With the recent stretch of nice fall weather, we continue to get questions about control of certain winter annual weeds, namely common chickweed and horseweed/marestail.
Horseweed has been a growing concern the past several years and although we have conducted research on this weed, Mark Loux and his colleagues at Ohio State have some helpful insight on this specific species. Instead of reinventing the wheel, here are a few highlights and references to his articles for further information.
- Keep cost of fall herbicide treatments in the range of $4–12/acre. Glyphosate + 2,4–D can be an initial low cost option to consider that provides control of a relatively broad spectrum of weeds.
- Products that contain chlorimuron (e.g., Canopy) tend to provide the most residual horseweed control into spring. (Keep in mind there are areas in Delaware, Maryland and in the Midwest that have ALS+glyphosate—resistant horseweed, so chlorimuron or other ALS herbicides would not provide control in these situations.)
- Fall only burndown/residual applications generally do not provide enough control of horseweed into next season. Two-pass burndown programs (fall followed by spring applications) are better at obtaining season-long horseweed control.
- If applying a fall burndown, make sure to select the correct product depending on what crop you intent to plant next spring. For example, don’t apply a Canopy product if you plan to plant corn.
For more details on the above points, please review articles about horseweed management from Ohio State:
Fall Herbicide Treatments - Focus on Marestail Management
Do We Need a Systems Approach to Marestail Management?
What We Learned About Marestail in 2010
Common chickweed is another weed we have been hearing more about not only in small grains but in forage crops as well. In wheat, barley, and oats, unless it is ALS-resistant, Harmony Extra (or equivalent generic product) is one of the better options. (On as side note, if you have horseweed in small grains, Harmony Extra usually provides good control.)
If you are one of the unfortunate who have an ALS-resistant chickweed population then programs that include Starane (fluroxypyr) appear to be providing the best control. We are currently conducting a couple studies on ALS-resistant chickweed in small grains, so we hope to have a better handle on this by next spring.
If chickweed is a problem in alfalfa, then some options include Pursuit, metribuzin, Chateau, and Gramoxone. Only Pursuit and metribuzin are labeled for use in alfalfa/grass mixtures.
Also, as we get later in the year, products like Pursuit tend not to be as effective due to colder conditions that affect weed growth. If you are struggling with chickweed in grass hay or pasture, a combination of dicamba (Clarity, Banvel, others) plus 2,4–D will provide some suppression (70–80% control). This tank-mixture works better on chickweed in the fall compared to spring applications where it typically provides only about 50–60% control.