Glyphosate-resistant marestail has infested about 2.2 million acres in Nebraska. Marestail is difficult to control due to its emergence pattern. Marestail primarily emerges in the fall though some will emerge from late March through early June, easily escaping herbicide applications and making it difficult to control.
This is the time of year when fall-emerged marestail starts bolting (stem elongation) and is more easily seen infesting corn and soybean fields in Nebraska. Ideally, the best time to control marestail is in the fall or early spring when marestail plants are young and actively growing (rosette stage) before corn and soybean planting.
With the cycling of cool-wet and hot-dry weather this spring and multiple rains over the last 10-14 days, early spring burndown and pre-emergence herbicide application for control of marestail have been less effective. Even in fields where 2,4-D and/or other burndown herbicides were applied for control of glyphosate-resistant marestail, wet cloudy weather has reduced the efficacy of these herbicides. This has resulted in marestail control escapes from early burndown herbicides. Additionally, due to continuous rain, corn and soybean planting were delayed in many fields, providing an extended opportunity for marestail growth and establishment.
Even after planting, many corn and soybean fields may have been too wet for a pre-emergence herbicide application. As a result, postemergence herbicides are the only option for control of marestail. Glyphosate applied alone will not be effective for control of glyphosate-resistant marestail in glyphosate-resistant corn and soybean; however, there are several postemergence herbicide options for corn and a limited number for soybean.
For postemergence control of glyphosate-resistant marestail in corn, dicamba, Liberty® (only for Liberty Link Corn®), and atrazine are effective options. Atrazine applied alone will not control larger marestail plants.
Effective postemergence herbicides are very limited in soybean for control of glyphosate-resistant marestail.
FirstRate can provide about 60-80% control, depending on height of marestail at the time of application.
Reflex, Flexstar, Harmony SG, Marvel, and Torment also may be good options.
If Liberty Link soybean is planted, a postemergence application of Liberty may be effective; however, Liberty can only be applied before Liberty Link soybean start flowering. Also, follow application recommendations listed above for greater efficacy.
Dr. Amit Jhala and Chris Proctor from the UNL Extension Weed Management office in Agronomy provided the information used in this week’s column.