Glyphosate-resistant marestail is the most problematic to control this year in Nebraska production fields. This is due to a number of reasons including wet spring weather and marestail’s emergence pattern. As marestail emerges from late March through June and from late summer to fall, it can easily escape herbicide applications and be difficult to control, especially plants that emerge in May and June.
A wet cool spring for most of the state reduced early spring and preplant application of herbicides for control of marestail. Even in fields where 2,4-D and other burndown herbicides were applied for control of marestail, wet cloudy weather has reduced the efficacy of these herbicides. We have seen marestail control escapes despite application of burndown herbicides. Additionally, corn and soybean planting were delayed in many fields, providing an opportunity for marestail to grow and establish. Even after planting, many corn and soybean fields were too wet for a pre-emergence herbicide application. Now, post-emergence herbicides are the only option for control of marestail.
For post-emergence control of glyphosate-resistant marestail in corn, dicamba, Liberty (only for Liberty Link Corn), and atrazine are really the only effective options and atrazine alone will not control larger marestail plants. More than 80% control should not be expected with POST herbicides, some of which are listed below:
- Status can be applied to corn up to 36 inches tall or V10 stage. It may provide about 80% control, depending on size of marestail.
- Clarity can be applied up to 16 oz/acre. Apply broadcast up to the 5-leaf stage or 8-inch corn. Apply 8 oz/acre or less broadcast or as a directed spray up to 36 inches tall or 15 days before tassel emergence, whichever occurs first.
- Corn larger than about 8 inches tall is more sensitive to dicamba, and use of a directed spray will reduce the risk of injury.
- DiFlexx is a new formulation of dicamba with CSI safner and it can be applied at a rate of 10 to 12 fl oz/acre up to V10 corn. It will provide better corn safety.
- Liberty can be applied only to Liberty Link corn. It is a contact herbicide, so effective spray coverage is required. (Use a spray volume of more than 15 gallons per acre.) It can be applied at a rate of 29 to 32 fl oz/acre. Liberty can be tank-mixed with DiFlexx to improve marestail control in Liberty Link corn.
- Hornet is a combination of clopyralid and flumetsulam. It can provide up to 80% control depending on size of marestail.
- Some other herbicides such as Realm Q, Capreno + atrazine also may provide about 75% control.
Effective post-emergence herbicides are very limited in soybean for control of glyphosate-resistant marestail. For example, FirstRate can provide about 80% control of marestail, while other soybean post-emergence herbicides such as Reflex, Flexstar, Harmony SG, Marvel, Torment may provide about 70% control. A sequential application of Liberty will be effective in Liberty Link soybean.
For other herbicide options, refer to 2015 Guide for Weed Management in Nebraska (EC 130). Always read herbicide label before applying herbicides.
Glyphosate-resistant marestail surviving two applications of glyphosate each at 24 fl oz/acre.