So this year I am getting even more calls and comments on run-away marestail. “Last year I killed it,” is often the remark I hear, too. And following is my response regarding horseweed (Conyza canadensis), or marestail as it is known in Ohio
This may be a new weed to you but the western side of the Ohio and particularly the southwest have been fighting it since about 2002. It takes a comprehensive effort, but it can be managed.
Depending on severity and tillage in your system:
For no-till soybeans — RoundupReady technology
1. Spray a combination of 2,4-D and glyphosate in the fall after corn harvest (the biggest problem with horseweed is from late summer and fall germinating seed), or you can spray a combination of 2,4-D and dicamba in the fall.
2. Spray a second burndown (this may be the glyphosate and 2,4-D as above or glyphosate plus Sharpen) in April and add your residual soybean herbicide — e.g. Trivence, Canopy, Authority, or your choice of many others, plus we usually add an extra two to four or six ounces of metribuzin. You can also spray a product like Boundary, again there are many options.
3. Spray glyphosate in crop at three- to four-leaf soybeans.
For tilled soybeans — RoundupReady technology
1. Till shortly before planting and that means with an effective tillage tool, not one that just stirs the top inch or so.
2. Before planting or immediately after, apply an effective pre-emergent herbicide.
3. Apply glyphosate at three- to four-leaf soybeans.
LibertLink technology — this is still a great option to manage horseweed and many other weeds that glyphosate has started to miss.
1. If no-till, apply fall turndown as noted above.
2. In the spring for no-till or tilled, apply pre-emergent herbicide before or at planting.
3. At three- to four-leaf soybeans apply Liberty or generic glufosinate, in some cases a post grass product may also be needed and added. A second application may also be used if needed. I went this way in 2017, but really needed the second application for pigweeds (as in waterhemp).
For conventional soybeans the practices are about the same as for RoundupReady technology. Most Marestail plants are resistant to glyphosate and we must eliminate them before planting. Use also a pre-emergent herbicide that is effective in preventing germination of new horseweeds.
Dicamba beans are now available, and dicamba works great as part of the burndown with soybeans with this technology. It still requires a pre-emergent residual though.
Find details in the Ohio, Indiana & Illinois Weed Control Guide, produced by our OSU Weed Specialist Mark Loux, from your local Extension office. The weed science team also has a blog posting some very well done videos on management: http://u.osu.edu/osuweeds/.