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Why was marestail so difficult to control this year in soybeans?

Oct 24, 2013

Question: I was really disappointed in my marestail control in soybeans this season, using a combination of glyphosate and 2,4-D.  I’d appreciate any advice you can offer for next year.

Answer:  You weren’t alone.  A lot of farmers had the same issue. 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 in the U.S. is making marestail more difficult to stop in its tracks.

Furthermore, marestail plants can produce an abundance of seed, up to 200,000 per plant. The individual seeds spread readily from field-to-field, Hager notes, because the plant’s design is adapted to wind movement. "If you have them in one field, chances are if they were able to make seed they’ll move within that field or out of that field to infest other geography or fields that perhaps never had the problem before," he says.

Bill Johnson, Purdue University Extension weed scientist, says his research during the past two years shows that farmers need to use multiple steps to control marestail in soybeans, especially 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," notes Johnson.

Hager agrees and encourages farmers to start their marestail control practices this fall with an application of 2,4-D (1.0 lb. acid equivalent per acre), which can be applied through late November. Additional herbicides can be included in the tankmix, he says, to increase the number of winter annual species controlled.

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


A Geographical Look at Weed Problems

Some weeds are prevalent in only certain areas of the country, while others seem to be everywhere. More than 1,500 farmers and ranchers shared their views in last week’s Farm Journal Pulse. The question was: What is your most significant weed problem? Waterhemp tops the list with 30% of respondents saying it was a problem. Marestail/horseweed was a close second with 22% of the votes.


Control Winter Weeds for Better Crops, Earlier Planting

Weather cycles of cold temperatures, abundant rain and short spurts of warmer, sunny days have allowed weeds such as chickweed, purple deadnettle, henbit, wild garlic, dandelions and marestail to flourish in the last few weeks.


Strategies to stay ahead of glyphosate resistance

Marestail is confirmed resistant to glyphosate, and Palmer pigweed also appears to be resistant. 


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