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.
Recent observations suggest marestail in Kansas will bolt in April throughout most of the state, so timing control in early spring before the end of March is highly 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 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. In addition to targeting smaller weeds, application of group 4 herbicides in March 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.
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 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.
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).
For more detailed information on post-emerge herbicide options go to www.cottonwood.ksu.edu to read this full article, or refer to the new 2022 Chemical Weed Control for Field Crops, Pastures, and Noncropland guide, they are available in your local county or district Extension Office or online at https://bookstore.ksre.ksu.edu/pubs/SRP1169.pdf
If you have any questions give me a call 785-628-9430 or email scampbel@ksu.edu anytime.
Stacy Campbell is an Agriculture and Natural Resources agent for Cottonwood Extension District. Email him at scampbel@ksu.edu or call the Hays office, 785-628-9430.
