No-till crop production in South Dakota is on the rise and new weed control measures are emerging. Some producers, mostly in soybeans this year, have had difficulty controlling marestail (also known as horseweed). Marestail is a native plant to the United States, and is considered either a winter or summer annual that is often difficult to identify.
In the Dakotas, most marestail populations will germinate in the fall and bolt in the spring (winter annual). 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.
Marestail bolts in the spring, and leaves are alternate, hairy, 1 to 4 inches long, linear in shape and attached directly to 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 percent of those seeds that germinate in the fall can survive through the winter.
A cost-effective fall burndown after soybean harvest and before a hard freeze could include dicamba (Clarity), glyphosate (Roundup) and 2,4D or a mixture of those, but dicamba should be in the mixture. Controlling marestail in the spring via burndown/pre applications can be the biggest challenge for producers. 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 Liberty-Link soybeans and use Liberty herbicide. Remember that Liberty can only be applied postemergence on Liberty-Link soybeans.
To help avoid further resistance in weed populations on your farm make sure not to over use curtain herbicides such as Liberty. This can be avoided by rotating crops and herbicide programs.
For any questions on marestail control or weeds in general please contact Gared Shaffer at 605.626.2870.