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Weeds: Big and Tall Edition
Wednesday, May 15, 2019 2:31PM CDT
By Emily Unglesbee
DTN Staff Reporter

ROCKVILLE, Md. (DTN) -- Leave that 4-inch soda can in the truck -- some of the weeds and cover crop cover in the Midwest right now will require a yardstick to measure.

The persistent cold, wet weather dogging much of the corn and soybean belt this spring has kept many farmers from applying their usual early spring burndown applications. Winter annuals like marestail and butterweed grew as woolly and wild as they pleased, and now early summer annual weeds are starting to sneak in as well, said Purdue University Extension weed scientist Bill Johnson.

"Ragweed, foxtail and lambsquarters are typically summer annuals that come up early and take off pretty quickly," he said. Some growers are also facing overgrown cover crops still standing weeks past their ideal termination date.

Many of these fields can still be controlled by burndown applications, but keep the following recommendations from Purdue's Johnson and Ohio State Extension weed scientist Mark Loux in mind.

INCREASE YOUR RATES

Controlling tall weeds and dense covers requires a heavy dose of glyphosate, Johnson said.

"If you're using a generic formulation with 3 pounds of acid per gallon, use at least 48 ounces or more per acre," Johnson urged. "If you're using more concentrated formulations that contain 4 pounds of acid or more per gallon, then you need to be at 32 ounces or more."

DON'T SKIMP ON THE TANK MIX

Yes, margins are tight and additional inputs are a tough decision, but you will need another tank mix ingredient or two to target weeds that don't respond to glyphosate as well or at all, Johnson said.

Depending on the weeds you're targeting, 2,4-D, dicamba, metribuzin (Sencor) or saflufenacil (Sharpen) are common additions. Remember that on weeds much beyond four inches tall, paraquat (Gramoxone) will not be an effective option, he added.

For more details on individual herbicide efficacy, see this guide: https://www.extension.purdue.edu/…

Tank mix additions can be trickier for grassy cover crops, Ohio State's Loux cautioned in a university newsletter. "Mixing glyphosate with other herbicides or ATS can reduce its activity on grass covers, especially when large," he wrote in an OSU newsletter. "Herbicides that can antagonize glyphosate include 2,4-D, metribuzin, atrazine, and flumioxazin and sulfentrazone products. Sharpen has not caused a reduction in glyphosate activity on grass covers in university research."

RESIDUAL CAVEATS

Residual herbicides are often recommended in burndown tank mixes because they can provide additional weeks of protection. But because of their longevity on the soil, they also come with larger plant-back restrictions that can push back a planting window or lock a grower into a certain crop, Johnson noted. Read the labels carefully.

In thick, waist-high cover crop stands, residual herbicides may not make it to the soil to do their job, added Loux. "Our experience with relatively small (less than 2 feet) covers is that they do not interfere with the activity of residual herbicides," he wrote. "We are somewhat unsure about the effect of taller covers on residual herbicide activity, but assume it could be reduced. This may be a situation where the residual could be omitted from the burndown and then included in an early POST treatment."

Keep in mind that depending on your trait platform, there are post-emergence herbicides for corn and soybeans that can help clean up a field after planting if residuals don't work, Loux added.

PLANTING GREEN IS AN OPTION FOR COVERS

Given tight planting windows and wet soils, this spring might become an unexpected experiment for some farmers in "planting green" -- planting row crops amid live cover crops and terminating the cover later, Johnson noted.

"Sometimes equipment works better if you plant into a living cover crop," he said. "One of the big issues guys are wrestling with now is do I terminate and wait until the cover crop has hit the ground to plant, or plant green and hope I can terminate later."

Planting green is probably a better option for fields planted to corn or soybean varieties with multiple herbicide-resistant traits, which will give growers a wider variety of post-emergence herbicides to terminate with, Loux said.

For more details on the rates and chemicals to use when tackling fields of overgrown winter and spring annual weeds, see this article from Purdue University: https://extension.entm.purdue.edu/…

For more details on controlling overgrown cover crops, see Loux's article here: https://agcrops.osu.edu/…

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

Follow her on Twitter @Emily_Unglesbee

(PS/BAS)


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