We conducted several studies this year that expanded our knowledge on management of marestail (horseweed).

Some of the findings reinforced what we already knew and validated our current recommendations, but we did learn some new things.

It seems as though we are still trying to figure out what the most consistently effective approach is, given that some years even good marestail management programs can fail to provide adequate control. 

Among the more general studies of marestail that we always conduct, we conducted the following that had more specific objectives:

1. Where the marestail population is both ALS- and glyphosate-resistant, is there any point in using residual herbicides in the fall?

The short answer is "no." Our research continues to support the fact that chlorimuron (Canopy/Cloak) is the only herbicide that persists long enough into spring to have value when applied in the fall.

Even where the population is not ALS-resistant, the control from fall-applied chlorimuron is variable into the spring. It generally won’t completely control marestail from fall through soybean canopy closure, so we recommend still using residual herbicide in spring.

Where the population is ALS-resistant, chlorimuron will no longer be effective. All of the other residual soybean herbicides lack substantial activity into spring. So where the population is ALS-resistant, the better approach will usually be to apply glyphosate + 2,4-D in the fall, and save the residual herbicide for spring.

It is of course still possible to go ahead and use a low rate of Canopy in the fall for residual control of other weeds, and apply the bulk of the residual herbicide in spring.

2. What are the most consistently effective marestail burndown treatments — especially where plants are big or have survived a previous treatment?

Marestail plants are always easier to control when small, and preferably before the stem elongates more than a few inches. We assume, though, that burndown treatments that work in difficult marestail burndown situations should also work well for relatively small plants.

We conducted studies at two sites with either very tall (single stalk up to 30 inches) plants, or plants that had survived an earlier glyphosate treatment (branched plants, up to 18 inches tall).

Results of these studies generally supported our previous recommendations. Burndown treatments that should generally work except in extreme situations include:  glyphosate + 2,4-D; glyphosate + Sharpen; Ignite; Ignite + metribuzin; Ignite + Sharpen, or Gramoxone + metribuzin + 2,4-D.  Burndown recommendations are listed in the OSU/Purdue fact sheet, “Control of marestail in no-till soybeans”, available at http://agcrops.osu.edu/specialists/weeds.

3. What is the most consistently effective overall approach to marestail management - is there a “systems” approach that we may be missing?  See accompanying article in this issue of C.O.R.N. for more on this.

4. What is the effect of marestail on soybean yield? 

While not a direct objective of this year’s research, information on yield was a byproduct of some control studies we conducted.

In a study where we were able to roughly separate the effects of the burndown vs residual herbicide, the numbers were as follows: ineffective burndown and no residual – 51 bu/A; effective burndown but no residual – 57 bu/A; and effective burndown and residual – 65 bu/A. 

We actually found that where the burndown was ineffective, it did not matter whether residual was included since the yield was already lost. We're working in a fairly small area with a consistently high marestail population, so this probably overestimates yield loss compared with a typical field that has a less dense and more variable population.

Nonetheless, these figures show the importance of effective marestail management, and the return on investment in the appropriate herbicides.