Fall Marestail Management in Soybean

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Marestail is a prevalent annual weed that can germinate in the spring, summer, or fall and can produce 200,000 seeds per plant with 80% of the seeds capable of germinating as soon as the plant is mature.1 The size and stage of growth at the time of herbicide application can have a significant influence on efficacy. Marestail is most susceptible to control when it is in the rosette stage of growth and less than 4 to 6 inches tall, but control is more difficult once it bolts in the spring. Because of the extended emergence time of marestail and limited number of postemergence (POST) herbicide options in soybean, it is important to use burndown, preplant, and preemergence (PRE) herbicides effectively.

With fall herbicide application time upon us, marestail control is on many growers’ minds. Those who planted Roundup Ready 2 Xtend® soybeans this season may be wondering whether the spring dicamba burndown was effective enough on overwintered marestail to reduce the need for fall-applied herbicides. While this may have been true in some cases, fall-applied herbicides should still be considered an important component of the weed control program in Roundup Ready 2 Xtend® soybeans to help minimize the potential for challenges in spring marestail control. Controlling marestail that emerge in late summer and fall can result in weed-free fields so that the spring burndown will be left controlling only the small, spring-emerged marestail.

According to Ohio State University, dicamba has proven more effective than 2,4-D on marestail in the spring, and has an important place in burndown programs to help control this weed.2 OSU research has shown:

  • Use of dicamba in burndown treatments prior to early May resulted in effective control of small emerged marestail; however, application beyond early May to taller, older marestail may result in reduced control. Many of these treatments also included residual herbicides and glyphosate, which may influence the activity of dicamba on larger marestail.2
  • When spring burndown was applied early enough and included comprehensive residuals to control the later-emerging marestail, there was little need for POST dicamba applications for marestail.

Fall herbicide applications will not eliminate the need for a residual herbicide program near or at planting, nor will they provide in-season control of summer annual weeds. However, fall conditions are generally more favorable for control of winter annual weeds than early spring because of the smaller weed size and more suitable days for herbicide applications.3 Furthermore, there are also agronomic benefits to consider:

  • The weed-free seedbed created by fall herbicide application results in better conditions for tillage and planting.
  • Controlling winter annuals in the fall can also increase soil temperature and soil moisture at planting.3
  • Finally, winter annual weeds are capable of removing available nitrogen (N) from the soil. Therefore, a delay in removing winter annual weed infestations until spring may result in reduced N uptake in developing plants.3

Sources: 1 Loux, M., Stachler, J., Johnson, B., Nice, G., Davis, V., and Norby, D. 2006. Biology and management of horseweed. GWC-9. https://www.extension.purdue.edu; 2 Loux, M. 2017. Are fall herbicide treatments necessary in Xtend soybeans? Ohio State University Extension. CORN Newsletter 2017-31. https://agcrops.osu.edu/newsletter/corn-newsletter/; 3 Bradley, K. 2013. Considering fall herbicides applications: It’s not just about the weeds. University of Missouri. Web sources verified 10/16/17. 171023123753