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.
Early spring options -recent observations suggest marestail in Kansas will bolt in April throughout most of the state, so timing control before the end of March is recommended. 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. Dicamba has provided 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. In addition to targeting smaller weeds, application of group 4 herbicides also generally allows adequate time ahead of planting soybeans to meet required pre-plant intervals.
Using herbicides with longer residual helps control weeds that germinate between treatment 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.
Pre-plant options - 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 group 4 herbicides ahead of soybean, which can range from 7-30 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.
Post-emergence options - controlling marestail in the growing soybean crop can be the biggest challenge for producers, especially in soybeans without herbicide-resistant traits or in glyphosate-resistant soybeans (if marestail is glyphosate resistant). The most successful treatments for large marestail in Roundup Ready soybeans have been tank-mixes of glyphosate with herbicides containing chlorimuron or cloransulam. However, marestail may also be ALS-resistant, and thus not controlled by those herbicides either.
If Roundup Ready 2 Xtend or XtendFlex soybeans are planted, Xtendimax, FeXapan, and Engenia should be some of the most effective herbicides for post-emergence control of marestail in soybeans. Remember that Xtendimax, FeXapan, 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.
Stacy Campbell is an Agriculture and Natural Resources agent for Cottonwood Extension District. Email him at firstname.lastname@example.org or call the Hays office, 785-628-9430.