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Take best shot at controlling runaway, resistant ragweeds

Lucky people hide the field they don’t want anyone to see in the back 40. Those less fortunate discover it’s the field right out front. If you’ve got a field of soybeans with glyphosate-resistant giant ragweed patches, maybe you don’t even want to see it.

By now those patches are wooly. Betsy Bower, an Indiana Certified Crop Adviser and agronomist with Ceres Solutions, Terre Haute, says it may still be worth one more shot at them. Bryan Overstreet, also a CCA, isn’t so sure. Overstreet is Purdue University Extension ag educator in Jasper and Pulaski counties.

Key Points

• Experts differ on the value of a late rescue treatment for ragweed.

• Brace yourself for bean burn and weed suppression, not weed kill.

• When weeds are bad enough, consider using harvest aids.

“A rescue treatment may still pay for itself,” Bower believes. That’s assuming soybeans are still in the vegetative phase to early flowering. Depending on planting date, that could be a stretch.

Why would Bower consider one more shot?

“Giant ragweed is one of the most competitive weeds,” she says. “Michigan State University notes that one giant ragweed plant per 100 square feet can hurt soybean yields up to 50%.” A hundred square feet is a 10-by-10-foot space. One weed cuts yield in half? Wow!

However, Overstreet thinks the horse may already be out of the barn. “It’s getting late for a rescue treatment,” he says.Bower counters that there may be more reason to spray than salvaging yield. “How will they affect harvest?” she asks.

Late options

Remember that the farmer determined these giant ragweeds were resistant to glyphosate. So it’s not an option. “The biggest question is whether they’re still controllable with a conventional herbicide,” Bower observes. “It depends upon how big the weeds are.”

Before glyphosate, products of choice were Flexstar or Cobra, Bower notes. Today, Flexstar GT has replaced Flexstar. Cobra is the same product today that it was years ago.

“Both Flexstar and Cobra can be applied up to 45 days before harvest,” Bower says. “FirstRate has been a good product in the past, but it can only be applied up to 50% flowering.” That likely takes it out of consideration now.

Cobra’s label says it can suppress giant ragweeds up to 36 inches tall, Overstreet adds. Before you decide to spray, Overstreet advises checking giant ragweed stalks for stalk borer damage. “If plants have been damaged, herbicides won’t work nearly as effectively,” he says.

Improve odds

To give yourself the best shot if you decide upon a rescue treatment with Cobra or Flexstar GT, use at least 15 gallons of carrier per acre, Bower advises. “Coverage is the key,” she emphasizes.

She recommends a crop oil concentrate of methylated seed oil as an adjuvant. Individual product labels should specify rates. Also use 10 pounds per 100 gallons of spray of AMS, she adds.

“Spray anytime after the dew clears in the morning until late afternoon,” she suggests.Both Bower and Overstreet know this is a tough, last-ditch effort.

“Be ready to see burned bean leaf tissue, too,” Bower says. Just in case, the 2010 Ohio and Indiana Weed Control Guide discusses possible harvest aids, Overstreet adds. Find it at www.btny.

This article published in the August, 2010 edition of INDIANA PRAIRIE FARMER.

All rights reserved. Copyright Farm Progress Cos. 2010.