I realize in Ellis County, we have few acres of soybeans grown most years. But since this newspaper reaches folks in other counties, I thought it would be worth writing about controlling a difficult weed in soybeans.

Controlling marestail in soybeans continues to be a big challenge for Kansas no-till producers. Because soybeans are usually planted later in the season, and marestail generally germinates in the fall or early spring, application timing and weed size are critical factors for successful control. 

Mother Nature is good at controlling some marestail throughout the winter. Research has shown up to 80 percent of marestail can die in the winter as a result of cold temperatures.  The marestail that do survive often are robust and can be difficult to control with herbicides. However, there are still a number of herbicides that do a good job of managing the surviving rosettes.

In the early spring, using a growth regulator herbicide such as 2,4-D or dicamba is an inexpensive and effective option to control rosette marestail. Dicamba has provided better marestail control than 2,4-D and also will provide some residual control, especially at higher-use rates. A combination of the two will give broader spectrum weed control than either one alone. Recent observations in Kansas suggests marestail will bolt in April throughout most of the state, so timing control before the end of March is recommended.  Application of dicamba and 2,4-D in March also generally allows adequate time ahead of planting soybeans to meet required preplant intervals.

Using herbicides with longer residual helps control weeds that germinate between treatment and soybean planting. Products include Canopy EX, Autumn Super, Classic, FirstRate, Sharpen, metribuzin or Valor can help provide residual control against several broadleaf species, including marestail. However, it is important to consult and follow the herbicide label guidelines for the required preplant intervals prior to planting soybeans.

As soybean planting nears, existing marestail plants can become difficult to control 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 careful to follow label directions when using 2,4-D prior to soybean planting. The plant-back restriction ahead of soybean can range from seven to 30 days depending on rate and formulation. Sharpen generally provides good marestail control and can be applied any time before soybean emergence. However, it is still most effective if applied before marestail starts to bolt, in a tank-mix with other herbicides, when used with methylated seed oil, and at spray volumes of 15 gallons per acre or more. Elevore is a new herbicide that has provided similar marestail control to dicamba, but needs to be applied at least 14 days prior to planting.

Preplant restrictions for dicamba products such as Clarity, Banvel and others range from 14 to 30 days depending on product, application rate, rainfall amounts and geography. However, with the introduction of Xtend soybeans, the new dicamba products Xtendimax, FeXapan and Engenia have no preplant interval restrictions applied ahead of Xtend soybeans and should be some of the more effective treatments for marestail control in that scenario. Xtendimax, FeXapan and Engenia are still most effective prior to bolting.

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 gallons per acre 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 often is not effective on larger plants or glyphosate-resistant marestail. The most successful treatments for large marestail in Roundup Ready soybeans have been tank-mixes of glyphosate plus FirstRate, glyphosate plus Classic, or glyphosate plus Synchrony. However, some marestail also might be ALS-resistant, and thus not controlled by those herbicides either. If Xtend soybeans are planted, Xtendimax, FeXapan and Engenia should be some of the most effective herbicides for postemergence control of marestail in soybeans. Remember that Xtendimax, FeXapan and Engenia only can be applied to Xtend soybeans.

Another postemergence option to control marestail in soybeans is to plant Liberty Link soybeans and use Liberty herbicide. It is important to remember Liberty only can be applied postemergence on Liberty Link soybeans.

• Information provided by Dallas Peterson, weed management specialist.

Stacy Campbell is a Kansas State Research and Extension agent in Hays for the Cottonwood Extension District Office.