The month after wheat harvest provides an opportunity to control marestail and prevent further increase in the soil seedbank, but coming up with the right strategy has not necessarily been easy. Keep in mind that the primary goal of marestail control here is preventing seed production, which doesn't mean that any treatment applied has to provide 100% control of the plants themselves. We have conducted several studies targeting tough marestail situations, representing plants that have been previously treated with herbicide or mowed, or survived tillage, and also one study in a wheat stubble situation. The reason all of these studies are pertinent is that there can be a couple different types of marestail plants in wheat stubble. The generally easier plants to control are those that small and have lurked within the wheat, below the height of the cutter bar. These plants grow taller in response to light once the wheat is harvested, but are free of prior damage that would make them more difficult to control. The other plant type is the one that was tall enough to be cut off by the cutter bar, and these also regrow following harvest, but the prior stress makes them much more difficult to control. And of course there can be emergence of new plants after harvest, depending upon moisture and the degree of soil cover by residue. A few things we have learned relative to this situation:
1. The goal of preventing or greatly reducing seed production can be accomplished without herbicides. Mowing is effective and can be timed for the early flower stage, before plants have produced seed, and late enough that substantial regrowth and potential for much additional seed is low. Downside – more time consuming than herbicide application. And of course tillage can work here too.
2. If both mowing and herbicides are going to be used, we would suggest using herbicides first and follow with mowing later as needed. Trying to control plants that have regrown after mowing will be difficult.
3. With regard to herbicide treatments, earlier is better. Some regrowth of damaged plants is fine, but allowing plants to get larger almost always reduces control and may require a more comprehensive treatment. In a 2014 wheat stubble study, we applied herbicides on either July 25 or August 7. These were mostly plants that had not been damaged by the wheat harvest. The combination of glyphosate plus 2,4-D applied on July 25 provided 85% control and resulted in only few plants being able to produce seed. All other treatments (Sharpen, Liberty dicamba, etc) provided 100% control. When application was delayed until August 7, control with glyphosate plus 2,4-D or dicamba dropped to 70 to 74% and 38 to 43% of the surviving plants produced seed. Control with dicamba plus 2,4-D also was reduced to 78%, but plants did not produce seed. The other treatments still provided upwards of 85% control and prevented seed production. We suspect that the difference in control between application dates would have been accentuated for marestail plants that had been damaged by the cutter bar and regrown.
4. Some growers try to apply late enough in summer to control both marestail and volunteer wheat. This is likely to make control of marestail difficult. The post-harvest treatment is also not likely to serve the purpose of controlling marestail plants that emerge in late summer into fall. A mid to late fall application will still be most effective for those.
5. In the other studies where we were trying to control marestail plants that had undergone a previous stress (such as cutter bar damage), it was difficult to obtain greater than 80% control with any treatment, and we actually rarely killed more than 10% of the plants. We did not follow surviving plants through to maturity to determine effect on seed production. The more effective treatments tended to be combinations of more than one herbicide that had activity on marestail (excluding glyphosate), such as glufosinate plus 2,4-D or Sharpen. Overall, the variability of control increases and predictability of effectiveness decreases for plants that have regrown following damage or prior treatment, and allowing them to get taller does not help.
6. Every year we get a few calls into late August about control of large marestail in wheat stubble and at that point we really don’t have a recommendation to offer. They will often have gone to seed by then anyway. Mowing is certainly still an option then, but we would encourage application of herbicides fairly soon to avoid this problem.
We will post tables of results for the studies mentioned above in the marestail section of the OSU weed management website – u.osu.edu/osuweeds.