Did you have trouble controlling marestail in soybeans this year?
Marestail — also known as horseweed — is on the rise in parts of South Dakota, especially in no-till fields, says Gerald Schaffer, South Dakota State University Extension weed specialist.
He says he received a lot of questions at the South Dakota State Fair about controlling marestail.
Marestail is native to the United States. It’s considered either a winter or summer annual, and it is often difficult to identify. In the Dakotas, most marestail populations will germinate in the fall and bolt in the spring. The first leaves of marestail have a broad, round end and have a whorled leaf arrangement that forms a rosette. Small plants may be purple or green during cool weather.
When it bolts in the spring, the leaves are alternate, hairy, 1 to 4 inches long, linear in shape and attached directly to the stem.
“Not letting marestail produce seed is of the utmost importance because they can produce up to 200,000 seeds per plant, and 20% to 91% of those seeds that germinate in the fall can survive through the winter,” he says.
Controlling marestail is typically not a problem in wheat and corn. But in soybeans, controlling it in the spring with burndown and pre-applications can be a challenge because the weed gets too big.
The most successful treatments for control of large marestail in Roundup Ready soybeans have been glyphosate tankmixes with FirstRate, Classic or Synchrony XP (if no ALS-resistance exists). Another option to help control marestail in fields with a history of marestail problems is to plant LibertyLink soybeans and use Liberty herbicide. Liberty can only be applied postemergence on LibertyLink soybeans.
A cost-effective fall burndown after soybean harvest and before a hard freeze could include dicamba (Clarity), glyphosate (Roundup) and 2,4-D or a mixture of those, but dicamba should be in the mixture, Schaffer says.
To help avoid further resistance in weed populations, make you don’t overuse certain herbicides, such as Liberty. Rotate crops and herbicide programs for best results.
For more information, contact Shaffer at 605-626-2870.
SDSU Extension provided information for this article.