Marestail (Conyza canadensis), also known as horseweed, has become one of the most difficult weeds to manage -- especially in no-till soybean systems.3
Marestail is a winter annual that can germinate in early fall or the following spring. Starting from a rosette cluster of leaves, stem elongation occurs quickly, with rapid growth up to 3-7 feet high. This occurs more quickly on the plants that germinated in the fall, as compared to spring germinating plants.1 Once mature, marestail seeds are distributed mostly by wind, and a single plant can produce up to 200,000 seeds with around 80 percent viability. These seeds can germinate immediately after falling from the mother plant and can do so on top of the soil.4
The primary goal of a fall herbicide treatment is to control small seedlings and rosettes. The best way to determine if one is necessary is to scout fields just before or right after harvest. Depending on your location, applications can be made through November, but daytime temperatures at least in the mid-50s are needed for adequate control with herbicides.3
Fall herbicide applications are also beneficial because inclement weather may interfere with early spring treatments, enabling plants to grow too large to be controlled by herbicides. Keep in mind, however, that these fall applications might not be a replacement for pre-plant or pre-emergence herbicide treatment the following spring.1
Herbicide resistance should be considered when dealing with marestail. If you have used certain herbicides in the past with limited success, you may have a resistance issue. When selecting herbicides, look at the mode of action group to confirm you are not using the same mode of action over and over. That would increase the chances of developing resistance. Marestail has confirmed cases of resistance with herbicide Group 2 (Classic®, Pursuit®), Groups 5 and 7 (atrazine, linuron), Group 9 (glyphosate) and Group 22 (paraquat)2, but not all locations will have resistance to all of these herbicides.
Remember that timely fall and spring tillage, as well as certain cover crops like cereal rye planted before or after crop harvest in the fall, can also help to control marestail.4
Application rates should be specific to marestail populations and plant height. If your marestail pressure is high or there is a history of heavy marestail pressure, mix 8 fl oz/A of dicamba, 16 fl oz/A of 2,4-D ester, 32 fl oz/A of glyphosate and a residual herbicide that contains active ingredients like metribuzin or flumioxazin.4 Whatever residual products you add in the fall, make sure these products will not interfere with your planned crop rotation the following spring.
If you feel your weed pressure is low to medium or if the field has had little marestail pressure in the past, a good post-harvest herbicide mix may be to apply 16 fl oz/A of 2,4-D ester with glyphosate at 32 fl oz/A. This mix can also be enhanced with 8 fl oz/A of dicamba DMA.
In all scenarios, always read and follow the label instructions. Adjuvant recommendations are specific to each herbicide, so make sure you are adding the label-recommended combinations. Check your state regulations specifically around dicamba use to ensure that you are in compliance with local regulations.
Marestail can be a tough weed to control, but timely scouting and application of approved herbicides can make it less of a challenge. Scout your fields this fall and take action where needed. Marestail is sneaky -- if you see a few now, you can bet you will see thousands in the spring.
With FBN Direct, you can purchase chemical applications online and have them delivered straight to your farm.
Pursuit® herbicide is a registered trademark of BASF Ag Products. Classic® Herbicide is a registered trademark of DuPont Crop Protection.
ALWAYS READ AND FOLLOW LABEL DIRECTIONS. It is a violation of federal and state/provincial law to use any crop chemical product other than in accordance with its label. The distribution, sale and use of an unregistered pesticide is a violation of federal and/or state law and is strictly prohibited. It is your responsibility to confirm prior to purchase and use that a product is labeled for your specific purposes, including, but not limited to, your target crop or pest or weed, and its compatibility with other products in a tank mix.
https://reader.mediawiremobile.com/USB/issues/200925/viewer?page=53 https://ag.purdue.edu/btny/weedscience/Documents/marestail%20fact%202014%20latest.pdf https://weedscience.missouri.edu/publications/50737_FINAL_FactSheet_Horseweed.pdf https://www.bookstore.ksre.ksu.edu/pubs/MF3014.pdf https://www.extension.purdue.edu/extmedia/GWC/GWC-9-W.pdf