Burndown Targets Resistant Weeds

By Carroll Smith
Senior Writer

Small PigweedA lot of time and effort has gone into developing in-season strategies to manage problem weeds such as glyphosate-resistant Palmer amaranth (pigweed) and Italian ryegrass. Now, research has shown that applying herbicides in the fall, particularly after corn harvest, can help knock down resistant weeds that emerge prior to the first frost and reduce the Palmer amaranth soil seed bank.

“In the past few years, we have seen more and more of our farmers coming in with a burndown herbicide including a residual as soon as the combine runs through the corn field,” says Larry Steckel, University of Tennessee weed specialist. “It has become a pretty common practice, and it’s a way to knock down the Palmer pigweed soil seed bank so that this weed is more manageable in cotton the next year.”

Bob Scott, University of Arkansas weed scientist, has seen much the same scenario in his area.

“We’ve added fall burndown herbicides to our recommendations over the last few years, especially behind corn, because it is such a good rotational crop with cotton and soybeans for resistance management,” Scott says. “One of the most common combinations is applying paraquat and Valor on corn ground to take care of the weeds that are there and also lay down some residual to keep them out.

“We don’t have quite the resistant Italian ryegrass problem that Mississippi does right now, but where we do have it, we’ve been looking into fall applications of Dual along with other products.”

Pre-Frost Treatment Warranted

For areas where glyphosate-resistant marestail, or horseweed, is a problem, the Arkansas weed scientist says Sharpen herbicide, a new PPO inhibitor, works well to control it and provides some residual. Scott points out that Sharpen is in the same family of chemistry as Aim and Flexstar.

He also notes that when Palmer pigweed emerges in the fall, the plant can tell biologically that the days are shorter, which triggers it to go reproductive really early. Consequently, when pigweed comes up in the fall, the plants may only be four or five inches tall, but they will put on a seedhead. Normally, pigweed that comes up in the spring would be more vegetative and grow to eight or nine feet tall and then put on many seedheads.

“The small pigweed plant that you see in your field in the fall can make at least several hundred seeds before a frost gets here,” Scott says. “That’s why you need to treat these plants even before there is any seed in the seedhead. If you give them another two weeks, they probably will have produced at least several hundred seed by that time.”

Managing Italian Ryegrass

Jason Bond, Mississippi State University (MSU) weed scientist says, along with several of his colleagues, “A glyphosate-resistant Italian ryegrass management program should begin with residual herbicides applied when weather permits between mid-October and mid-November. Fields containing glyphosate-resistant Italian ryegrass not controlled at burndown will have significant residue at planting. Residue will impede planting practices, contribute to competition between crop seedlings and established glyphosate-resistant Italian ryegrass and hinder herbicide programs due to inadequate coverage.”

MSU also notes, “Regardless of the crop to be planted, paraquat (0.5 to 0.75 lb ai/A) should be included with the fall residual herbicide to control glyphosate-resistant Italian ryegrass emerged at time of application. During years where activating rainfall is inadequate, research indicates that shallow tillage will effectively incorporate fall residual herbicides.”

Glyphosate-resistant weeds continue to challenge cotton farmers, especially those who are in a cotton/corn rotation. However, incorporating a fall burndown program into the equation can provide another “tool in the box” to keep these troublesome weeds at bay.

Contact Carroll Smith at (901) 767-4020 or csmith@onegrower.com.


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