Review strategies to control marestail and other weeds before planting double-crop soybeans.

With barley harvest occurring and wheat coming off soon after, double-crop soybean planting will soon follow in some areas of the state. Weeds are generally less of a problem in double-crop soybean because of the delayed planting date. But even with fewer weeds emerging, when they are present and in combination with potential dry weather, impacts on yield can be noticeable. A weed-free start is one of the most critical aspects of weed management in double-crop soybean. One of the biggest weed concerns is in managing glyphosate-resistant marestail or horseweed. Some of these populations may also be ALS-resistant. Marestail that are cut off during small grain harvest will recover, branch out, and probably be even more difficult to control. In full season soybeans, we have been strongly advocating the use of 2,4-D LVE in the burndown program. However, most growers are not willing to wait the necessary 7 days after 2,4-D application in double-crop. So, what are the options?

• For glyphosate susceptible marestail, apply glyphosate at 1.5 lb ae/A (44 fl oz of Roundup PowerMax, 48 fl oz or Touchdown Total or Durango DMA or 64 fl oz of a 3 lb ae/gal product).
• Include 1 pt per acre 2,4-D LVE in the burndown and wait 7 days to plant.
• Add 1 oz of Sharpen (or equivalent rates of Optill or Verdict) to glyphosate. Keep in mind, Sharpen helps with burndown but does not provide much residual control.
• Add FirstRate (or Authority First or Sonic) or a chlorimuron containing product (Authority XL, Canopy, Envive, Synchrony, Valor XLT, etc.) to improve control if the population is not ALS-resistant. (Valor, Authority, and metribuzin provide residual activity; but not much burndown activity.) Also, root uptake of Valor XLT/Envive may help kill horseweed stumps.
• Use Liberty at 29 to 36 fl oz in the burndown application. This is fairly good on marestail, but not that effective on some other weeds such as large annual grasses and Canada thistle. You can tank-mix with other herbicides to improve performance and provide some residual control (Sharpen, metribuzin, etc.)
• Plant a LibertyLink soybean variety and apply Liberty POST as needed.
• Plant a Roundup Ready soybean and apply glyphosate POST if marestail is not a problem.
• For growers interested in non-GMO soybeans, a number of POST herbicides are still available, but many of the broadleaf products have some potential for soybean injury and they are not effective for control of marestail.

Dwight Lingenfelter is an Program Development Specialist in Plant Science with the Pennsylvania State University.  William S. Curran Ph.D.is a Professor of Weed Science with the Pennsylvania State University. This article originally appeared in the Penn State Extension’s website and has been reposted with permission.