Weed Season Begins

Top Spring Weeds to Watch in 2018

Emily Unglesbee
By  Emily Unglesbee , DTN Staff Reporter
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Marestail is the primary driver for spring burndown treatments across the Corn and Soybean Belt, and the weed's herbicide resistance could complicate your options this spring. (DTN photo by Pamela Smith)

ROCKVILLE, Md. (DTN) -- "Start clean; stay clean."

That's the mantra University of Nebraska weed scientist Greg Kruger wants running through every grower's head this spring.

Herbicide resistance has made many spring weeds difficult to reign in once they get a foothold in your fields, Kruger said. Growers who missed fall herbicide applications are especially vulnerable, added Ohio State University weed scientist Mark Loux.

"Using $5 to $8 of herbicide (per acre) in the fall goes a heck of a long way," he said. "If they haven't done a fall application, growers need to know they're going to have to use a more comprehensive and expensive burndown treatment."

Remember that when it comes to spring herbicides, an early start is key.

"Getting out there at least 30 days before planting opens up your options in terms of chemistry you can use and lets you get good chemistry rotation in your burndown," Kruger said.

Here's a look at the top culprits coming to a field near you this spring:

MARESTAIL/HORSEWEED

This winter annual has earned its place as the most problematic spring weed across the Corn and Soybean Belt.

Left untreated in the fall, marestail overwinters as small rosettes and then springs into action as soon as temperatures rise. It can carry multiple types of herbicide resistance, most commonly glyphosate and ALS-resistance.

Marestail is particularly difficult to manage in soybeans.

"If marestail is not completely killed before planting soybeans, it cannot be successfully controlled in the crop," Penn State University weed scientist Dwight Lingenfelter warned. "Glyphosate, gramoxone or Liberty are all options, and make sure to use 2,4-D and/or Sharpen in the burndown tank mix, and now, with the advent of Xtend soybeans, you can use certain dicamba products."

If you aren't growing Xtend soybeans, watch the plantback restrictions that come with dicamba products, Loux added.

Keep in mind that glyphosate is largely ineffective against marestail, and 2,4-D shows variable control once the weed has bolted, Loux added. So the old burndown standby of glyphosate and 2,4-D is unlikely to work if you are facing an established marestail population in your field this spring.

Because marestail can continue to emerge after crop planting, Loux and Lingenfelter recommended adding residual herbicides to the burndown.

RAGWEED

Ragweed, long despised by allergy sufferers, is fast becoming a major target of spring burndowns.

Despite being summer annual weeds, common and giant ragweed have a long emergence window from early spring into the summer, Lingenfelter noted.

Although glyphosate resistance is becoming more common in giant and common ragweed populations, 2,4-D still works well on them, Loux said.

"Although the triazine herbicides have historically been effective for common ragweed control, resistance or tolerance or environmental conditions have allowed for ragweed escapes over the last several years," Lingenfelter warned. "Higher atrazine rates -- 1.5 to 2 pounds active ingredient per acre -- are now necessary for adequate control."

KOCHIA

For Western Corn Belt growers, kochia, "the weed of the west," will be a primary target of spring burndowns, Kruger said.

Kochia is fairly cold tolerant, which makes it one of the earliest emerging weeds, Kruger noted. "Kochia also really does well under drought condition, so it's more competitive than a lot of weeds in the High Plains," he said.

Like marestail, glyphosate and ALS resistance are common in kochia populations. Triazine and 2,4-D resistance have also surfaced in some areas of the High Plains, which takes more chemistries off the table, Kruger said. Isolated pockets of dicamba-resistant populations in western Nebraska and eastern Colorado have also been reported, he added.

Emily Unglesbee can be reached at Emily.unglesbee@dtn.com

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Emily Unglesbee