Controlling marestail in soybeans has been a big challenge for Kansas no-till producers. Last season, because of the dry fall and spring, there were generally much fewer marestail that germinated and survived the winter. From scouting fields this winter it appears there will be a more significant population of marestail than last year and it will need to be addressed. Because soybeans are generally planted later in the season, and marestail generally germinates in the fall or early spring, application timing and weed size are critical factors to successful control.
In the early spring, using a growth regulator herbicide like 2,4-D or dicamba is an inexpensive and effective option to control rosette marestail. In addition, using a herbicide with residual control of marestail helps with weeds that germinate between the fall and early spring burndown and soybean planting. Products that include Canopy EX, Classic, FirstRate, Sharpen, or Valor can help provide residual control against several broadleaf species including marestail. Consult the herbicide labels for the required preplant intervals prior to planting soybeans.
As soybean planting nears, marestail control can become difficult because plants will have bolted and be considerably larger. Herbicides to apply as a burndown prior to planting include tank mixes of glyphosate with FirstRate, Classic, Sharpen, Optill, or 2,4-D. Be very careful to follow label directions when using 2,4-D prior to soybean planting because the plant-back restriction with these herbicides ahead of soybean can be from 7-30 days. Sharpen is a relatively new herbicide that has provided good marestail control and can be applied any time before soybean emergence. Maximize marestail control by applying Sharpen in combination with methylated seed oil and at spray volumes of 15 gallons per acre or more.
One additional herbicide to consider as a rescue burndown application to control bolting marestail prior to soybean planting is Liberty. Although, it would be better to control marestail at an earlier stage of growth, Liberty has been one of the most effective herbicides to control bolting marestail. Liberty also has broad spectrum non-selective activity on other broadleaf and grass species if treated at a young growth stage. Liberty, is primarily a contact herbicide, so a spray volume of 15 gpa or greater generally provides the most consistent weed control. Liberty tends to work best under higher humidity and warm sunny conditions at application.
Controlling marestail in the growing soybean crop can be the biggest challenge for producers. Glyphosate alone is often not effective on larger or glyphosate-resistant marestail. The most successful treatments for large marestail in Roundup Ready soybeans have been with combinations of glyphosate + FirstRate, glyphosate + Classic, or glyphosate + Synchrony. Another option to control marestail in soybean is to plant Liberty Link soybeans and use Liberty herbicide. It is important to remember that Liberty can only be applied postemergence on Liberty Link soybeans.