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Glyphosate-resistant horseweed can be managed

The phone has been busy with questions about burndown of glyphosate-resistant horseweed (marestail) before cotton and soybean planting. Over the past couple of years, cotton and soybean producers in west Tennessee have been trying to put together a weed control program that will consistently control horseweed in no-till cotton and soybeans.

To date, the burndown protocol that has provided the most consistent control of this troublesome weed has been 0.75 pounds of glyphosate and 8 ounces of Clarity. Some have also used 0.75 pounds of glyphosate plus 16 ounces of a 4-pound-per-gallon 2,4-D material. Though this provided good control of horseweed on some fields, it was not as consistent as the glyphosate plus Clarity treatment.

Many have inquired about incorporating a newly registered herbicide, Ignite, into their spring burndown program. Ignite herbicide, better known as Liberty (glufosinate), recently received a label to be used on Liberty Link cotton varieties, hooded application to non-Liberty Link cotton varieties and for burndown prior to general crop planting. The best attributes about Ignite herbicide is that cotton and soybeans can be planted anytime after an Ignite application.

This provides more flexibility over a Clarity application where cotton and soybeans cannot be planted for 21 days. In addition Ignite will provide good control of many winter annual broadleaf weeds like henbit, purple deadnettle and chickweed.

In regards to glyphosate resistant horseweed, recent research conducted by Dr. Robert Hayes, a weed scientist with the West Tennessee Experiment Station at Jackson, has shown that 32 ounces of Ignite will control horseweed up to 6-inches tall. However, he found that control is sometimes inconsistent once horseweed grew taller than 6 inches. Moreover, it is weak on winter annual grasses such as annual bluegrass and ryegrass.

A very good fit for Ignite in a burndown program is cleaning up any horseweed that emerges after or escapes a Roundup Weather Max plus Clarity application. Unlike Roundup Weather Max, Clarity and 2,4-D, Ignite is a contact-type herbicide, so good coverage is essential for consistent control of horseweed.

Use a minimum of 15 gallons of water and for dense weed stands 20 to 40 gallons is recommended. The label specifically states to not use drift reduction nozzles. Flat fan nozzles at an 80- or 110-degree angle are recommended to obtain good control.

Horseweed is considered to be a winter annual. That is, it emerges in the late summer or fall, overwinters and then flowers in the early spring and goes to seed in June. Research conducted last year at Jackson and Knoxville found that horseweed has the ability to emerge throughout the year with peak emergence events in the late summer and spring.

As a result, horseweed has the ability to emerge after a successful burndown program has been implemented. Therefore weed control programs designed to control this weed should also include at least one herbicide that provides residual control.

Valor herbicide can provide residual control of horseweed at the 2-ounce rate. Valor must be applied 30 days prior to cotton planting. Valor may be applied preplant or pre-emergence in soybeans.

Glyphosate-resistant horseweed can be successfully managed. However, it takes some planning and utilization of several different herbicides.

Larry Steckel is an Extension weed specialist with the University of Tennessee.

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