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Corn+Soybean Digest

Marestail Jumps Glyphosate Fence

Roundup Ready soybeans, probably the hottest, most rapidly adapted herbicide technology ever, could be in jeopardy — unless farmers start exhibiting some restraint.

Why? Because what many weed scientists predicted, based on what happened to some triazine and ALS-inhibitor herbicides, is starting to occur with Roundup and other glyphosate herbicide brands.

Roundup-resistant marestail, a troublesome weed also known as horseweed or stickweed, has popped up in the Mid-Atlantic states of Delaware, New Jersey and Maryland. Waterhemp in Missouri also appears to be pointing in that direction — its biotypes seemingly shrug off normal rates of glyphosate.

It could be just the beginning of the problem, where resistant biotypes may differ genetically or have an enhanced ability to degrade the herbicide. But there's plenty of research behind why many weed scientists are worried.

Mark VanGessel, a University of Delaware researcher, has identified glyphosate-resistant marestail. It also has been confirmed by Roundup's manufacturer, Monsanto, and by Syngenta, which produces Touchdown brand of glyphosate.

VanGessel first encountered what appeared to be resistant marestail biotypes in three grower fields in Delaware in 2000.

“Marestail control was random throughout the field, which ruled out sprayer problems or applicator error,” VanGessel recalls. “And with ideal weather conditions, we also ruled out environmental stress factors.”

From there, they transplanted those plants in the greenhouse, then sprayed them with over 10 quarts/acre of Roundup Ultra. The plants were injured but could not be killed. The plants produced viable seed.

“At this point, however, we still weren't comfortable saying we definitely had resistance,” VanGessel recalls. “So we collected seeds from plants in the field, planted them in the greenhouse and at four weeks after planting sprayed them with various rates of both Roundup Ultra and Touchdown.

“We found we had to spray eight to 10 times the recommended rate of glyphosate to get the same level of injury as with non-tolerant plants,” he adds. “So this isn't the level of resistance seen with atrazine or Pursuit, but it's in the neighborhood of a 10-fold difference.”

Most weed scientists say that if you put enough selection pressure on a weed population with a very effective herbicide long enough, it will lead to weed resistance problems. It happened with triazine herbicides, mainly atrazine, and ALS-inhibitor chemistry, principally Pursuit.

Both were king-of-the-mountain-type herbicides. Both ran into resistance problems after very heavy use.

“If you do it wrong enough long enough, you're going to be in trouble. I first said that with atrazine in corn — monotonous corn and monotonous atrazine,” says Jerry Doll, University of Wisconsin weed scientist. “And what happened? We found a bunch of weed species, but primarily common lambsquarters and pigweed, which developed triazine resistance.

“Admittedly, Roundup is in a much safer category in this regard, so it's a lower risk. But nobody says it's no risk and that has obviously been proven,” he adds.

Since Roundup went off patent in 2000, glyphosate is now marketed under brand names such as Roundup Ultra, Touchdown, Glyphomax and Glyphos, among others. It can be used postemergence on Roundup Ready soybeans, corn and cotton, and is often used as a burndown herbicide as well.

Many scientists, including VanGessel, believe that the situation will worsen if growers continuously use glyphosate. “This is kind of a wake-up call that we can't simply rely on glyphosate year after year for weed control. That's especially true when it's used both as a burndown and then postemergence in the crop and possibly in other Roundup Ready crops, such as corn and cotton,” he cautions.

Brad Majek, weed scientist at Rutger's University in New Jersey, has seen resistance. “In no-till soybeans, which are very common here, horseweed is a major problem, and the Roundup resistance is real. The plants have the ability, after being sprayed with Roundup, to come back and be competitive.”

Because corn can't be grown well on New Jersey sandy soils without irrigation, beans after beans are typical. And with a high percentage of no-till beans, Roundup as a burndown magnifies the usage.

“Those of us in universities in the Mid-Atlantic region were not enthusiastic about the exclusive, continuous Roundup approach without anything else,” Majek explains. “We really need to rotate our burndown herbicides, and we would like to see a residual herbicide with a different mode of action go out with Roundup Ready beans in those continuous soybean fields.”

Reid Smeda, University of Missouri weed scientist, found waterhemp with probable signs of glyphosate resistance. But, at this point, he calls the biotypes that seem to shrug off glyphosate treatments — even at higher-than-normal rates — “insensitive.”

The reason he's not calling it resistance is that he has been unable to reproduce offspring from the 4-5% of tolerant waterhemp with greater degrees of resistance than their parents. The percentage has stayed about the same, not what one would expect in truly resistant plants.

Typically, weed resistance begins with a few biotypes of a species in a field. Those biotypes show resistance to a herbicide, but that generally multiplies rather rapidly, sometimes exploding in a few years. So far, that has not been the case in Smeda's research.

Nevertheless, Smeda acknowledges that the Roundup resistance threat is there and that farmers will need to rotate their herbicides. Ultimately, that means planting something other than Roundup Ready crops continuously. “The Roundup Ready technology is so good, we don't want to lose it by abusing its use,” Smeda adds.

What do officials at Monsanto, the sole supplier of Roundup Ready crops, have to say about this development?

“With respect to marestail in the Delmarva region, we have confirmed that some marestail is resistant to Roundup,” says David Heering, Monsanto's Roundup herbicide technical manager. “At this time, we are investigating the mechanism of resistance.”

Heering says that resistance to Roundup, which has been used worldwide for more than 25 years, is extremely rare. Nevertheless, he acknowledges that greatly increased usage of any product increases the likelihood of resistance.

Monsanto has adopted recommendations for marestail control where growers have had problems that include atrazine on conventional corn. In burndown, whether in front of corn or beans, a tankmix of Roundup and 2,4-D is suggested. For soybeans in-crop or burndown, Monsanto recommends a tankmix of Roundup plus Amplify, one of its products, which has a different mode of action.

Bill Beutke, Touchdown brand manager for Syngenta, recommends limiting glyphosate to one crop (soybeans, corn or cotton) and then using a different mode of action the next year. Where a crop is grown continuously, rotate out of glyphosate every other year. “We also recommend crop rotation, mechanical control where practical, and using full labeled rates of glyphosate when it is applied,” Beutke says.

The bottom line: With Roundup and other brands of glyphosate, resistance is a real threat — if heavy and consistent use continues. “No scientist who really knows about weed resistance would argue with that assumption,” Smeda says.

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