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Why battle weeds?

Agriculture.com Staff 02/14/2016 @ 10:57am

How about a show of hands for those who think glyphosate-tolerant crops have made weed control simple, effective and inexpensive?

If you've got your hand raised, you're right.

"I agree, as weed control looks awful easy now," says David Duncan, an Alcester, South Dakota, farmer. "Today's seed and herbicide technology has really simplified farming compared to the 1980s. Back when I first farmed, we used atrazine, Treflan, Lasso and Bladex. The only time we'd use Roundup was for spot spraying."

His wide-eyed awe of technology change in agriculture is because Duncan left farming for 10 years in 1989, when he sold cars and mobile homes. His dad's stroke brought him home again. Today, Roundup Ready technology forms a major part of his weed-control plan.

"It's just amazing the technology we have for weed control and the seeds we have nowadays," he says.

But has weed control become too easy? Rather than buy into the one-application-only scenario, Duncan has done his homework.

"You can't let weeds get too far, or grass and broadleaves will suck the yield right out of the crop. And corn gets hit the worst," he says. "My corn acres get a half-rate, soil-applied herbicide to keep early weeds down and to save yield until my postspray cleans up the rest. I use two postapplications on my beans since a lot of bushels can be lost while I'm waiting for most of the weeds to emerge."

Kevin Bradley, University of Missouri Extension weed scientist, winces at the mention of a single glyphosate application. "Growers wait too long to apply glyphosate in soybeans. Sometimes they spray 10- to 12-inch weeds," he says. "Many growers don't realize it is more economical to adopt a two-pass glyphosate or pre/post approach due to lost yield."

Not only is yield lost, but also some taller weeds can fight off glyphosate. Mark Wolters, DuPont's Midwest marketing manager, says "Growers who rely on a single glyphosate application on taller weeds are seeing their weed-control effectiveness drop."

To help growers understand yield loss from weed competition, Bradley often steers them to a yield loss estimator tool called WeedSoft.

"It allows a grower to take real situations, plug in the crop and weed height, and quickly learn about yield already lost, as well as future yield losses as weeds get taller," he says. Use it free on the Glyphosate Stewardship Working Group's new Web site at www.glyphosateweedscrops.org.

The other issue that furrows the brows of weed scientists is the lack of concern among growers -- and even some industry folks -- toward glyphosate-resistant weeds.

"Perhaps it's too easy to blame poor herbicide performance instead of pulling the canopy back in late season to find what weeds survived glyphosate and are still going to seed," says Bill Johnson, Purdue University Extension weed scientist.

"Growers won't find glyphosate-resistant weeds as easily as ALS resistance. When a respray of an ALS product hit ALS-resistant weeds, nothing happened. So we knew they were resistant.

"When you respray with glyphosate, it can injure the weeds enough to make growers think the respray worked. But resistant weeds will survive under the canopy and produce seeds -- especially cocklebur, pigweed, waterhemp and lambsquarter.

"One simple solution that should halt lambsquarter resistance is to get residual control back into a weed-control program. This is because most soil-applied herbicides work well on it; many postproducts don't work," he adds.

Jim Flater, DuPont corn product manager, says "We're hearing from growers -- loud and clear -- about tough-to-control weeds in glyphosate-tolerant crops, such as lambsquarter, marestail (horseweed), ragweed and others. We feel the market is changing, especially as more acres of glyphosate-tolerant corn are managed. To lengthen the glyphosate system, growers cannot forget residual programs. No one wants to run out of the ability to control some of these tough weeds."

Another important resistance-management tool is to tankmix a broadleaf herbicide with a postglyphosate application, says Bill O'Neal, technical product manager for Amvac Chemical Company. This year, the company is promoting its corn postproduct, Impact, to partner with glyphosate.

"This can improve control of both small- and large-seeded broadleaf weeds and can reduce selection pressure for tolerant and resistant biotypes," he says.

Bayer CropScience is making a big push this year, along with seed partners, to offer more LibertyLink hybrids with big discounts on Liberty herbicide as an alternative to glyphosate-tolerant corn.

"Growers will see more LibertyLink hybrids from more companies this year to help with weed shifts in glyphosate-tolerant crops," says Tim Zurliene, herbicide and fungicide manager for Bayer. "It is now a viable alternative to glyphosate, given its performance, economics and convenience."

How about a show of hands for those who think glyphosate-tolerant crops have made weed control simple, effective and inexpensive?

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