Weed Resistance: Understanding the Problem and the Solutions

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The ‘Glyphosate Honeymoon’ is over. Weed control is complicated again, not to mention more expensive than ever. Here’s what you need to know to understand the problem, and win the battle.

Across North America, a generation of farmers has been enjoying the power of herbicide-tolerant crops to withstand specific broad-spectrum treatments, killing the weeds and leaving the cultivated crop intact.
Monsanto introduced the herbicide glyphosate in the 1970s and marketed it as Roundup. In this so-called Glyphosate Era, growers and even ag-related scientists stalled somewhat in their efforts to attend to the problem of weed resistance.
That’s because for the most part, it was a fairly simple fix, thanks to crop rotation basics and herbicide-tolerant crops.
It has been a successful run — until recent years, when growers and agronomists began to notice that more and more weeds were surviving. Their tolerance to the herbicides was growing.
According to one industry insider, the ‘Roundup Generation’ is coming to the end and weed control has become lot more complicated.

Last July, driving back 450 miles from a conference in Tennessee, Dr. Stephen Duke was impressed by the clean crops of cotton and soybeans. Clean — despite huge issues with herbicide resistance. Farmers were coping, at a cost.
Later, from his office as leader of a natural products research unit for the USDA Agricultural Research Service in Oxford, Mississippi, the senior plant physiologist pondered how herbicide resistance has impacted both sides of the border, and what growers are doing about it.
“You haven’t had as big a problem with resistance in Canada as we have in the U.S.,” Duke says. “In the mid-South, resistance is a horrendous problem. We’ve got so much cross-resistance to all kinds of modes of action. Farmers’ options are getting more and more complicated, and more expensive. For some farms in the mid-South to mid-West, the cost of herbicide alone has gone from $30 an acre to $120 an acre. It’s a problem — a major problem.”
This won’t surprise many successful, progressive growers who have been watching this trend escalate in their crops every year.

More Complicated = More Expensive

Synthetic herbicides began to be marketed in the 1950s. When the original Roundup herbicide was introduced in 1974, it contained the new active ingredient, glyphosate. Glyphosate had an unique, mode of action known as EPSP, which inhibits a certain plant enzyme. Two years later, it provided the foundation for the new zero tillage concept that used glyphosate instead of tillage to control weeds. The practice of zero till or minimum tillage spread rapidly as it provided numerous benefits to both the producer and the environment.

Hugh Beckie, Agriculture Canada’s weed resistance expert, has made a career of tracking what followed the universal adoption of glyphosate, especially as resistance to other modes of action began to build. Farmers stopped looking for other ways or products to control weeds. Research dried up.
Between 1980 and 2009, according to Duke, the world saw 137 new herbicide active ingredients introduced. However, not one used a new mode of action.
The world now had 19 accepted sites or modes for action by herbicides. The last new mode of action was introduced in 1982, when triketone HPPD inhibitors became available. Roundup was just eight years old.
“New modes of action are definitely needed. Before this, a new mode of action was introduced approximately every two to three years,” says Duke.
Beckie’s resistance surveys have now identified 21 weed species with herbicide resistance. Resistance to the Group 1 (A) or Group 2 (B) mode of action is by far the most common and economically damaging. Some weeds now have stacked resistance two or three modes of action.

Beckie estimates that in 2014, 38 million acres of farmland in Western Canada had herbicide resistant weeds. The cost to farmers was $1.1 to $1.5 billion in increased herbicide use and decreased yields. “Without meticulous management, resistant weeds will continue to spread and the effect on grower profits will only get worse,” says Beckie.

His weed surveys show annual increases of resistant weed acres in the range of 2.2 to 2.5 million acres. In theory, by 2025, virtually all 65,000,000 acres of Prairie crop land may have resistant weeds.

Enter Glyphosate Resistance

Now, wild oats is identified as ‘at risk’ of developing glyphosate resistance. Several suspected cases have occurred in North Dakota and Minnesota. If proven, this would be the first example of a grassy weed showing resistance to glyphosate on the northern Great Plains or Prairies — and would further complicate weed control.
Currently, in Western Canada, glyphosate resistant kochia has the most attention but it is far from the only herbicide-resistant weed to watch.

In his 2013 weed survey report, Beckie wrote that kochia resistant to glyphosate plus Group 2 herbicides is increasing rapidly across southern Alberta and Saskatchewan, primarily selected in fields dedicated to cereals and chem-fallow rotations.
“It was common to see putative GR kochia populations in fields adjacent to the survey-targeted field, suggesting seed spread via tumbleweed movement or by farm equipment. The ease of mobility from field to field demands a collective regional response, proactively or reactively, to manage this multiple-resistant weed biotype,” Beckie wrote.


There never was or will be a silver bullet. But the good news is that with a multi-pronged strategy growers can get a handle on their weed challenges. Here’s what you need in to do:

1. Assume you have herbicide resistant weed populations on farm.
If you have not been diligent in step 3, chances are in some weed population you have or are developing a resistance to one or more of the herbicide chemistries you have been using. Better to assume you have and act accordingly than to deny and end up in a wreck.

2. Get educated
Agri-Trend Senior Agri-Coaches Dr Jim Harbour, Dr Bill Hamman and Markus Braaten say the new approaches for coping with weed resistance will take more knowledge, more time and more money.
Hamman was one of Monsanto’s research and development lead scientists for 28 years before setting off with his own company, Hamman AG Research Inc., in 2003 at Lethbridge, Alberta. Today, he specializes in small plot research contracts with fertility, herbicides, fungicides and insecticides.
It is knowledge, not herbicides, says Hamman, that is really the key to managing weed problems. Integrated Pest Management (IPM) is the popular term for it.
“It is becoming necessary for farmers to be very knowledgeable about the biology of the weeds they are dealing with as well as all the control options,” Hamman says.
“For example, for a farmer with glyphosate-resistant kochia, learning that kochia seed is viable in the soil for only a couple of years can be very useful in long term management strategies.”
“The direct experience of a consultant or Agri-Coach with a new herbicide like Authority or Aim, or the new herbicides in groups 14, 15, 19 and 27, can help a farmer to keep a step ahead of the weeds,” he says.
Managing herbicide-resistant weeds isn’t new; it’s the variations that are new. In short, if you understand the biology involved and if you know the options, you can keep a step ahead.

3. Rotate crop species and herbicide chemistries
Beckie continues to advocate a simple management principle: Rotate crops and rotate active ingredients. In addition, use tank mixes containing multiple modes of action or sequential treatments of herbicides having a different site of action but which are active on the same target weeds. This tool is critical in preventing the development of herbicide resistant weeds and managing should a resistant weed population
Recently, Monsanto Canada and public weed scientists got on the same page when it comes to basic weed management advice related to glyphosate. From similar menus, they say: Use it, but don’t depend on glyphosate alone to do the job you need.
“Monsanto now often recommends using other chemical manufacturers products so that we don’t have exclusive reliance on Roundup,” says Joe Vink, Monsanto Canada weed management technical lead, Winnipeg. “We have our own weed management platform [Roundup-Ready Weed Management Solutions]. The backbone of that platform is to have relevant economical, efficacious, practical recommendations for growers to use other herbicides within Roundup-Ready crops.”
The message of Agriculture Canada on the subject of weed resistance — and of the world’s most powerful crop protection company — is really about diversity, according to Vink.
“We have gotten into trouble where there’s a lack of crop rotation. We have increased selection pressure for glyphosate-resistant weeds where we just rely on Roundup for weed control,” says Vink. “We’re strongly encouraging adding diversity in the cropping system, within a good sound crop rotation. With a diverse set of crops you can utilize different herbicides groups that are applied at different timing and keep the weeds off-balance every single year.”
“It’s nice to have that non-Roundup-Ready crop within the rotation, like wheat or corn, so you don’t have exclusive reliance on Roundup,” says Vink. “For true herbicide diversity or stewardship, we are in full support of Hugh [Beckie] when he talks about making sure you are tank mixing effective, overlapping modes of action on target weeds.”
Monsanto’s mantra now is: Start clean, stay clean. Start the crop by using a burndown tank mix, not just glyphosate or Roundup. The ideal mix would include herbicide with residual that will continue to provide weed control while the crop germinates and emerges.
“Resistance is a numbers game,” Vink says. “If you start clean, have a pre-emergence residual herbicide with broad spectrum activity on narrow-leaf and broadleaf weeds. Then when you come back in crop with, for example, Roundup to clean up a few escapes that have come through that pre-emergence residual, the numbers are on your side. There are fewer weeds, they are smaller and there is less selection pressure to find that rare individual weed with potential to be resistant to glyphosate.”

4. Check herbicide efficacy post application
American Senior Agri-Coach Dr. Jim Harbour worked with DuPont crop protection development for 15 years. Today, he is 
a Post-Doc at the University of Nebraska-Lincoln in extension plant pathology.
His Golden Rule on the subject of weed resistance? “Know Thy Enemy,” Harbour says. “If a weed seems to pop up more and more, get your agricultural service or Agri-Coach to look at it. If you have to, send it in and see if it actually is tolerant to the herbicide you’re using.” Scout couple of weeks post application, check herbicide efficacy, track any misses, follow-up once suspect weed seed is viable and send sample to the lab. While you’re out in the combine, make note of any weed patches, especially those with known resistance in your area.

5. Re-introduce the cultural techniques
Cultural (or non-chemical) weed control methods do not exert a chemical selection pressure and assist greatly in reducing the soil seed bank. Tillage, mowing, grazing, silageing, even burning used judiciously can help in the management of resistant weed populations.
Agri-Trend’s US director of Knowledge Team, Markus Braaten, who is based in Kalispell, Montana, has been a consulting agronomist for 10 years and has trained Agri-Coaches for four years.
He says the most important thing for coping with herbicide resistance is to understand that growers are dealing with a dynamic system.
“We need to take a systems approach to weed management and consider the crops we’re growing, the crop protection products we use — specifically the modes of action — and be as judicious in the selection of modes of action as we are in the selection of the crops we grow,” says Braaten.
Just understanding the challenges can be challenging in itself, as plans are made to place crop seed into a field loaded with weed seed.
The big issue could be cheatgrass, or Canada thistle, or resistant kochia. Then, to go after the most challenging weed in that field, the goal is to understand the tools available before reaching for any jugs to knock out the challenge.
“For a lot of years, glyphosate was the ‘easy button’,” Braaten says. “It took care of a lot of things. But I don’t see that now. Glyphosate is going to continue to be an important part of our weed management program, for sure, but we are incorporating other chemistries. Good stewardship and judicious use of the herbicides are paramount if we’re going to conserve their efficacy long term.”
He warns that just ‘hurting’ the weed, i.e. setting it back for a while, should not be an option.
“Anytime you subject a weed to a sub-lethal dose of herbicide, you potentially inoculate that particular plant; you create some selection pressure toward developing resistance. We want our rates to be on the upper edge of that label, with the water quality and amounts and surfactants we need to drive the efficacy of that product. Understand the factors that affect effectiveness — environment, coverage rates, surfactants — and do everything 
to make sure we utilize those as effectively as possible.”
And while you’re working through those plans, think about the longer term, beyond the coming year.
“The crop rotation we use needs to account for the weed pressure we’ve got. Move away from over-reliance on a single commodity and a single mode of action. Scout fields. Understand the nature of the weed spectrum. Choose the appropriate mode of action based on that weed spectrum.
“Then, do the application and keep watching. Do we have escapes? Where do we have escapes? What weeds are escaping? Confirm whether the escapes are a function of resistance. If they are, take steps to knock it down.”
It may sound like a daunting task, but Braaten remains optimistic.
“I think we understand the risk associated with resistance,” he says. “It’s just a matter of developing a course of action to take care of it. I don’t think our resistance issues are insurmountable. We’re gaining ground through diversifying the cropping system and chemistries. We do have some resistance issues associated with kochia and wild oats, but we are not stumped or ready to give up.”


Weed control tools used by the post-Roundup generation of farmers are likely to look a little different, according to industry and academics. Check out these new tools we discovered in Saskatoon, Arkansas and Mississippi.

• At the first Ag in Motion event at Saskatoon in July, crowds studied the latest guided weed control implement out of England - the Garford Robocrop, a tool for mechanical control of weeds, resistant or not.
Pulled on a 3- point hitch, the Robocrop’s high-speed, robotic hoeing implement, mechanical weeded about 1,000 acres of organic wheat, canola, beans and other crops in Manitoba, at row spacing of 6.5-inches, at up to 10 mph.
The Robocrop’s implements are custom manufactured by Garford Farm Machinery Ltd. To date, most have been made for vegetable growers. Garford, a seasoned British manufacturer, was asked to develop precision guidance for inter row hoes in 1997 and introduced 
the first Robocrop in 2001.
Sales associate Keith Campbell says, “The same guidance system is used in row crops and vegetable crops. In vegetables, as long as the plants are eight inches apart or more, the Robocrop will go between each plant weeding at a rate of two plants per second per row or more.”
Robocrop uses a camera-guided twin hydraulic side-shift, with lateral movement up to 24 inches. Computer software interprets the image to accurately place hoes within one inch of the plant row. The company claims it can work effectively and safely in rows as narrow as 4.5 inches. Normal working depth is about an inch below the ground; hydraulic down pressure maintains a constant, even hoeing depth.
The hoes take out weeds to within two-centimeters of the crop plants, travelling at up to 12 mph, Campbell says. The widest system to date is comprised of three 20-foot sections on a pull-type hitch.

• For the weeds that do go to seed, another option may be to harvest and remove them, or simply burn them. A team at the University of Arkansas led by Jason Norsworthy, Elms Farming Chair of Weed Science, has studied both recently.
The University of Arkansas is looking for strategies to reduce weed seed returns to the soil seedbank. Over the course of three years, the Arkansas team harvested a 20-acre site in continuous soybean production, using different methods of weed control. Most recently, they have focused on two methods, narrow-windrow burning and use of a chaff cart.
“For our system, we captured the material with a chaff cart, which may be the oldest implement for capturing harvest material, and physically removed it from the field. It was very effective,” Norsworthy says.
The second approach began with a simple chute at the back of a combine. It directed all the chaff, including weed seeds, into narrow windrows. Later, they burned the windrows and determined that the team had “100 percent control” of all weed seeds coming through 
that combine.
Burning residue has issues in some areas, but it is effective. Norsworthy says, “You’re building another layer of complexity into that weed management system. That, in itself, is going to reduce the risk of weeds evolving resistance. By bringing down the soil seedbank, there will be fewer weeds that need to be controlled and hence, the risk of herbicide resistance 
is reduced.”

• Bio-pesticides are also getting a lot of attention these days.
Stephen Duke is looking for ‘natural’ chemicals from plants to serve as new bio-pesticides, using new modes of action.
“Now that we’re getting all this resistance, companies are reinvesting in discovery. His ARS research team is looking for mode-of-action options in natural, plant-based compounds. Chemists refer to them as ‘allelochemicals,’ products that a plant produces to fight off competitors.
“In the realm of natural compounds that kill plants, there are quite a lot,” Duke says. “Industry has used these natural compounds as starting points for the development of some important commercial herbicides.”
Sorgoleone is one of the most studied allelopathic compounds. It is produced in the roots of sorghum. It has 
a mode of action that interferes with several molecular 
target sites. In the lab, it can inhibit photosynthesis in germinating seedlings.
“People have worked to formulate it for an over-the-top 
type of herbicide. I doubt that it’s going to be competitive or very useful in that way, but we have a project where we’re trying to engineer a crop to make its own sorgoleone. It would be transgenically enabled allelopathy,” Duke says. We’ll see,” he adds.
The idea of genetically manipulating a plant to defy or resist weeds is pretty attractive. Similar work is underway in lines of rice, but nothing has been done to this point in corn.
“There are allelochemicals produced by rice that are pretty effective against weeds. They’ve worked hard to develop varieties that make more of those compounds,” he says. “If it were significantly effective, you’d be able to either eliminate or significantly reduce the use of synthetic herbicides on those crops.”

About the Author
John Dietz

Freelancer John Dietz and wife Angie left the city in 1980 to live in the country at Arden, the Crocus Capital of Manitoba. Dietz became a farm writer while a communications specialist for Manitoba Agriculture (1975-1980). His news releases introduced zero tillage in Western Canada, the Manitoba Weed Fair, Manitoba Agricultural Hall of Fame and much more. He regularly contributes to a major American farm magazine, Successful Farming, and is author of three tractor books.

Dietz enjoys news, sciences, photography, wild saskatoons and garden-grown raspberries.


His website is: www.prairie-stock.smugmug.com.

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