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6 steps to halt herbicide-resistant weeds

Two years ago, Bruce Stripling sat in FMC’s booth at the Commodity Classic trade show, viewing companies pitching the latest agricultural technology. 

“All this technology, and here we are, still pulling pigweeds,” says the regional technical service manager for FMC. 

Stripling hails from Georgia, where Palmer amaranth started to devastate soybean and cotton fields in the mid-2000s. At that time, glyphosate-tolerant Roundup Ready technology still worked well in the Midwest. 

Before Roundup Ready, Georgia farmers managed weeds in cotton and soybeans with a mix of tillage and overlapping residual herbicides. 

“Roundup Ready was one of the biggest technologies since the Green Revolution, but it did have one bad side effect,” says Stripling. “It made us lazy.”

Bit by bit, weeds like glyphosate-resistant Palmer amaranth and marestail escaped. Georgia farmers dealt with it by a mix of tillage, overlapping residual herbicides, and postemergence herbicides used in other herbicide-tolerant systems. In extreme cases, they hired weeding crews who pulled Palmer amaranth plants and loaded them in a wagon to destroy outside the field. (Palmer amaranth can reroot itself if tossed aside.)

Paging Jimmy Durante

In a sense, herbicide resistance is akin to the trademark line of the late comedian Jimmy Durante: “Everybody wants ta get inta da act!” 

Following this situation faced by Georgia farmers, Midwestern farmers soon struggled with glyphosate-resistant weeds. “Mother Nature always wins,” says Stripling. 

That’s a point to consider when forming weed management plans for 2021 and beyond. Farmers have a host of preemergence and postemergence herbicide options to consider. Still, using the same herbicide strategy year after year will trigger what happened with glyphosate-resistant weeds. 

“We now have [herbicide-tolerant postemergence] dicamba, glufosinate, and 2,4-D-choline technologies,” he says. A key to keeping these technologies effective involves steps like including overlapping residual herbicides in tank mixes, he adds. 

“Without such steps, we will see them fall to the same circumstances that eroded glyphosate in post applications,” says Stripling. “Whatever comes in the marketplace will not last forever.” 

What to Do

Here’s a list of recommendations on how farmers can forestall herbicide resistance with those technologies and improve weed control. 

1. Don’t use just one herbicide program.

So far, the only weed that resists glufosinate – used in several postemergence herbicide-tolerant systems – is Italian ryegrass that surfaced in Oregon and California orchards and vineyards. That doesn’t mean glufosinate is immune to resistance in row-crop production, says Mark Storr, BASF technical services representative. 

“Certainly, we don’t want to use Liberty [glufosinate] followed by Liberty followed by Liberty,” he says. Instead, farmers should aim to apply such products in tank mixes incorporating different effective herbicide sites of action and also use preemergence chemistry, he adds. 

2. Nix early-emerging weeds with preemergence herbicides.

Ever get a bit grumpy when you get a fertilizer bill? This minor annoyance could morph into ear-steaming anger at the thought of 1- to 2-inch-high weeds consuming up to 9 pounds per acre of nitrogen. 

“This also reaches into other nutrients, like P and K and micronutrients,” says Mark Kitt, Syngenta herbicide technical product lead. Ditto for the inch of water that 3-inch-high weeds can remove from the soil in three days, he adds. “It’s important to use broad-spectrum preemergence residual herbicides to control weeds before they emerge,” he says. 

“One of the best recommendations is the old adage of start clean, stay clean,” says Randy Niver, a DeKalb/Asgrow agronomist. “Let’s think about adding one more phrase: ‘Start clean, spray clean, stay clean.’ If we can overlap residual herbicides, we can keep weeds from emerging and never see a weed in the field.”

Preemergence residual herbicides aren’t bulletproof, though. 

“Depending on the product, you can expect the residual to last from 10 days to two weeks up to maybe more than a month,” says Meaghan Anderson, an Iowa State University Extension agronomist. 

About 1/2 inch of precipitation is also needed for activation, she adds. If rainfall is not received, weeds can shoot past early-growth stages to the point where they’re more difficult to control with postemergence herbicides. 

The alternative of relying solely on postemergence soybean herbicides is not a good one, though. “You are putting weed control at risk because you have fewer options,” she says. 

3. Don’t be penny-wise and pound-foolish when it comes to weed management. 

“In a year when everyone wants to cut costs, it’s tempting to cut out a residual preemergence herbicide,” says Aaron Hager, University of Illinois Extension weed specialist. “That is a terrible mistake.”

Without a residual herbicide, numerous weed seeds are exposed to post-emergence herbicides like glufosinate, dicamba, glyphosate, and 2,4-D choline. A herbicide selects for resistant weeds during each application. 

“Having more weeds only speeds it up,” says Hager.

4. Tank mix effective multiple herbicide sites of action.

Applying several herbicides with a different site of action in a tank mix is less likely to result in weed resistance than applying different sites of action between years, says Hager. 

In 2015, USDA-ARS and University of Illinois weed scientists released a study that found fields in which 2.5 herbicide sites of action per application were applied were 83 times less likely to select for glyphosate-resistant waterhemp within four to six years compared with fields in which only 1.5 herbicide sites of action per application were used.

However, the researchers stressed this works only if each component of the tank mix is effective against the target species. They also emphasized that effective, long-term weed management will require even more diverse management practices.

5. When in doubt, opt for a premix or branded product.

These days, farmer-applicators are donning the equivalent of a chef’s hat as they mix complex combinations of chemicals, adjuvants, pH modifiers, and other spray mix components. Forty-two components are used in Syngenta’s Acuron herbicide, says Kitt.

“Developing a high-quality premix formulation is a complicated process,” he says. 

Farmers who form complex herbicide combinations on their own can save money, but it also increases risk. Crop injury can accompany improperly mixed combinations, says Kitt.

“Sometimes, components may not play well together in the tank, and crop injury will occur,” he adds. “We’ve also seen some generic companies that make a premix similar to one of ours, but have a reduced active ingredient.” This can lead to less effective weed control, Kitt adds. 

6. Use nonchemical weed techniques.

“A great man once said, you can’t solve problems created with a herbicide by using another herbicide,” says Richard Zollinger, northwest product development manager for AMVAC.

Cultural weed practices like narrow rows and cover crops can help squelch weeds before they start, says Wen Carter, AMVAC southern region technical service manager.

Ditto for harvest weed seed control (HWSC) strategies pioneered in Australia that include combines that grind weed seed at harvest. Another technique funnels harvest residue containing weed seed into narrow windrows at harvest. Farmers can then burn it, spray it, or leave it to rot. 

By reducing weed seed numbers, Australian farmers increase herbicide effectiveness on surviving weeds. Weed scientists at several land-grant universities are testing that adaptation of these techniques for U.S. farmers. 

There’s a hitch, though. 

“Steps like investing in a Harrington Seed Destructor (a combine weed seed grinder) or a cover crop plan or implementing multiple strategies may cost more money up-front,” says Carter, “but they may prevent farmers from being out of weed control options five years down the road.”

Dicamba Moves Forward

The Environmental Protection Agency (EPA) in October approved the use of dicamba formulations for dicamba-tolerant soybeans and cotton for the 2021 to 2025 growing seasons. 

Approved herbicides include Engenia (BASF), Tavium Plus VaporGrip Technology (Syngenta), and XtendiMax with VaporGrip Technology (Bayer Crop Science). Bayer is the registrant for the dicamba formulation that Corteva Agriscience previously marketed as DuPont FeXapan herbicide with VaporGrip. At presstime, Corteva had applied for federal registration of FeXapan.

Changes from previous registrations include: 

  • A national cutoff date of June 30 on soybeans. This differs from the 2018 label that prohibited over-the-top (OTT) application of dicamba on soybeans 45 days after planting.

Slight differences exist in cutoff wording between chemistries. Bob Hartzler, Iowa State University Extension weed specialist, says XtendiMax application is prohibited after R1, even before the June 30 cutoff date. If the R1 stage has not been reached, XtendiMax can be applied up to June 30. Tavium application is allowed through V4, but not after June 30, he says.. 

There is no mention of growth stage with Engenia, Hartzler says. This may enable Engenia to be applied later than XtendiMax in some cases, but not after June 30, he says.

  • Mandated use of a pH buffering agent in all dicamba formulations applied to dicamba-tolerant crops. Adjuvants like these reduce acid content of tank mixes by pushing pH above 5 [acid content increases as a solution moves downward on the 0-to-14 pH scale]. The higher the acid content, the more prone a herbicide is to volatilize and move off target. 
  • Increased the required downwind buffer from 110 feet to 240 feet, with an increased buffer of 310 feet in counties where endangered species exist.

States may be able to enact additional regulations, but will first need to work with EPA, says Alex Zenteno, Bayer dicamba product manager.

Waterhemp seeds in a farmer's hand
Photo credit: Gil Gullickson

Waterhemp seed is small, shiny, and black.

Know your waterhemp

Waterhemp double-teams farmers who battle it. That’s because this pugnacious pigweed is dioecious, meaning there are male and female plants that team up to inflict misery upon farmers. Males provide the pollen, while females produce the seed that can tally around 500,000 seeds per plant. Farmers who rogue a few stray waterhemp plants before harvest should target their efforts toward seed-bearing females. Although male and female differences can be difficult to discern early in the year, they appear later in the season.

“Females are usually highly branched, whereas the males have fewer branches and the individual stems are longer,” says Meaghan Anderson, Iowa State University Extension agronomist. “Male flowers are also quite a bit bigger.”

Seeds can also be gleaned from female plants later in the season. “If you rub a male plant’s head, you may get some anthers and other male flower parts,” she says. “Tiny black seed will appear when you rub the heads of female plants.”

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