by Mark VanGessel, Extension Weed Specialist; mjv@udel.edu
If no-till soybean fields have not been sprayed yet, we are facing an uphill battle. Most of the horseweed (or maretail) is now well over 6” tall, and so are a lot of the other winter annual weeds.
It’s too late to rely on 2,4-D to control marestail because you need the 1 qt rate to provide effective control. The 1 qt rate of most 2,4-D products require 4 weeks before planting and in too many places sensitive plants have emerged. If you are on appropriate soil type, Sharpen with all the required adjuvants is an option (see the label). Sharpen is not an option for coarse-textured soils because it requires 4 weeks between application and soybean planting due to concerns with safety to soybeans. A chlorimuron-based herbicide is another option, but you need to use rates that will provide good suppression/control. Chlorimuron rates equivalent to 1.7 oz of Classic are what we have found for more consistent activity (see table). This is probably a higher rate than is typically used, but you will need these higher chlorimuron rates to provide the best level of suppression.
| Herbicide | Rate oz wt/A | Classic Rate | Other |
| Valor XLT | 4.0 | 1.7 | Valor |
| Envive | 4.6 | 1.7 | Valor+Harmony |
| Canopy | 4.0 | 1.7 | Metrbuzin |
| Canopy EX | 1.9 | 1.7 | Express |
We have found populations of horseweed that are resistant to ALS-inhibiting herbicides, so this approach is not a recommendation for use every year.
Two other considerations are Liberty 280 and Gramoxone. Liberty 280 is in tight supply and maybe hard to get. Gramxone SL will require two applications. One of the Gramoxone applications should be applied with a metribuzin containing-product (Canopy, Boundary etc) because it will improve the performance of the Gramoxone. Both Liberty 280 and Gramoxone should be applied with at least 20 gallons per acre of water and flat fan nozzles to provide the best coverage of the weeds.
All of these products mentioned above perform best on small plants. Given the size of the weeds, I would not expect complete control. Following up with an early postemergence application of Liberty (for Liberty Link soybeans), FirstRate, or glyphosate will further suppress the marestail and allow the soybean canopy to get over the top of the marestail.
Source: Read More from Weekly Crop Update
