Resistance on the Rise

March 25, 2010 04:30 AM

It's not nice to fool Mother Nature. Sure, the line comes from a 1970s Chiffon margarine commercial, but the truth of that statement is playing out in nearly every agricultural commodity that depends on herbicides.

There are now 341 herbicide-resistant weed biotypes reported globally, and 130 of those are in the U.S. A rash of resistance to ALS inhibitors, ACCase inhibitors and synthetic auxin transport inhibitors caused concern back in the 1990s, but nothing has set the weed world on edge like resistance to glyphosate.

Nine weed species in the U.S. now have confirmed resistance to glyphosate. These weeds are strains of common ragweed, common waterhemp, giant ragweed, hairy fleabane, horseweed, Italian ryegrass, johnsongrass, Palmer amaranth and rigid ryegrass. Most of the species that have evolved resistance to glyphosate also demonstrate multiple resistances to other herbicide mechanisms of action.

When glyphosate was first introduced for weed control, its unique way of inhibiting protein synthesis and growth in plants led many to believe that resistance would not be an issue. "The herbicide is not the cause of the problem,” says Micheal Owen, Iowa State University Extension weed specialist. "The problem is that we mismanaged the system. The convenience and simplicity of the use of the product have resulted in an escalating problem.”

Fingers point to the commercial success of herbicide-tolerant crops, consolidation of the agrochemical industry, increased regulatory hurdles, generic herbicide production, lack of crop rotation and improper herbicide applications as other contributing factors. Owen says surveys of farmers show a growing awareness of herbicide resistance but a continued reluctance to see glyphosate-resistant weeds as a serious problem.

"The belief that a new active ingredient will save us is hopeful thinking,” Owen says. "Yes, we have new herbicides coming through the pipeline, but a new herbicide mode of action hasn't been commercialized since 1985.”

To date, no weed species has officially been found to be resistant to glutamine synthetase inhibitors (glufosinate) and HPPD inhibitors. However, scientists say the writing is on the wall: Manage these products well or lose their effectiveness in the future.

Those who doubt the seriousness of weed resistance need to listen to Stephen Powles, an 1,100-acre wheat, barley and canola farmer and University of Western Australia professor who has been active in herbicide resistance research since 1983.

"The lesson to the U.S. farmer can be found in Australia, where we have a catastrophe with regard to resistant weeds,” Powles says. He says the dominance of wheat as a crop has contributed to issues in that country.

"Diversity is essential for sustainability,” he adds. "What would Darwin have said about glyphosate being applied on every crop, every field and every year?

"He would have said nature will win. It is the essence of evolution,” he says.

Problems in the southern U.S. have led weed scientists to rank weed resistance as important as the next boll weevil. "Most of the fields still look pretty good across the Midwest. For the most part, the problems can be seen at field edges,” Owen reports.

"However, currently the majority of the corn and soybean acres grown across the Midwest contain glyphosate-resistant cultivars, and glyphosate represents the primary if not sole weed management tactic. Given this incredible selection pressure, it's clear that unless proactive tactics are taken toward stewardship of this technology, we will see widespread issues,” Owen says.

Get Ready to Manage Glyphosate When the Roundup Ready system hit the market 14 years ago, it seemed so easy. "Now, we can no longer assume that all Roundup Ready soybean or corn fields will be free of weeds at the end of the season,” says Mark Loux, weed scientist at The Ohio State University. Resistance issues didn't happen overnight, though.

He notes that giant and common ragweed, horseweed (marestail) and waterhemp are problematic in the eastern Corn Belt. He finds lambsquarters populations getting tougher to control, but he hesitates to label it resistance.

"Still, I see growers who have maintained effective control of weeds in a Roundup Ready system,” Loux adds. "They practice glyphosate management and include other herbicides in their weed management programs.”

He says these farmers are also more apt to practice crop rotation, use proper rates and plan programs based on the biology of the weed.

"The experience most weed scientists and growers had with resistance to triazines or ALS inhibitors led us to believe that a resistant weed would exhibit no response to whatever herbicide it was resistant to,” Loux notes.

"Not so with glyphosate—resistant populations often show at least some response when sprayed and can often be partially controlled,” he adds. Stunting, discoloration and abnormal growth are common characteristics—even in plants that eventually regrow.

Residuals to the rescue. Rick Cole, Monsanto Company technical development manager for U.S. markets, agrees the popularity of Roundup and Roundup Ready technology has put a huge amount of selection pressure on the compound and says we need to steward the technology.

"Regardless of whether the crop is corn, soybeans or cotton, you may need to use all of the tools available to control the weed situation in your field,” Cole says. "There are good reasons to use residual herbicides. Not only do they offer agronomic benefits, but residuals can also
help spread the workload and increase timeliness of your glyphosate application.”

Cole notes that there's a whole generation of farmers who only know the Roundup Ready system. "Some don't remember that there are other herbicides that can be used in the weed control system that can provide great economic value,” he says.

Tech fee qualms. Some farmers question why a technology fee still exists if the product value is diminishing. Cole asks customers to remember that glyphosate controls more than 150 weeds. "In some cases, we've forgotten many of the weeds that were once a problem,” he says.

"I think we need to look at the bigger picture. That tech fee is providing us the opportunity to fund future research and provide additional solutions,” he says. Cole points to dicamba, an effective postemergence herbicide option for control of glyphosate-resistant waterhemp in corn that
cannot currently be used in soybeans. Soybean varieties that are resistant to both glyphosate and dicamba are currently in Monsanto's pipeline.

The company also recently released a new encapsulated formulation of acetochlor for soybeans and cotton to be marketed for the 2010 growing season as MON 63410.

The product is aimed primarily at areas of the Southeast, Mid-South and Midwest where growers have experienced resistance or control problems.

University of Illinois weed specialist Aaron Hager points out that use of Ignite in LibertyLink systems can be an effective tool to manage glyphosate-resistant weed popu-lations and extend the viability of both postemergence technologies. "Some growers rotate post programs—they'll use LibertyLink in one crop and switch to Roundup Ready in the next,” he says.

The new SmartStax hybrids are resistant to both glyphosate and glufosinate (Ignite). This combination reduces the worry of using the wrong product over the top, but it also reinforces the importance of record keeping and planning to safeguard both technologies and avoid volunteer corn situations.
"In soybeans, we can manage volunteer corn from stacked hybrids with ACCase herbicides, but we won't be able to manage volunteers in second-year corn,” Hager says.


Waterhemp Worries
This past summer, I rode shotgun with Aaron Hager, University of Illinois Extension weed specialist, down the back roads of central Illinois in search of waterhemp in farm fields. We didn't have to look far.

Soybean fields that a few years ago would have been clean as a whistle are increasingly polka-dotted with the wily weed. Hager and fellow weed researcher Pat Tranel confirmed this past year that at least one waterhemp patch in Illinois is now resistant to four herbicide classes: triazine, PPO inhibitors, glyphosate and ALS inhibitors.

That leaves HPPD inhibitors and glufosinate-based herbicides as the remaining mode of action herbicides to control the problem. Weed scientists are already starting to worry about what will happen if farmers start to depend too much on those herbicide classes.

Common waterhemp is a member of the pigweed or amaranth family—which explains some of its stubborn ways. Like corn and grain sorghum, it is a C4 plant that is very efficient at fixing carbon and well-adapted to high temperatures and intense sunlight. It's capable of producing 500,000 seeds per plant that tend to germinate throughout the summer. Waterhemp has separate male and female plants, and cross-pollination between plants increases the genetic diversity of a population and favors development of resistance.

"The long emergence window means it's unlikely a pre-emergence treatment alone will provide complete control,” Hager says. "A postemergence treatment will also more than likely be necessary to manage late-emerging individuals.
"We usually see resistance problems pop up first in small strips or patches along field edges or near the field entrance,” he says.

DNA revealed. There is some good news in the waterhemp war. Tranel, a molecular weed scientist, has been able to use sequencing technology to reveal nearly all of waterhemp's genes. This information is being used to develop assays that will help diagnose herbicide-resistant populations more quickly and contribute to the management of this sophisticated weed.

"If you understand how the weed evolves, that helps you devise strategies that hopefully at least slow the rate at which it happens,” Tranel says.

Weed management programs that use both pre-emergence and postemergence treatments offer the best control of waterhemp in corn and soybeans, but you need to consult your state weed specialist for specific programs. "What's really important is to treat weeds at the size specified,” Hager says. "Delayed post applications of glyphosate are many times to blame for poor results.”

A variety of pre-emergence and postemergence herbicide options are available for controlling glyphosate-resistant waterhemp in corn. Fewer alternatives are available in soybeans, where waterhemp is competitive enough that research has shown up to 40% soybean yield loss.

"If you know you have resistance, you definitely need to switch to glufosinate [provided you are in a Liberty-Link system] or a PPO inhibitor,” Hager says. If you suspect glyphosate-resistant waterhemp in soybeans, he recommends the following weed control measures:

  • Apply a full rate of a soil residual herbicide no more than seven days before planting.
  • Apply postemergence glyphosate treatment when waterhemp plants are 3" to 5" tall.
  • Scout the field seven days after glyphosate treatment. If waterhemp control is inadequate, consider applying a full labeled rate of a PPO-inhibiting herbicide.
  • Rescout the field within 10 to 14 days. Hand rogue any surviving plants before they reach reproductive growth stage.


You can e-mail Pam Smith at

Southern Exposure

Southern farmers call the early stages of a Palmer pigweed infestation "the red tide." The weed seedlings come on so fast that the row middles appear to take on a red hue before your eyes.

"Leave them be and it doesn't take long for fields to become so consumed you can't tell what's growing except for the pigweed," says Charlie McClenny, a Mt. Olive, N.C., farmer. "A zero-tolerance approach is the only thing that works with this weed."

McClenny, like many growers in the South, fell in love with the ease and effectiveness of glyphosate. He still appreciates the attributes of the herbicide, but long gone are the days of two shots over the top. His current weed control program is now a complicated full-season plan of attack that takes into consideration herbicide mode of action. Glyphosate is now a team player that controls grasses and a lot of broadleaf weeds beyond pigweed.

"Anyone who doubts weed resistance or the impacts of it should come down and take a little southern tour," McClenny says. "I'm luckier than most because rotation between corn, soybeans, cotton and wheat keeps me rotating herbicides."

Six weed species are now confirmed resistant to glyphosate in the southern U.S. Horseweed is the most prevalent. Giant and common ragweed are problematic and recently, Italian ryegrass and johnsongrass have started causing headaches. Still, nothing is quite as competitive as Palmer amaranth. Once thought to be mechanically spread by seed, recent research from Georgia indicates the glyphosate-resistant trait can be moved by pollen, too.

Alan York, North Carolina State University crop science professor emeritus, says resistance is driving almost all weed control decisions in the Southeast right now. "It's forced us to go back to pre-emergence applications and directed applications and, in some cases, mechanical weed control," York says.

"We are pushing residuals anyplace we can get it in the system. In addition to preplant or pre-emergence that includes residuals mixed with glyphosate and more traditional directed layby applications in cotton."

York says it took the literal abandonment of some fields in the South due to resistant pigweed populations before some farmers got the picture. "It's taken a lot of education, but our growers now understand that resistant Palmer demands a new approach."

Supplemental measures to manage horseweed are costing growers around $11.50 per acre for extra herbicide and another $3 per acre for application. "For Palmer, the added cost depends on how much extra herbicide is included in the program—it could range from $23 to $38 per acre, with $30 per acre as a good average," he says.

Larry Steckel, University of Tennessee weed scientist, has seen Palmer amaranth grow 1' in 13 days after receiving full rates of Roundup PowerMax over the top. "PPO herbicides in soybean and glufosinate in cotton or soybeans offer good control options if timed properly," Steckel says. "However, I'm concerned that overreliance will promote resistance in these classes of chemistries."

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