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Combat glyphosate-resistant weeds with the old & the new

Herbicide resistant weeds -- not just those resistant to glyphosate -- are continuing to wreak weed-management havoc.

“There are biotypes no longer controlled by previously effective herbicides,” says Aaron Hager, University of Illinois (U of I) Extension weed specialist. In one Illinois case, a waterhemp biotype is resistant to not only glyphosate, but also an additional three herbicide action modes.

The good news? Glyphosate (used on over 95% of soybean acres and 70% of corn acres) continues to be the cornerstone of most weed-control systems.

An online survey by Successful Farming magazine and Agriculture.com found that 92% of respondents are either satisfied or very satisfied with their glyphosate-tolerant cropping systems.

“In most Midwestern fields, Roundup Ready technology and glyphosate are still doing the job,” says Mike Owen, Iowa State University (ISU) Extension weed specialist.

Still, there’s apprehension about glyphosate’s performance. Of those responding to the survey, 55% felt it is as effective as it used to be; 45% noticed a decrease in its effectiveness. Meanwhile, 57% noticed an increase in hard-to-control weeds, while 44% noticed an increase in glyphosate-resistant weeds. 

“We are at the edge of a slippery slope over a deep valley,” says Owen. “It will get to be a serious problem very quickly if the system continues as it has without consideration toward stewarding the Roundup Ready technology.”

A disconcerting fact is that herbicide-resistant biotypes may be spreading via pollen. Research by Pat Tranel, a U of I weed scientist, indicates weed resistance to glyphosate can be transferred by both pollen and seed.

Pollen may move over a larger geography than seed does. Therefore, a herbicide-resistant biotype could be spread over a large area, just as herbicide-resistant weed seed can by hitching a ride on harvesting equipment.

How It Spreads

Doug Doolittle, Story City, Iowa, adopted Roundup Ready soybean technology early on. For years, two postemergence applications of glyphosate provided excellent weed control.

In a couple fields, however, he’s wrestled with waterhemp that’s become increasingly tougher to kill each year with glyphosate. He’s cooperating with ISU weed scientists with plots on his farm to find ways to manage it.

“You can brown it, but it keeps coming back,” he says.

Fortunately, the resistant waterhemp hasn’t been a problem in corn, as he applies Lumax preemergence. On soybeans, though, hard-to-kill waterhemp (which likely is glyphosate-resistant) prompted him last year to apply a rescue treatment of Flexstar when soybeans were beginning to bloom. This was after two glyphosate applications failed to kill the waterhemp.

The good news is the Flexstar worked. Those soybeans went on to be Doolittle’s top yielders on his farm.

However, the application dinged the beans and temporarily turned them brown. “Crop safety is important to me,” he says. That’s why he’s continuing to search for additional solutions.

What’s coming up

There are new tools coming later this decade that provide ways to manage glyphosate-resistant weeds. They include:

  • Dow AgroSciences Technology (DHT) for soybeans by Dow AgroSciences in 2015, pending regulatory approval. This features tolerance to phenoxy auxins like 2,4-D and “fop” grass herbicides.
  • Dicamba-resistant soybeans from Monsanto’s Roundup Ready 2 Yield soybeans. Dicamba, a synthetic auxin, will offer a second mode of action to glyphosate.
  • HPPD inhibitor-resistance soybeans from Bayer CropScience. These soybeans would be tolerant to isoxaflutole (Balance).

These compounds will join Optimum GAT, a new glyphosate-tolerant trait with ALS inhibitor action from Pioneer Hi-Bred and its parent company DuPont.

A new trait/chemistry that’s an alternative to glyphosate-tolerant systems, Liberty Link soybeans, was launched by Bayer in 2009.
BASF also launched Kixor, a new PPO inhibitor compound for corn and soybeans, this year.

It’s important to remember that continual use of any of these technologies can sprout weed resistance. “Growers may see them as a silver bullet, but they are just another tool,” says Owen.

What To Do Now

That’s why using a mix of chemistries now and in the future is recommended to forestall weed resistance to herbicides.

“The key to managing a resistant population is to reduce the intensity of selection,” says U of I’s Hager.

This includes moving away from a total postemergence glyphosate plan and incorporating these strategies:

  • A preemergence residual herbicide.
  • When feasible, tankmixing herbicides with a different action mode with a postemergence glyphosate application.
  • Rotating away from the same active ingredient used year after year.

“You can slow the progression of herbicide weed resistance by using a more integrated approach,” says Hager.

One benefit of using a preemergence strategy is you’ll forestall glyphosate resistance and also boost yields.

University of Wisconsin WeedSOFT calculations show soybeans treated with glyphosate at the V2 (second node) stage with 2- to 4-inch-high lambsquarter and 4- to 8-inch giant foxtail showed an early-season yield loss of 0.5 bushels per acre. When the glyphosate application was delayed until V5 (fifth node), early-season yield losses tallied 4.4 bushels per acre.

Thus, the extra $10 to $15 per acre (the value of between 1 to 2 bushels of soybeans) spent for a preemergence product and application returns at least 4 bushels per acre or even more in increased yield.

“The way to make money is to eliminate early-season competition,” says Owen.

An attractive preemergence option is early preplant. This normally occurs a couple weeks prior to planting. Nixing early-season weed competition via residual early preplant treatments takes the weed-pressure heat off a later postemergence nonselective treatment.

It also has a time-management benefit.

“The single most important thing farmers do in the spring is plant,” says Owen. Applying a residual preemergence herbicide two weeks prior to planting enables farmers to solely seed at planting.

Early preplant treatments aren’t foolproof. Excessive rainfall, for example, can curb the length of their residual protection. Weather can influence the length of their residual protection.

For the most part, though, they are an effective tool.

Still, it’s a harder sell convincing farmers to spend $10 to $15 per acre in advance for a postemergence chemical. Another hurdle these treatments face is declining glyphosate prices.

“The farmer is worried about tomorrow, not three to four years from now,” Owen says. Thus, the concern about farming amid tightening margins can outweigh concerns about forestalling glyphosate resistance.

Practicing Sanitation

The trends toward fewer farmers, more acres, and more distance between these acres make it more difficult for farmers to practice sanitation as a weed-control practice.

“Lots of things have to be compressed into a short period of time these days,” says Owen. Farmers don’t have as much time to clean equipment between fields as they once did. This nixes a method of preventing the spread of glyphosate-resistant weeds.

“If you have a field with a herbicide-resistant weed problem, you may want to attend to that field last,” says Owen.

Doolittle takes his resistant weeds serious. He tried a preplant herbicide this spring on a couple of his problem waterhemp fields. Although he won’t know the yield results until this fall, he’s searching for tools to help him deal with glyphosate resistance. “That’s why these (ISU) plots are here,” he says.

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