff Stachler is now reporting glyphosate-resistant horseweed. “We have nine populations from four counties in southwest Ohio surviving four times the labeled glyphosate rate,” says Stachler. The glyphosate-resistant horseweed is also in Indiana, halfway between Indianapolis and the Ohio River. He’s calling for strong resistance management tactics.

Resistance vs. Weed Shifts

Likewise, Hoverstad says he’s seeing more waterhemp than ever before in Minnesota. “Velvetleaf also seems to have an edge over glyphosate,” he notes. “But this is probably the result of weed shifts and not true resistance.”

Like resistance, weed shifts are caused by repeated glyphosate use. But there’s no genetic change in the weed population, just a build-up of weeds that are naturally more tolerant to glyphosate chemistry.

“Glyphosate has never been as consistent on weeds like black nightshade, yellow nutsedge, and velvetleaf,” says Bob Hartzler, weed management specialist with Iowa State University. “In a RR system where you’re relying heavily on glyphosate and little else, these weeds can survive, produce seed, and increase in density.”

In Ohio, dandelion is a good example. Stachler reports that it’s gone from a minor weed to a major problem on just about every acre of no-till soybeans. “It’s even hard to control it with two tillage passes,” he says.

He’s also seeing a shift to common lambsquarters in RR soybeans, where glyphosate no longer gives consistent control of the weed.

Dallas Peterson, weed specialist with Kansas State University, says he is seeing more morningglory where farmers grow continuous RR soybeans. Like his counterparts in Ohio and Minnesota, he says this is more of a weed shift than resistance. “But the end result is the same,” he notes. “We’re left with a weed that our current program no longer controls.”

What Causes Resistance?

Researchers say the culprit in glyphosate resistance and weed shifts is continuous use of glyphosate in Roundup Ready cropping systems. In Tennessee, it’s continuous RR cotton, or RR cotton rotated with RR soybeans. In Delaware, it’s continuous RR soybeans. These rotations allow for glyphosate use every year, with few if any other herbicides in the rotation.

“In the Corn Belt there hasn’t been as much continuous glyphosate because many growers have been rotating RR soybeans with conventional corn,” says Hartzler. But an increase in RR corn threatens to upset that balance.

Introduced in 1999, RR corn took off slowly, but Hartzler says it’s gaining popularity in pockets of the Corn Belt, especially where corn is grown for domestic use instead of export.

“If farmers keep planting more RR corn, we’ll be in a continuous glyphosate situation too,” Hartzler says. “Granted, many farmers would probably use a pre-emergence treatment on RR corn and that would offer us some protection against resistance, but probably not enough. There would still be a lot of selection pressure.”

Rotate to Fend Off Resistance

Despite the potential consequences of resistance, researchers say resistance management is a tough sell with some growers. They like the simplicity of Roundup Ready technology and want to use it every year. Many figure they’ll make a change after resistance becomes a problem. But once that happens, the damage is already done.

“Once you apply glyphosate or any herbicide, the clock starts ticking,” says Smeda. “If you want to protect the chemistry, you must be proactive.”

The only way to preserve glyphosate is to avoid overusing it, says Hartzler. He advises growers who plant RR soybeans to avoid RR corn, and vice versa.

“It’s just too easy to fall into a trap of overusing glyphosate when you’re growing one RR crop after another,” says Bryan Young, assistant professor of weed science with Southern Illinois University.

KSU’s Peterson agrees. “If you’re relying heavily on glyphosate in RR soybeans, then we encourage you to use conventional weed control systems in corn,” he says.

Rotation figures heavily in resistance management guidelines issued by Syngenta for growers using glyphosate-based herbicides - including Syngenta’s Touchdown, which is commonly used in RR crops.

“We’re advising growers to use no more than two applications of Touchdown or any other glyphosate-based herbicide over a two-year period,” explains Chuck Foresman, technical brand manager at Syngenta. “If they’re using two sequential sprays on RR soybeans or corn, then we recommend using conventional seed and herbicides the following year.”

Corn vs. Soybeans

Which crop should be Roundup Ready? Hartzler leans toward soybeans. “First, growers are already using RR technology in soybeans but most haven’t started yet in corn,” he says. “Second, we have good alternatives for waterhemp control in corn, but there is no existing chemistry or new chemistry in development that would give us the same flexibility for controlling waterhemp in soybeans.”

Corn is also more susceptible to early-season weed competition than soybeans, notes Hoverstad. That makes post-emergence glyphosate weed control a better fit for soybeans.

For now, weed specialists say the biggest challenge will be convincing growers to use less RR technology instead of more. “We can’t keep using glyphosate until it doesn’t work anymore, because there’s nothing else out there,” concludes Stachler. “It’s essential to start addressing resistance problems now -- before we wind up with super- weeds.”