Weed Answers for 2020 Start this Fall

By: Harold Watters, CCA, Ohio State University Extension

So this year I am getting even more calls and comments on run away marestail.

“Last year I killed it, this year not so much” is often the remark I hear. 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 corner have been fighting it since about 2002. It takes a comprehensive effort, but it can be well managed.

Depending on severity and tillage in your system:

For no till soybeans and RoundupReady technology alone, it doesn’t work anymore.

1. Spray a combination of 2,4-D and glyphosate in the fall after corn harvest or you can spray a combination of 2,4-D and dicamba in the fall.
2. In the spring spray a second burndown (this may be the glyphosate & 2,4-D as above or glyphosate plus Sharpen) add your residual soybean herbicide – e.g. Trivence, Canopy, Authority, or your choice of many others, plus we usually add an extra 2-4 or even 6 ounces of metribuzin.
3. Spray glyphosate in crop at 3- to 4-leaf soybeans – there should be no marestail at this point.

For tilled soybeans with RoundupReady technology:

1. Before plant or immediately after, apply an effective pre-emergent herbicide.
2. Apply glyphosate at 3-4 leaf soybeans — again there should be no Marestail.

LibertyLink technology is a great option to manage horseweed and many other weeds that glyphosate misses.

1. If no-till, apply fall turndown as noted above — 2,4-D and glyphosate.
2. In the spring use pre-emergent herbicide before or at planting. If no-till apply burndown.
3. At 3- to 4- leaf soybeans apply Liberty or generic glufosinate. A second application may also be useful — the second application is for pigweeds (as in waterhemp).

For conventional soybeans the practices are about the same as for RoundupReady technology (since it isn’t working on the weed anyway). We must eliminate them before planting, use also a pre-emergent herbicide that is effective in preventing germination of new horseweeds.

Dicamba and 2,4-D herbicide tolerant soybeans are now available, and both can work well as part of the burndown for soybeans with this technology. This still requires a pre-emergent residual though. And you have the post option which works particularly well for giant ragweed and waterhemp management. The post- dicamba or 2,4-D options should not be needed at this point for marestail control. Your burndown and residual package took care of that.

Find details in the Ohio, Indiana & Illinois Weed Control Guide, produced by our OSU Weed Specialist Mark Loux, from your local Extension office.

Keep up with us at http://agcrops.osu.edu.