Controlling herbicide-resistant weeds begins with weed seed bank management – Successful Farming


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Jim Schwartz was nervous.

Schwartz, who directs Beck’s Practical Farm Research, was meeting with BASF executives with a message he thought ran contrary to one of BASF’s main businesses.

“I told them we were trying to figure out how to control weeds without opening a [herbicide] jug,” he says. “I thought that would go over as well as a porcupine in a balloon factory.”

It didn’t.

“They said, ‘You know, that’s interesting you say that. We think that’s important moving forward, too.’ ”

Herbicides have been and always will be part of weed management, says Scott Kay, vice president, U.S. crop agricultural solutions for BASF. Still, they aren’t the only component. Change is needed.

“We need to have a different mind-set, a different approach,” says Kay. That’s the thinking behind BASF’s Operation Weed Eradication, which stresses chemical, cultural, and mechanical means in an integrated weed management approach, he adds.

The $2.5 Billion Weed

Two pigweeds — Palmer amaranth and waterhemp — have prompted this switch. Waterhemp alone has caused farmers and industry to burn through $2.5 billion worth of chemistry since 1990, 
says Kay. 

Continual flushes throughout the growing season frustrate management of these pigweeds, says Aaron Hager, University of Illinois (U of I) Extension weed specialist. “There’s no residual herbicide that will last the entire growing season,” he says. “That’s why all these other things, such as cultural and mechanical means, come into play.”

Still, chin up.

“You can battle out-of-control populations by targeting the most sensitive life cycle stage — the seed,” says Hager. “Their seeds do not remain viable in the soil seed bank indefinitely.”

A 1997 to 2000 trial conducted at the 
U of I by graduate students and USDA-ARS scientists studied seed viability of a 1996 field with dense waterhemp populations. 

“The waterhemp was so thick that you literally could not see it was a corn crop,” says Hager. 

The research team measured seed viability from 1997 to 2000 in the top 6 centimeters of soil (2.36 inches) after waterhemp shed its seed in 1996. Seed viability levels in each year were as follows: 

1997: 39% 

1998: 28% 

1999: 10% 

2000: .004%

“If you limit seed production, waterhemp numbers will plummet,” says Hager. “Anything you can do to reduce seed deposition is a win.”

Throw Shade

Herbicides pack a
 powerful weed-killing punch.
 Still, none compare to a natural weed slayer that doesn’t come in a jug.

“One of my professors 
at Purdue, Dr. Merrill Ross, would tell us that the best 
herbicide is shade,” says 
Jim Schwartz, director of Beck’s Practical Farm Research. 

Two tools farmers can use to induce shade are cover crops and narrow rows. 

“Waterhemp needs sunlight to germinate, so having large amounts of cereal rye biomass helps prevent sunlight from reaching it,” says Mandy Bish, a University of Missouri Extension weed specialist. 

Iowa State University (ISU) weed scientists found that a cereal rye cover crop suppresses waterhemp 40% to 45% when terminated at anthesis (full flower) following “green planting” soybeans into the standing cover crop. This generated 3,500 to 4,500 pounds per acre of cereal rye biomass. 

“This is a level where you can get weed control benefits,” says Prashant Jha, an ISU Extension weed specialist. Besides waterhemp, a cereal rye cover crop also suppresses winter annuals, such as marestail, he adds. 

Cover crops can also aid postemergence herbicide performance. 

“The weeds we targeted were smaller and had less density when we applied Liberty (Group 10), 2,4-D choline (Group 4), and dicamba (Group 4),” says Jha. 

Using a cover crop to control weeds isn’t foolproof, though. It works best on cleaner fields.

“If you have a field with extreme waterhemp densities, you will be disappointed,” says Bish. 

Narrow Rows

Planting in narrow rows further stifles weed growth through early canopy formation. Beck’s trials show waterhemp density declined from 5.9 plants per square foot in 30-inch rows to 4.0 plants per square foot for 15-inch rows, says Schwartz. 
Narrow rows also make soybean farmers more money. A multiple-location 10-year Beck’s trial showed soybeans planted at 100,000 plants per acre in 15-inch rows generated gross returns of $694 per acre. This eclipsed the $650 per acre for soybeans planted at the same population in 30-inch rows. •

Shocking Death

Kevin Bradley disappoints farmers who ask what herbicide they can apply to waterhemp towering over mid-summer soybeans.

“Nothing,” says Bradley, a University of Missouri (MU) Extension weed specialist. By then, only roguing waterhemp will nix its shedding seed for future infestations.

Enter The Weed Zapper. It’s akin to rope wick applicator technology that kills weeds by contact with a herbicide-soaked canvas. Rather than chemical, The Weed Zapper kills weeds by generating a capacity of 110,000 to 200,000 watts of electricity. Operators must be careful,  but company officials add the units are equipped with multiple safety sensors that stop electrical output when necessary.

The tractor-mounted Weed Zapper Annihilator units range in size with 10-foot- to 40-foot-wide booms. It will fit any row spacing and tractor tire width used by the operator. The self-propelled Weed Zapper Terminator units with 40- to 60-foot booms are run on 14-inch tires and can be hydraulically adjusted to the desired row width.

The technology aims at both conventional and organic farmers, say Ben Kroeger, a company spokesman. MU weed scientists are testing it for zapping soybean weed escapes.

MU tests show control of 90% or more results for broadleaves that include waterhemp, giant ragweed, common ragweed, and marestail three days after treatment and at year’s end. It’s less effective on grass species. “It may be due to regrowth or that there is not as much surface area to contact,” says Haylee Schreier, an MU weed science graduate student.

Because grasses contain less water, additional contact via slower speed is necessary for control, say company officials. “A double pass application will increase effectiveness on grasses like johnsongrass, says Kroeger. “Foxtail and other grasses are usually killed in a single pass with a follow-up pass in two to three weeks.”

Weed Seed Viability

The unit also sliced nearly all seed viability of common ragweed and giant ragweed. Performance was not as good on waterhemp, but it still reduced 70% of waterhemp seed viability, says Bradley. It’s difficult to curtail all seed shed from pigweeds such as Palmer amaranth and waterhemp because some seeds can be viable six days after flowering, he adds.

Prices for the 2021 Annihilator series range from $42,000 for the smallest unit to $72,000 for the widest one. Terminator series prices range from $165,000 to $200,000. The firm has not announced 2022 prices yet, but material price spikes and fluctuating equipment costs will likely increase prices at least 10%, says Kroeger.

Increased effectiveness for conventional farmers would occur with 60- to 90-foot or wider units, Bradley believes.

“It’s not a home run, but the good news for us is it does a good job of killing waterhemp,” says Bradley.

Harvest Weed Seed Control

Combines harvest grain well.

Unfortunately, they also can be decent weed seeders, too. Seeds of late-season emerging weeds such as waterhemp and Palmer amaranth can exit the combine, germinate, and fuel future infestations.

Harvest weed seed control (HWSC) systems aim to change this. HWSC includes combine-mounted units that grind weed seeds exiting the combine. University researchers are testing units that include the Harrington Seed Destructor, the Seed Terminator, and the Redekop Seed Control Unit.

Not all seed shed at harvest exits the combine. In tests of the Seed Terminator, University of Missouri (MU) weed scientists found 27% of waterhemp seed escapes at harvest due to combine head shattering.

Of the remaining 73% of weed seeds, one-third exited the straw chopper without passing through the Seed Terminator. MU researchers also found weed seeds escaped into the grain tank.

Still, the Seed Terminator destroyed on average 95% of waterhemp seeds that passed through it in MU tests. Waterhemp densities in evaluated fields also decreased in subsequent years.

“Our results indicate continued use of the Seed Terminator will result in a decrease of waterhemp seed going back to the soil’s weed seed bank,” says Travis Winans, an MU weed science graduate student.

Torching Weeds

Propane isn’t just for heating your home or shop. It’s also a way to deter herbicide-resistant weed seeds from making it to the soil seed bank.

Although organic farmers have traditionally used propane-fueled flaming technology, more conventional farmers are adopting it because of herbicide-resistant weeds, says Mike Newland, Propane Education & Research Council director of agriculture business development. Hoods on a tractor-mounted frame shield row crops, such as corn and soybeans, from a unit’s burners. These units kill weeds by flames that rupture cell walls.

Flaming can control up to 90% of targeted weeds, Newland says. Compared with cultivation, no soil or placed fertilizer is disturbed.

Systems range from using handheld torches for building-site spot treatments to tractor-mounted 16-row units used on large row crop fields, he says.

Cost varies depending on the unit. Propane costs typically follow gasoline and diesel fuel prices. The council has awarded incentives to farmers wanting to transition to propane for uses like flaming.

“The technology is much more scientific than what it was 30 years ago,” Newland says. “There have been advances in hood designs and flaming angles that didn’t exist back then.”

Operation Weed Eradication

BASF’s Operation Weed Eradication program uses integrated steps to nix weeds. These include:

Cultural Practices

Tillage and cultivation

Crop rotation

Narrow rows and planting date

Cover crops and crop residue

Chemical Control

Multiple herbicide sites of action

Layered residual herbicides

Label rates to target small weeds

On-Farm Diligence

Hand weeding

Attention to weeds in or near field edges, ponds, and ditches

Seed destruction

Clean machinery and equipment

What’s Coming Up?

One tool that complements herbicides has its origins in a central Kansas shop. This mechanical option, along with other advances to apply herbicides more efficiently, will be featured in the December issue of Successful Farming magazine.

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