Glyphosate-resistant horseweed or marestail has made liars out of many weed scientists. The first recommendations were early applications of growth regulator herbicides. Later recommendations included Ignite at planting time. November applications have been recommended. This year, residual tank mix herbicides in March have been recommended. None of these recommendations have offered reliable control.

With funding from the Cotton Incorporated State Support Program we have an extensive horseweed research program underway, including seven separate studies.

Currently, our research is supporting the following recommendations:

Fall applications of traditional preemergence herbicides provide relatively little horseweed control; however, fall-applied Envoke appears to have potential to control horseweed through cotton planting.

2, 4-D and dicamba (Clarity) both have provided adequate horseweed control in our research; however, some states recommend dicamba instead of 2,4-D. Mixtures of dicamba and 2, 4-D may allow slightly reduced rates of each herbicide to be used.

Target burndown applications as late as reasonable possible. In our data, late March applications of 2, 4-D and dicamba have had minimal problems with new germination.

In-crop Envoke applications provide good suppression of smaller horseweed.

Do not tank mix "burner" herbicides with "systemic" herbicides. Some of our data hint that "burn-type preemergence herbicides like Caparol, Direx and Cotoran, may reduce the activity of dicamba. Additional research is needed; however, there were some failures this year from this type of treatment. Although there are exceptions, A general rule of thumb is that you shold not mix herbicides which cause burning symptoms with herbicides that are systemic.

2006 Field Day Report