Controlling marestail in soybeans continues to be a big challenge for Kansas no-till producers. Application timing and weed size are critical factors for successful control of this weed that germinates in the fall or early spring. Research has shown that up to 80% of marestail can die over the winter as a result of cold temperatures and/or lack of adequate moisture. In addition, a well-established cover crop in the fall can further reduce marestail establishment and survival and often is quite effective for marestail control. However, marestail that does survive is often robust and can be difficult to control with herbicides, especially later in the spring. Herbicide options are also limited by widespread resistance to glyphosate and/or ALS-inhibiting (group 2) herbicides in marestail.
Figure 1. Glyphosate-resistant marestail in soybeans. Photo by Dallas Peterson, K-State Research and Extension.
Early spring options
In the early spring, using a Group 4 (growth regulator) herbicide such as 2,4-D and/or dicamba is an inexpensive and effective option to control rosette marestail (Figure 2, left). Dicamba provides better marestail control than 2,4-D and will also provide some residual control, especially at higher use rates. Haluxifen (Elevore) is a newer group 4 herbicide that can provide similar marestail control to dicamba. Making these applications in March generally allows adequate time ahead of planting soybeans to meet required pre-plant intervals, but more importantly, spraying weeds before they bolt (Figure 2, right) will result in greater control , In general, marestail in Kansas will bolt in April, so now is the time for these applications.
Using herbicides with longer residual helps control weeds that germinate between early spring applications t and soybean planting. Products that include chlorimuron (Classic, Canopy), cloransulam (FirstRate), flumioxazin (Valor, others), saflufenacil (Sharpen, Optill, Verdict), or metribuzin, can help provide residual control against several broadleaf species, including marestail. However, it is very important to consult and follow the herbicide label guidelines for the required pre-plant intervals prior to planting soybeans as well as the proper rate for your soil. Also keep in mind that resistance may reduce the effectiveness of ALS-inhibiting herbicides such as chlorimuron and cloransulam.
Figure 2. Marestail in the rosette growth stage (left photo) versus bolted (right photo). Photos by Sarah Lancaster, K-State Research and Extension.
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 2,4-D, and the residual products listed above.
Be very careful to follow label directions regarding plant-back restriction when applying 2,4-D or dicamba ahead of soybean varieties that are not resistant to the herbicide you use. Enlist soybean varieties have no plant-back restriction for Enlist One or Enlist Duo and Xtend varieties have no plant-back restriction for XtendiMax, Engenia, or Tavium. However, non-resistant varieties have plant-back restrictions that range from 0 to 30 days depending on the herbicide rate and formulation, as well as soybean variety, precipitation, and geography.
One additional herbicide to consider as a rescue burndown application to control bolting marestail prior to soybean planting is glufosinate (Liberty and others). Although, it would be better to control marestail at an earlier stage of growth, glufosinate has been one of the most effective herbicides to control bolting marestail. Glufosinate also has broad spectrum non-selective activity on other broadleaf and grass species if treated at a young growth stage. Glufosinate is primarily a contact herbicide, so a spray volume of 15 gallons per acre or greater generally provides the most consistent weed control. Glufosinate 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, especially in soybeans that are not resistant to 2,4-D or dicamba. If Roundup Ready 2 Xtend or XtendFlex soybeans are planted, Xtendimax and Engenia should be some of the most effective herbicides for post-emergence control of marestail in soybeans. Remember that XtendiMax and Engenia can only be applied to Xtend soybeans. Similarly, Enlist One or Enlist Duo will be effective control options in Enlist E3 soybeans. One final post-emergence option to consider is glufosinate. Glufosinate resistance is in Liberty Link, Enlist E3, and XtendFlex varieties. However, glufosinate-containing herbicides cannot be tankmixed with XtendiMax, Engenia, or Tavium.
For more detailed information, see the “2023 Chemical Weed Control for Field Crops, Pastures, and Noncropland” guide available online at https://bookstore.ksre.ksu.edu/pubs/SRP1176.pdf or check with your local K-State Research and Extension office for a paper copy.
Sarah Lancaster, Weed Management Specialist