Manage marestail and henbit this fall

Farmers who want to keep marestail and other winter annuals like henbit and purple dead nettle from overrunning their 2021 fields can get a head start this fall. 

“Marestail is much easier to control in the fall compared with the spring or in-season,” says John Pawlak, Valent U.S.A. senior project development manager. Fall herbicide applications can help nix a spring salad bowl of weeds from forming and also erase plant-back restrictions that accompany spring burndown treatments. 

“It not only reduces weed density but also weed seed production,” says Jake Hoxmeier, a Pioneer field agronomist. “It’s also nice to come back into the spring [with growing-season herbicides] and not be overwhelmed by heavy weed growth.”  

Meanwhile, no weed cover means soils warm up quicker in the spring, which helps ensure earlier planting, adds Pawlak. 

“One problem we have is that we [in Illinois] are big on applying anhydrous ammonia in the fall,” says Aaron Hager, University of Illinois Extension weed specialist. “Most farmers and custom applicators focus on that rather than winter annuals.”

Still, time often exists for fall herbicide treatments even after farmers apply anhydrous ammonia. “Most years, there can be good success in controlling winter annuals with applications made up to Thanksgiving,” Hager adds. 

Winter Annual Damage

Winter annuals may not garner the attention of weeds like waterhemp and Palmer amaranth, but they still can curtail crop production. Common chickweed can form a dense mat that shades the soil and delays field drying, causing planting delays. Winter annuals also provide a haven for black cutworm to lay eggs that lead to hatching larvae, which damage corn after emergence. 

Ditto for soybean cyst nematode (SCN). Although no summer annuals like waterhemp host SCN, it’s a different story with winter annuals such as henbit and field pennycress, which can support significant SCN reproduction levels. 

“They don’t necessarily attract soybean cyst nematode because it can’t move very well on its own,” says Greg Tylka, an Iowa State University Extension nematologist. Once SCN is in the field, though, winter annual weeds can be hosts for the pests. “They complicate the problem by providing food for the nematode, even if farmers grow a nonhost crop like corn,” says Tylka. SCN numbers will continue to rise, even in the alternative crop year in a corn-soybean rotation. 

“The earlier we control them, the better off we are during the growing season,” adds Hager. 

More Options in Fall

Since many marestail populations now resist glyphosate, fall applications of that herbicide will be no more effective than spring or summer ones. However, 2,4-D works well as an inexpensive fall application, says Hager. It also does not have the seven- to 30-day plant-back restrictions for soybeans and five- to seven-day ones for corn that accompany spring 2,4-D applications. 

Farmers can also use higher 2,4-D rates in the fall, which can lead to more effective control of marestail and other winter annuals, says Hager. In soybeans, spring burndown applications can only use 1 pint of 2,4-D, and seven days must pass before planting can commence, says Hager. Since planting isn’t an issue in the fall, farmers can hike 2,4-D rates up to 1 quart per acre. 

“If you apply 2,4-D a week or two after corn has been shelled in mid- to late September, and winter annuals have not yet fully emerged in mid- to late October, you may want to add residuals like metribuzin [Group 5, Sencor] or chlorimuron [Group 2, Classic],” says Hager. 

If weather thwarts fall application plans, farmers can always implement early-spring herbicide applications instead, says Ron Geis, a Corteva Agriscience market development specialist. 

“There’s no disadvantage to making a fall application plan,” says Geis. “We may not be able to execute the plan, but you can just push it back into spring if you can’t. What you gain in the fall is easier control of weeds because they’re smaller.”

Geographic differences can come into play, says Geis. In Iowa, for example, Interstate-80 is a transition zone. 

“North of I-80, we can usually apply a fall burndown with residual activity that can give us a clean planting season and several weeks of residual protection before a postemergence application is made, assuming it’s a no-till environment,” he says. “All we have changed is the timing.”

However, a second preemergence application south of that line will be needed, due to weed growth potential spurred by an earlier arriving spring, he says.  

Farmers need to consider carryover concerns when selecting residual products, reminds Hager. Many herbicides are not labeled for fall application, so check labels first. For example, atrazine is widely used before and after corn emergence, but is not labeled for fall application. Some herbicides approved for fall application have application timing and geographical restrictions, says Hager. 

The problem is complicated when Midwestern farmers plant soybeans early in April in the Midwest, Hager says. Although early soybean planting can maximize yield potential, early planting can make soybeans more prone to herbicide carryover than later planted ones, says Hager. 

No Guarantee

Weed-free spring fields generated by fall treatments may not always result. 

“Delays in spring fieldwork may allow the fields to green up before the crop can be planted,” says Hager. Another drawback is that summer annuals like common lambsquarters and smartweed can emerge sooner than if winter annuals were still present.

Fall applications won’t curb summer annual headache weeds like waterhemp, either.

That’s why farmers need to remember the merits of spring preemergence residual herbicides, says Lance Tarochione, an Asgrow/DeKalb technical agronomist. That especially was true in a wet year like 2019, when rampant rainfall prevented timely postemergence applications. 

“That was a great year to have residual preemergence products down, as fields with no residual treatments turned into a jungle,” he says.

Dandelions on steroids

Thunderstorms aren’t the only thing that move from west to east. Farmers in Montana and western North Dakota are seeing a similar phenomenon with the winter annual narrowleaf hawksbeard. 

“I think of it as a dandelion on steroids,” says Bridgette Readel, Corteva Agriscience market development specialist. “You think it’s probably just a dandelion, and it will be easy to control with clopyralid [Stinger] or 2,4-D [both Group 4 herbicides], and then it gets big fast and shoots up multiple flower heads that stand above the crop canopy.”

So far, it’s been limited to cereals and pulse crops like lentils and field peas. Narrowleaf hawksbeard emerged in Saskatchewan and also through Montana and into western North Dakota. Readel expects it to keep migrating eastward this year in North Dakota. 

So far, herbicides have limited activity on it, she says. Fall treatments are an option, but consider crop restrictions when choosing herbicides, say North Dakota State University (NDSU) weed scientists. NDSU guidelines for fall treatments include the following:

  • Glyphosate + Express (or Panoflex) +/- 2,4-D 
  • Glyphosate + Sharpen +/- 2,4-D 
  • Glyphosate + 2,4-D 
  • Glyphosate + dicamba 
  • Glyphosate + Valor +/- 2,4-D 
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