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The new math of weed management

Boot up your smartphone’s calculator. That’s because we’re flashing back to a problem akin to one you’d encounter in a high-school math class.

It’s based on estimates provided by Pat Tranel, a University of Illinois (U of I) weed scientist. Ready? 

In 2017, Illinois had 22 million acres of corn and soybeans, 75% of which — 16.5 million were infested with waterhemp. So how many waterhemp plants will take root and grow the next year? 

Read More: Soil Lives!

Here are the numbers. 

• 825,000. The amount of those infested acres (5%) that have waterhemp escapes at year’s end. 

• 25. The amount of waterhemp plant escapes per acre on those infested acres. 

• 500,000. The amount of seeds that is potentially produced per escaped plant. 

• 50,000. (10%) The amount of seeds actually produced for the next year per escaped plant.  

• 5,000. (10%) The amount of seeds that actually survive, germinate, and form plants the following year.


Don’t be. Tranel says this scenario just created 100 billion viable waterhemp plants for Illinois farmers in 2018.  

It’s the explosion in weeds like waterhemp and Palmer amaranth that’s prompted weed scientists to revise weed management recommendations.  

Herbicide-tolerant systems are still part of the equation, of course. Still, weed scientists say farmers need to look beyond herbicide-tolerant postemergence options to ones that halt weeds from emerging in the first place. This includes tools like burndown and preemergence herbicides and weed seedbank management. 

“It’s just not a popular message, frankly,” says Bryan Young, a Purdue University weed scientist. “Growers don’t want to hear weed management will be more difficult and complex and more expensive.”

It’s a stark departure from 1996, when the Roundup Ready system first debuted for soybeans. 

“We watched our neighbors try it out first,” recalls Lynnet Talcott, who farms with her husband, Norris, near Bennet, Nebraska. Besides being nervous about applying non-selective glyphosate over soybeans, they also had concerns over yield drag, she says. 

“We finally switched over to Roundup Ready soybean production in 2000,” says Norris. “It was much easier and cheaper.”

All this came with another perk. 

“Twenty years ago, we were told resistance to glyphosate would not happen,” recalls Aaron Hager, a U of I Extension weed specialist

It happened. 

Besides glyphosate, waterhemp now resists six other herbicide sites of action. Palmer amaranth resists five herbicide sites of action. Cracks are also appearing in the dicamba-tolerant system.

“I wouldn’t call it resistance yet, but dicamba is starting to fail on Palmer amaranth,” says Travis Gustafson, a Syngenta  agronomic service representative in Grand Island, Nebraska.

Young cautions farmers not to revolve weed control solely around herbicides. Instead, he advises farmers to use herbicides in an integrated program that also includes strategies like narrow-row soybeans that canopy earlier and crop rotation. 

“We need a lot of little hammers, as opposed to big hammers, when it comes to controlling weeds,” he says. 

Here’s a mix of those hammers you’ll need to consider for 2020 and beyond for managing weeds.

1. Think Linemen, Not Quarterbacks 

If herbicides were football players, herbicide-tolerant traits and accompanying herbicides would be the quarterbacks. You know the type – the pretty boy types that garner the eight-figure annual salaries.

Paving the way for them, though, are the offensive linemen of the herbicide world — burndown and preemergence residual herbicides. Like linemen, they do hand-to-hand combat in the trenches and snuff out weeds before they become a problem. 

“We have to get away from postemergence applications,” says Bryan Young, a Purdue University weed scientist. “Research at the University of Illinois, Western Illinois University, and Southern Illinois University funded by the Illinois Soybean Association indicate the best return on investment is early-season management. Later post applications should be used to clean up anything that doesn’t work out early on.”

2. Bet on Burndown

Farmers who weren’t able to apply herbicide last fall should consider a burndown treatment as soon as possible next spring spring.

“The earlier we get them out on small weeds, the better,” says Joe Bolte, a Practical Farm Research (PFR) operator and herbicide specialist for Beck’s Hybrids based in Effingham, Illinois.

Fall treatments are particularly preferable for winter annuals like marestail, as they nix small weeds that otherwise would come back in the spring.

However, it was all many farmers could do last fall to harvest their crops, let alone apply herbicides to control winter annuals like marestail. Although time is tight, a spring burndown application can kill marestail before it bolts, he says. 

3. Build a Better Mousetrap

Homeowners who set mousetraps in their houses are ready if any rodent ventures indoors. That’s akin to spraying a residual preemergence herbicide that nixes unemerged weeds. 

“It is really hard to make that shift from waiting for weeds to emerge vs. applying a herbicide before emergence,” says Travis Gustafson, a Syngenta agronomic service representative based in Grand Island, Nebraska.

Still, it’s easier to stop unemerged Palmer amaranth than trying to catch up to an actively growing one with a postemergence one, he says. 

“Palmer can grow 2 inches in a day,” says Gustafson. A several-day rain delay that nixes an early post application when Palmer amaranth is 2 inches tall can morph into hard-to-control foot-high monster weeds, he adds. 

Read More: The Road to Restoring Soil Health

4. Pre’s Aren’t Foolproof

Preergence herbicide performance hinges on weather. Too much rainfall can leach preemergence herbicide from the root zone. Too little rainfall can nix incorporation of the herbicide. (Irrigators have an edge, as they can use a center pivot to ensure incorporation.) 

“Last year, I thought we would see a big dropoff in performance because we had so much rain,” says Gustafson. “Overall, though, the pre’s did a good job. They held a lot better than I thought they would with all the rain that we had." 

Pre’s are starting to resist herbicides, too. In February 2019, resistance was confirmed in two Illinois waterhemp populations to S-metolachlor (Dual Magnum), a Group 15 herbicide.

Still, Group 15 herbicides remain a viable tool for managing weeds, says Gordon Vail, Syngenta technical product lead. “In the vast majority of cases, Group 15 herbicides like S-metolachlor still give excellent control.” 

5. Spray By The Calendar

Jeff Vinzant, agronomy services manager for Norder Supply, Elwood, Nebraska, advises his customers to apply an early preemergence residual followed 21 to 28 days later with another overlapping residual. He recommends a  final post treatment 21 to 28 days later.

Unfortunately, a calendar and Mother Nature often aren’t in synch. “We have been in the business 20 years, and last year was the most difficult year you could ever imagine,” says Vinzant. 

It’s here where establishing alternate plans prior to spraying season can help farmers prevail. There’s a price to pay, though, when switches are made. Moving from Plan A to Plan B or C can decrease the number of herbicide sites of action in a tank mix that help forestall resistance, says Vinzant. Ditto for the length of residual protection. 

“The cost to the grower can increase dramatically, too,” adds Vinzant. “Ten years ago, you could spray glyphosate anytime and it would work. Today, you can spend anywhere up to $30 to $40 per acre up to $70 to $80 per acre..”

Still, the cost of not having an alternative plan in advance is a weedy field and a seed bank full of weed seeds that can fuel future infestations.

6. How Resistance Starts

A good example is Palmer Amaranth’s resistance path in Nebraska. 

“We first started seeing problems in 2010 and 2011,” says Mark Stutterheim, a Syngenta seed adviser and crop consultant from Eustis, Nebraska. “It started in turn rows. Some guys would say, 'Well, I wasn’t going full bore on the turn rows, that’s why we didn’t kill the weed.'”

In truth, these escapes were setting seed, akin the waterhemp seed drop that caused waterhemp populations to explode in Illinois. (See previous page.)

“At first, we thought glyphosate didn’t kill it because we thought it was too dry or too windy or that the plant was too tall to kill,” says Stutterheim. “Resistance never hit home until the 2013-to-2015 time frame. Conditions were ideal to spray and we still weren’t killing it." 

7. Sprayer Ownership Perks

Once a weed resists a herbicide, it’s resistance. If not, though, one way to boost post postemergence performance on weeds where resistance has not surfaced is early treatment. Owning a sprayer can cut time spent waiting on a commercial sprayer and enable farmers to get a jump on fast-growing weeds like waterhemp and Palmer amaranth.   

“We were able to spray faster and be more timely,” says Lynett Talcott, who farms with her husband, Norris, near Bennett, Nebraska. 

Affordability is a big hurdle. Gustafson says he’s seeing a trend where several farmers jointly buy a sprayer. “It may not work with just one farmer, but if they go in with a couple other farmers, they may be able to justify the cost,” says Gustafson. 

8. Enlist is Here

The big news this year the full-scale launch of the Enlist Weed Control System. It confers herbicide tolerance to a new 2,4-D formulation – 2,4-D choline – and glyphosate in corn, soybeans, and cotton and fop herbicides in corn. Herbicide options include Enlist Duo, a mix of glyphosate, and 2,4-D choline. Enlist One is straight 2,4-D choline that can be tank-mixed with approved label herbicides.

9. Homebody Herbicide? So Far, So Good

Synthetic auxin (Group 4) herbicides are prone to off-target movement via volatility. However, Corteva Agriscience officials say the new 2,4-D choline formulation is much less volatile than older 2,4-D amine and 2,4-D ester formulation. 

“Everything I have seen in either plot research or in published literature about the 2,4-D choline is that there is not the issue so far with volatility that some of the dicamba products (labeled for dicamba-tolerant soybeans) have had,” says Kevin Bradley, University of Missouri (MU) Extension weed specialist. “Now, if the wind is blowing (toward the crop) and you make a bad decision to spray 2,4-D, it will hurt (neighboring nontolerant vegetation) just like dicamba. There still is a risk there. But if an applicator does everything right, so far, I have not seen those problems.”

Corteva announced in November that Liberty is the preferred tank mix partner for its standalone (just 2,4-D) Enlist One herbicide. 

“Being able to tank mix Liberty in the tank and have two effective modes of action is a huge draw for many farmers,” says Shawna Hubbard, Corteva product marketing manager. “There are no state cutoff dates and time-of-day restrictions for spraying, as long as no temperature inversions exist.”

10. Yield Drag is a Drag

“You always worry when something is introduced, it might not be very good,” says Lynnet Talcott. “Or, it might be good in another area and not in your area.”

So far, though, no significant yield differences have occurred in University of Wisconsin (U of W) trials comparing varieties with Enlist E3, LLGT27, and Roundup Ready 2 Xtend traits, says Shawn Conley, a U of W Extension agronomist. 

That said, some herbicide-tolerant varieties took a yield hit in 2019 due to disease susceptibility to certain diseases. 

Several Roundup Ready 2 Xtend varieties appear to be heavily affected by white mold, according to evaluations by University of Minnesota (U of M) scientists including James Kurle, plant pathologist; Bob Koch, Extension entomologist; Dean Malvick, Extension plant pathologist; and Bruce Potter, Extension IPM specialist. 

11. Why White Mold Susceptibility?

Two possible explanations exist, say the U of M scientists. 

1. The gene conferring dicamba tolerance may be associated with increased susceptibility to white mold. 

2. The background genetics of these varieties may be susceptible to white mold.

When the first Roundup Ready varieties debuted in 1996 and 1997, white mold outbreaks occurred similar to those that U of M scientists saw in 2019 with several Xtend varieties. The Roundup Ready gene may have increased susceptibility, resulting directly or indirectly in increased risk of white mold, according to the U of M scientists.

12. The Scoop on GT27 Soybeans

So far, GT27 soybean seed is all gussied up to go with no accompanying Group 27 herbicide. (Examples of Group 27 herbicide are Balance Flexx and Callisto.) Initially, the seed was developed by MS Technologies, Bayer, and Mertec LLC. BASF replaced Bayer as the chemical company partner as part of divestments Bayer made when it bought Monsanto in 2018. 

GT27 soybeans tolerate glyphosate and HPPD inhibitor (Group 27) herbicides. BASF also offers soybeans in a LibertyLink GT27 stack that tolerates glyphosate, a yet unapproved Group 27 herbicide, and glufosinate in its Credenz varieties. 

Although farmers planted 8 million acres of GT27 and LibertyLink GT27 stacks in 2019, they could not apply the system’s Group 27 herbicide, Alite 27. At press time, federal regulators have not approved its use. All other Group 27 herbicides are off-label and illegal to apply. 

13. How is Dicamba Technology Working?

Overall, dicamba gives excellent weed control.

“Having the dicamba option really works well for farmers, as there were some pretty challenging weed-control scenarios last year,” says Ryan Rubischko, U.S. soy portfolio lead for Bayer CropScience. 

Cracks are appearing, though. Control is fading in some Nebraska fields, says Golden Harvest seed adviser Stutterheim. 

“We don’t have dicamba resistance yet, but we’re not killing the Palmer in some cases,” he says. “We’ll kink and curl it and make it look pretty sick, but two weeks later it will come back with some new foliage coming out of that ugly-looking plant.”

Read More: How to Better Bean Yields

14. Herbicide Resistance Also Plagues Grassy Weeds

“Grasses are adapting (to weed-management practices) from a biological standpoint,” says Drake Copeland, FMC technical services manager. Weed grasses are trending toward germinating at different times of the season, increasing control challenges, he adds. If not controlled, they can also shatter seed earlier in order to ensure survival into the next year. 

“Fall panicum is a big problem, along with giant foxtail, barnyard grass, and jungle rice in the South,” he says. 

That’s likely the case with herbicide resistance, as well. 

In 2018, dicamba applications failed on about 40 Tennessee Xtend soybean and Xtend cotton fields, says Larry Steckel, University of Tennessee (U of T) Extension weed specialist. The failures occurred on barnyard grass or jungle rice. 

“In 2019, I fielded no less than 200 weed-control failures, 40 of which were fields where Palmer amaranth control failed to some degree,” says Steckel. “Most of the rest were grasses or prickly sida.” 

U of T scientists will evaluate samples later this winter for resistance confirmation. 

15. Is Dicamba Still Moving Off-Target? 

Off-target dicamba complaints declined overall in 2019, says Liam Condon, Bayer Crop Science president. In Illinois, though, off-target dicamba complaints set a record in 2019 at 728 – more than double the 2018 total of 330, says Jean Payne, executive director of the Illinois Fertilizer and Chemical Association. 

In response, the Illinois Department of Agriculture has enacted a June 20 dicamba cutoff date for 2020. It also is prohibiting application of dicamba if temperatures at application exceed 85°F. or if the National Weather Service’s forecasted high temperature for the nearest available location on application day exceeds 85°F. 

“In a broad-based program (like dicamba), you would expect off-target inquiries to be everywhere,” says Condon. “So, we need to look at specific agronomic conditions to evaluate what the root cause is and do whatever is possible to avoid it.” 

16. Mix it Up With Multiple Sites of Action

Tankmixing herbicide sites of action is an excellent way to forestall resistance. In 2015, USDA-ARS and University of Illinois weed scientists released a study that found a field in which 2.5 herbicide sites of action per application were used was 83 times less likely to select glyphosate-resistant waterhemp within four to six years than a field 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 tankmixture is effective against the target species. They also emphasized that effective, long-term weed management will require even more diverse management practices.

17. Rotate Traits, Too

That’s because rotating traits can forestall resistance. 

“We encourage customers who have been planting LibertyLink (glufosinate-tolerant) beans to switch to Xtend beans (for a time), because we do not want to lose the effectiveness of Liberty,” says Jim Schwartz, director of Practical Farm Research and Agronomy for Beck’s. 

So far, weeds have not resisted glufosinate. 

“Liberty is our best hope for a while,” he says. 

18. More Trait Stacks Are Coming

In mid- to late spring 2020, Bayer CropScience plans to launch XtendFlex on soybeans, pending regulatory approval. It’s a triple soybean stack that tolerates glyphosate, dicamba, and glufosinate. Bayer is also developing a four-way stack by the mid-2020s and five-way stacks by 2028.

Xtend Flex will aid farmers, says MU’s Bradley. There may be cases where weather adversely affects dicamba applications (due to label restrictions like wind). Farmers may then be able to spray glufosinate on the Xtend Flex soybeans due to less-stringent label restrictions, he says. 

It’s still early, but Bayer is developing dicamba-tolerant corn that’s slated for debut next decade, says Condon. 

19. Are New Herbicide Sites of Action Coming? 

HPPD inhibitor (Group 27) herbicides (Callisto, Corvus) were the last corn and soybean herbicide sites of action to be commercialized in the late 1990s and early 2000s. That may be changing, though. 

“It remains to be seen whether those survive all the way through the remaining research and development stages. But we have some promising things in the pipeline,” says Bob Reiter, who heads research and development for Bayer CropScience. “The average time for developing (herbicide) technology is 14 years, so we’re probably a good decade out.”

BASF also has a new herbicide site of action for the corn and soybean market that’s slated for the early 2030s. FMC plans to unveil new herbicide sites of action, including a new corn and soybean one, in five to 10 years.

Read More: Corteve Agriscience Submits New Pasture Herbicide to EPA

20. New PPO Inhibitor Tech Is Coming 

BASF has a new PPO inhibitor (Group 14) technology that will control some current PPO-resistant weeds, says Rick Van Genderen, BASF global lead for soybean and corn seed and trait strategy. Slated for the early 2020s, one of these herbicides will initially be launched for preplant and burndown situations. The trait technology that allows it to be used postemergence is slated for the early 2030s. 

“This uses a different binding site that differs from current PPO inhibitor herbicides like Valor or Flexstar,” says Young. 

21. Throw Some Shade

“We don’t have such heavy-hitting herbicides that you can forget about cultural control,” says Purdue’s Young. For example, narrowing soybean rows from 30-inch to 15-inch rows results in early canopying that can curtail the growth of Palmer amaranth and waterhemp.

“Shade is the best herbicide there is,” says Beck’s Schwartz. 

22. Cover Weeds With Cover Crops

Cover crops can nix weeds by squelching sunlight to would-be germinating weeds, says Anita Dille, a Kansas State University (KSU) agronomist. Others can inhibit weed seed germination by releasing chemicals from roots or decaying residue. Cover crops can also smother and outcompete weeds.

A 2013-2014 KSU study showed a winter rye cover crop suppressed 94% to 96% of the winter annual marestail. Another perk: Cover crops also enhance efficacy of other weed-control tactics like herbicides. This slices selection pressure of practices used, which ultimately extends their time of effectiveness, she says.

23. Room for Wheat? 

Wheat’s been the cellar dweller of Midwestern crops, with economics even more dismal than corn or soybeans. Still, a major perk it has is breaking up weed cycles when rotated with corn and soybeans. 

“You see less resistance when you have multiple crops and change up your chemistries,” says Norder Supply’s Vinzant. 

24. Cultivation Comeback

“They’re still few and far between, but some farmers in this area have gone back to cultivation,” says Vinzant. 

One drawback is finding a cultivator and then finding an operator to run it. Still, no weed yet resists cold, hard steel. 

Another perk: Guidance systems make cultivation much easier today than in your father’s and grandfather’s day, when a sore neck from looking back was the price paid for a weed-free field.

25. Paranoia Will Destroy Ya 

That is, if you’re a weed seed in the path of a seed destroyer. 

That’s the premise behind Australian companies that market technologies like the Seed Terminator or the Harrington Seed Destructor. These tools attach to a combine and then pulverize and grind weed seeds at harvest. 

They have potential to manage weeds like waterhemp that have not dropped all their seeds at harvest. Grinding some weeds like giant ragweed or giant foxtail at harvest won’t work well, since only 30% of those weed seeds remain with R8 (full maturity soybeans), says Joe Ege, an MU weed science graduate student who is researching the Seed Terminator. 

So far, these units are expensive. Prices for the Harrington Seed Destructor may range between $85,000 and $117,000, says Bob Hartzler, Iowa State University Extension weed specialist.

Long term, farmers also won’t catch a break when it comes to resistance, even with this tool.

“You will select for early shattering weeds,” says Mandy Bish, an MU weed scientist. She says the only surefire way to manage weeds is through an integrated approach using a number of tools.

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