Timely herbicide applications are key for post-emergence control of marestail. Marestail is difficult to control even when the plants are in the rosette stage, but becomes even tougher when plants are greater than 6 inches tall (Figure 1). That is why fall and early burndown treatments are critical to the long-term management of marestail. Unfortunately, those applications don’t always happen. Management is also complicated by the fact that some marestail populations in Kansas have developed resistance to glyphosate and ALS-inhibiting herbicides. However, there are some options for control of large marestail. These options are generally associated with the herbicide-resistance trait of your soybeans.
Figure 1. Herbicides control marestail more effectively in the rosette growth stage (left) than after the plant has bolted (right). Photos by Dallas Peterson and Sarah Lancaster, K-State Research and Extension.
In Xtend soybean, Xtendimax, FeXapan, or Engenia are some of the most effective herbicides for post-emergence control of marestail. Be aware of label restrictions associated with application of these herbicides, including nozzle selection and environmental conditions.
If you planted Enlist E3 soybean, Enlist One or Enlist Duo can effectively control marestail. Similar to Xtendimax, FeXapan, and Engenia, Enlist products have label restrictions on nozzle selection. Careful attention to tank-mix order has also been shown to be important to prevent physical incompatibility with Enlist products. In addition, Enlist could be tank-mixed with Liberty for multiple effective herbicide modes of action.
Liberty (glufosinate) can be effective for marestail control in soybean varieties with the Liberty Link trait, including Enlist E3 and LLGT27. Liberty requires higher spray volumes and good coverage for best results. Ammonium sulfate is essential to optimize Liberty performance.
A tank-mix of glyphosate plus FirstRate is a good option for marestail control in soybeans with the Roundup Ready trait. Some marestail populations are still susceptible to glyphosate, and even resistant plants will be affected to some degree. However, the combination of the two herbicides seems to work better than either herbicide alone, even on resistant plants. It is important to use ammonium sulfate to optimize control. Other tank-mixes to consider with glyphosate for controlling marestail would include Classic and Synchrony herbicides. Unfortunately, some marestail may also be ALS-resistant, in which case FirstRate, Classic, and Synchrony would also be fairly ineffective.
With all herbicides, it is important to use the full rates and observe label requirements to maximize weed control, prevent crop injury, and slow the development of herbicide resistance.
Sarah Lancaster, Extension Weed Science Specialist
slancaster@ksu.edu