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Tall marestail taking over farm fields

University of Missouri looks at burndown treatments for extremely large weeds.

Can you kill 50-inch-tall marestail growing in the field? Contrary to popular weed scientist recommendations, you can.

Kevin Bradley’s mantra has always been that to achieve effective control, farmers should spray weeds before they reach 4 inches in height. However, the University of Missouri Extension weed specialist says over the past few years, farmers faced wet conditions where they simply could not get into the fields to plant, let alone spray weeds.

This year was no different. While farmers waited for fields to dry out, weeds grew and grew. Farmers need treatment options.

As luck would have it, a small 150-by-150-foot area at MU’s Bradford Research Farm outside of Columbia became an impromptu test plot of burndown treatments for extremely large marestail, also known as horseweed.

Soil from construction on the farm was spread in that location. Passing by, Bradley noticed an abundance of the weed. So, he sectioned off the plot and began spraying.

Creating concoctions

Using herbicides such as Roundup PowerMax and Enlist Duo, Bradley created mixes that included burndown herbicides like Sharpen and Liberty.

Ultimately, six treatments produced control of marestail at 95% or greater. Click through the slideshow above to see photos of the plot and a few control mixtures, complete with product and rates.

Burndown basics

Bradley offered three key takeaways for farmers facing large marestail this year.

1. Don’t skimp on adjuvants or proper spray application parameters. Bradley applied these herbicides at 20 gallons of water per acre and used the most effective adjuvants. In some cases, he added a methylated seed oil or MSO. Most of the mixes included at least three products.

2. Add a burning herbicide product. Examples include Sharpen, Gramoxone or Liberty. This is a must to kill taller plants, he notes.

3. It isn’t cheap. It can take multiple products to achieve results. One trial mix alone includes Roundup PowerMax; Sharpen; 2,4-D Ester; Methylated Seed Oil; AMS; and a drift retardant. “Controlling these large weeds is heavy on the pocketbook,” Bradley adds.

Still, left untreated, marestail can produce as many as 200,000 seeds per plant. Farmers must weigh the options. “If you have part of a field that is wooly,” Bradley says, “pencil out what it costs to gain control.”

Waiting to spray at 50 inches will never be part of a weed management plan from Bradley. “It is always best to spray when they are at the rosette stage,” he says. “But many growers did not have a choice this year.” In these cases, he says, it is good to offer some solutions.

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