Fall Weed Control

What's Your Plan for Weed Seeds Lurking in Harvested Fields?

Emily Unglesbee
By  Emily Unglesbee , DTN Staff Reporter
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Many winter annual weeds, like this young marestail plant, are starting to emerge across the Midwest. Do you have a plan to manage them? (DTN photo by Pamela Smith)

ROCKVILLE, Md. (DTN) -- This time of year, Illinois farmer Nathan Wentworth is on the phone to his chemical retailer as soon as the dust from the combine settles.

"If we're in the middle of October or later, as soon as a bean field is harvested, we're having it sprayed with herbicides," he said. "It's a core component of our weed control program, because we're minimum-till."

Fall burndown applications may be more important than usual this fall, after the challenging spring many growers experienced, noted Ohio State University Extension weed scientist Mark Loux.

"If you missed spring burndown or had prevent plant, there was probably an increased frequency of winter annual weeds going to seed -- I know there was more marestail," he noted.

Summer annual weeds are still plaguing some growers, too, added Wentworth, who farms in central Illinois near Warrensburg, where herbicide-resistant waterhemp escapes are a growing problem. "It's a frustrating sight, because you know you're just perpetuating the problem, but we're limited to how late we can spray them," he said.

Weed control isn't always the first thing that comes to mind during the harvest season, but fall burndowns and careful weed seed management can be integral to a successful 2020 growing season. Here are four top things to keep in mind this fall:


Most winter annual weeds -- chickweed, henbit, purple deadnettle -- go to seed in May or June, although marestail can hold onto its seeds into late summer.

Given the historic fieldwork and planting delays of 2019, some growers didn't to get into their fields to spray before midsummer, Loux noted. Growers in these situations should brace for a "potentially increased population" of these weeds, particularly with aggressive herbicide-resistant marestail populations, he said.

Fall burndowns can help growers control these weeds, which are emerging now and will overwinter and begin growing again in the early spring, when weather conditions can make it difficult to control them in a timely fashion.

Burning down these populations in the fall can keep wooly, green weed blankets from trapping moisture and encouraging insect infestations in the spring as well, Wentworth added.

The addition of fall herbicide applications to their minimum-till operation in Illinois has actually allowed them to drop one, sometimes two, herbicides from their spring applications, which saves time and money during tight spring planting windows, he said. "So those dollars per acre that you spend in the fall get rolled into your larger herbicide budget and can actually offset costs later in the season," Wentworth said.


While Loux has seen successful fall burndown applications creep into December, they do their best work when weeds are still actively growing. Generally, between the beginning of October through Thanksgiving is prime time for fall weed control, he noted.

Keep in mind that spraying herbicides on frozen ground is usually a label violation, so watch temperatures and soil conditions carefully.

Adding metribuzin, dicamba or glyphosate to 2,4-D are common and cost-effective two-way fall burndown mixes that won't lock a grower into a certain crop the following season, Loux noted.

See more details on chemical options from Loux here: https://agcrops.osu.edu/…

Growers can include other products with residual activity to help lengthen control in the fall, but they should consult plant-back restrictions carefully. And Loux cautioned against adding residual products with the expectation that they will always maintain their efficacy through the winter and control weeds in the spring. "They often peter out before that," he said.

Wentworth credited his fall herbicide program with suppressing some early weed emergence in the spring, but agreed that winter conditions play a key role, namely, that the increased biological activity in the soil during warmer winters can allow residual products to break down before springtime.


The use of fall-planted cover crops to control and suppress weeds is a growing practice. Cereal rye is a popular choice, based on its ability to produce a sizeable biomass in both the fall and spring, Loux noted. "If you get a good stand, it can and does replace fall herbicides," he said. "It definitely helps with marestail."

Density is also key -- a higher rye seeding rate can provide more suppression than a thinner one, depending on when it is planted, he added.

Getting into fields after harvest to seed the rye and get a good stand is essential to the success of this method, Wentworth noted. His operation had one very successful year when a September-sown cereal rye cover suppressed weed emergence all fall and -- after it was terminated in the spring -- suppressed weeds deep into the summer, saving on multiple herbicide passes.

Subsequent falls proved too wet, cold or delayed to replicate that experience, so fall herbicides remain a staple on their operation for now, Wentworth added. "I think there's a place for it on some operations, though," he said. "If you can consistently get it well enough established in the fall, a thick blanket of cereal rye will suppress winter annual weeds and do a nice job on waterhemp."


Don't forget about the summer annual weed escapes, like waterhemp, Palmer amaranth, and ragweed, which still lurk in many fields. These weed escapes will either deposit their seeds into the field's weed seedbank or, worse yet, get trapped in the combine and then blow out in other fields as harvest progresses, Loux said.

An Iowa State University study found that waterhemp escapes were still holding onto 80% of their weed seed by Oct. 15, so Midwest combines have the potential to spread a lot of seed, ISU Extension weed scientist Bob Hartzler noted in a recent publication.

See it here: https://crops.extension.iastate.edu/…

Consider leaving heavily infested fields for last, to limit spreading that seed, Loux suggested. Wentworth said on their operation, they take care to blow combines off and run them empty to try to shake off these tiny invaders.

For more information on managing weed-infested fields during harvest, see this article from Iowa State University: https://crops.extension.iastate.edu/…

Emily Unglesbee can be reached at Emily.unglesbee@dtn.com

Follow her on Twitter at @Emily_Unglesbee


Emily Unglesbee