'Horse' on the Loose

November 8, 2013 06:26 PM
'Horse' on the Loose

Marestail gallops through Midwest soybeans 

Marestail kicked up its weedy heels in many Midwest farmers’ soybean fields this past summer. Like the proverbial horse that runs out of the barn, marestail, or horseweed, was tough to catch and corral.

Aaron Hager, University of Illinois Extension weed scientist, says soybean growers in the state saw an abundance of marestail for a couple of reasons. For one, inclement weather conditions when spring burndown herbicides were applied minimized control results. Second, the spread of glyphosate-resistant populations is making marestail more difficult to stop in its tracks.

"Many farmers reported poor marestail control from herbicides applied prior to planting (primarily no-till soybeans), especially when these burndown applications contained only glyphosate or glyphosate and 2,4-D," Hager says.


Scientists say a fall burndown treatment for marestail is a good first step in your management program for this weed.

On the move. Glyphosate-resistant marestail has rapidly spread since it was first identified in 2000 in Delaware. Today, glyphosate-resistant populations are confirmed in 21 states including Illinois, Indiana, Iowa, Kansas, Michigan, Nebraska, Ohio and Wisconsin. Along with glyphosate, some marestail populations have developed resistance to other herbicide modes of action, includ­ing paraquat, atrazine, acetolactate synthase (ALS) inhibitors and diuron.

Biology has also contributed to the rapid spread of marestail, as it is able to germinate in spring, summer or even fall. From 14% to 84% of fall-emerging marestail plants typically survive until spring, according to the multi-univer­sity Extension publication, "Biology and Man­age­ment of Horseweed."

Furthermore, marestail plants produce an abundance of seed, up to 200,000 per plant. The individual seeds readily move from field to field, Hager adds, because the plant’s design is adapted to wind movement.

The potential cost of uncontrolled marestail populations was demonstrated in a 2010 study conducted by Mark Loux, Ohio State University Extension weed scientist. He found a range of soybean yields resulted under three different herbicide-application scenarios, with a 14-bu. spread between the low and high average yield:

  • 51 bu. per acre average yield where the burndown treatment failed to control emerged plants;
  • 57 bu. per acre average yield where the burndown was effective but no residual herbicide was used;
  • 65 bu. per acre average yield where the burndown was effective and residual herbicides were used.

Start this fall.
Weed scientist Bill Johnson, Purdue University Exten­sion, says his research shows that farmers need to use multiple steps to control marestail in soybeans, especially any herbicide-resistant species: "Two-shot burndown programs, which include either two shots in the spring or fall plus spring burndown treatments are needed in areas with a history of poor marestail control due to glyphosate and ALS-herbicide resistance."

Hager agrees and encourages farmers to start their marestail control practices this fall with an
application of 2,4-D (1-lb. acid equivalent per acre), which can be applied through late November.
Include additional herbicides in the tank mix, he says, to increase the winter annual species control.

Hager cautions that fields populated by marestail and treated with fall-applied herbicides won’t necessarily be free of the weed by the following spring.

"Scout fall-treated fields before spring planting and plan to use supplemental herbicides and tillage to control any existing marestail plants. Do not plant soybeans into an existing marestail population," he adds. "Apply residual herbicides close to planting time to control summer annual species, including spring-emerging marestail." 

You can e-mail Rhonda Brooks at rbrooks@farmjournal.com.

For more information on weed issues, from resistance to control measures to stewardship, visit www.FarmJournal.com/weed_warriors

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