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Forge ahead with fall weed control

Call it marestail or horseweed, this cantankerous weed is mighty tough to corral. Fall control through herbicides, though, can help farmers tame it and other winter annuals before planting starts next spring. 

Agronomists and weed scientists say fall herbicide applications help farmers access historically better weather and field conditions to help attain clean fields at spring planting. Nixing fall growth of henbit, purple deadnettle, and other hosts for soybean cyst nematode (SCN) can also help reduce SCN problems next year.

Fall weed management benefits abound. They include:

• More suitable days for herbicide application.

“Like other Midwestern states, Missouri typically sees fewer suitable workdays in the spring than in the fall,” says Kevin Bradley, University of Missouri Extension weed scientist. “If you can’t apply herbicide timely in the spring, the field turns into a green weedy mat.” He adds that it’s easier to control smaller weeds in the fall. 

“It [fall weed control] will likely be needed if there have been weed issues early in the previous growing season,” adds Aaron Hager, University of Illinois Extension weed specialist. “Depending on late summer field conditions, it’s a balancing act to manage winter annuals like marestail. We assume many fields won’t have fall tillage. Growers need to watch them closely for winter annuals.”

29523 marestail

Marestail is best controlled in its early stages.

• Wider workload windows. 

Fall herbicide applications control weeds while also reducing spring workload, Bradley says. “During planting, you won’t have to also deal with weeds like marestail,” he adds. 

• Reduced stubborn winter annuals on no-till and reduced-till fields. 

Marestail can dominate many no-till or minimum-till fields, says Ron Geis, Corteva technical service manager in northern Iowa.

“Think of the way crops behave,” he says. “In the fall, they take the nutrition from leaves and pull it down into the root systems. If we can have an herbicide accompany that down into the roots, control is easier than in the spring. 

“In the spring, weed nutrients are pushing up, while we’re trying to push herbicide down,” he adds. “With fall herbicide applications, we’re right in harmony with the plant cycle.”

29523 henbit

Henbit is another winter annual that can cause springtime planting problems if not controlled. Photo courtesy of the University of Illinois.

Cover Crops

Cover crops can also bolster herbicide applications for managing weeds. Iowa State University (ISU) and other university research shows cereal rye is an effective complementary strategy to manage resistant marestail and waterhemp seed banks in the following soybean crop. 

“This cultural strategy should be considered as part of the fall weed management program to reduce sole reliance on herbicides and reduce selection pressure for resistance development,” says Prashant Jha, ISU Extension weeds specialist.

• Aid for early-planted soybeans. 

Fall herbicide applications also help nurse the trend of earlier planted soybeans in the Midwest. 

“A clean seedbed is critical for successful crop emergence, says Nick Hustedde, FMC technical service manager in Illinois and Indiana. “Fall herbicide treatments facilitate improved soil conditions for early planting dates.” 

John Pawlak, Valent USA senior product development manager, also advocates for fall application. 

“With a fall application, growers can minimize weed growth in the spring, allowing the soil to warm up and dry faster for earlier planting,” he says. “Fall weed management pays dividends when growers want to get their crops planted early in the spring and not worry about heavy weed pressure. Spring burndown programs take time to apply. Then there’s the wait for the weeds to die. There’s a wait for the soil to dry and warm up.” 

Want proof? 

“Data from Missouri and other universities shows the removal of winter annual weeds through residual fall herbicide applications increased soil temperatures by as much as 5 degrees [F] in corn experiments and as much as 8 degrees [F] in soybean experiments,” Bradley says.

29523 biennial wormwood

Biennial wormwood is a new problem for Upper Midwest farmers.

• Water conservation.

Weeds rob soil of water, says Ryan Bryant-Schlobohm, technical services manager for UPL. 

“Weeds are opportunistic,” he says. “Although drought has been a major issue across key geographies this summer, recent rains will allow for a wide range of weeds to germinate. Many of the most troublesome weeds are easiest to control before we even see them, and a preemergence herbicide is one of the strongest tools in battling weeds.”

Forget About Pigweeds

Most crop specialists say fall spraying for pigweeds, such as waterhemp or Palmer amaranth, is a waste of resources. 

“We don’t recommend fall applications to control summer annuals,” says Aaron Hager, University of Illinois Extension weeds specialist. “You can’t put on enough material in the fall to control pigweed. It’s not that good of an investment.” 

However, pigweed escapes in the fall may require spot sprays or manually removing the weeds to prevent seeds from overwintering. 

“A single waterhemp plant left in the field this fall can contribute thousands of seeds to the soil seed bank for infestations in 2023,” says Prashant Jha, Iowa State University Extension weeds specialist. “Hand pull and remove waterhemp survivors from the field to prevent the spread of herbicide resistance.”

29523 waterhemp

Except for spot sprays on escapes, save your money by skipping fall spraying of pigweeds such as waterhemp.

Don’t Till

Geis advises growers not to till after fall herbicide applications. “Once you have applied a residual [herbicide] and Mother Nature activates it with moisture, tillage is usually an enemy, not a friend,” he warns. “Tillage can disrupt the whole weed control zone. You will see weed escapes.

“We need to rely more on residuals,” he adds. “These herbicides need to be in the soil where they’re ready to work. Fall timing is assurance that is going to happen. It gives us five to six months to receive the activating moisture to get chemicals placed in the soil to handle spring-germinating weeds.”

Hager notes that skeptical farmers may consider fall weed control on part of their ground. “See for yourself if fall applications can help you get a head start into next spring,” he says. 

Of course, weather will dictate whether fall herbicide programs are successful. If cold, damp weather persists and transforms into early snow, fall weed control plans will likely yield to early spring action. 

A wealth of regional weed management information — from Extension specialists, crop consultants, and supplier technical experts — along with sufficient herbicide products support fall weed management to keep fields free from marestail, henbit, purple deadnettle, and other weeds when the first seeds are sown in 2023.

Corral horseweed now, not next spring.

Other Weed Worries

Glyphosate-resistant kochia and Italian ryegrass should also be confronted in the fall, crop specialists say. A newcomer, biennial wormwood, also needs watching. 

Kochia should be controlled in the fall like a winter weed, says John Pawlak, Valent USA senior product development manager. “Valor and Fierce brands provide excellent residual control,” he says. “Italian ryegrass is also a growing issue that can be controlled in the fall with several residuals, including Fierce brands.” 

“Kochia is moving west to east in Iowa,” adds Ron Geis, Corteva technical service manager in northern Iowa. “It will germinate through a wide window. If it germinates in mid-March and you plant in mid-May, the weed has an advantage over the crop, so it’s important to start clean. If you apply residuals in the fall, they’re already in the soil to control kochia when it germinates in early spring.” 

Alfalfa can double as a weed if a field is being rotated into a row crop, Geis says. 

“Apply herbicide to actively growing weeds or alfalfa,” he adds. “If it’s just after the last cutting, you’ll need a little regrowth to have more of a target above ground. That will help move more chemical below ground.”

Biennial Wormwood

Biennial wormwood, which often resembles ragweed, is more of a menace in northern parts of the Midwest. Atrazine; Stinger; dicamba; 2,4-D; and glyphosate all provide postemergence control when applied to weeds under 3 inches tall,” says Pawlak. 

“Residual options are limited, but include products containing flumioxazin [Group 14, such as Valor],” he says. 

Biennial wormwood naturally tolerates many soil and postemergence herbicides, including ALS inhibitors [Group 2, such as Pursuit], according to Nick Hustedde, FMC technical service manager in Illinois and Indiana. 

“When managing this weed postemergence, apply herbicide before bolting,” he says. “Including glyphosate plus 2,4-D or dicamba provides foliar activity on this species.” 

Ryan Bryant-Schlobohm, technical services manager for UPL, recommends growers use an herbicide containing sulfentrazone and metribuzin groups, such as Preview 2.1 SC, ahead of soybean planting. Sulfentrazone and metribuzin are two active ingredients North Dakota State University has documented for providing good preemergence control of biennial wormwood.” 

Yet another troublesome weed in eastern Iowa is wild cucumber, says Geis. “When corn is at the brown silk stage, there has been in-season success in treating it with 2,4-D, or glyphosate, or your early-season residuals,” he says. “Treatments should be made up to the field edge, as most wild cucumber germinates outside of the field near the river or creek.” 

For henbit and other winter annual control, crop specialists agree that producers should consider applying glyphosate and/or auxin herbicides and possibly residuals. 

Glyphosate should work in controlling these and other winter annuals, says Aaron Hager, University of Illinois Extension weeds specialist. “If there’s a lot of marestail, you’ll have to work in a tank mix of either 2,4-D, or dicamba.”

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