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Spray-and-go-away strategy leads astray

For years, relying on postemergence applications of glyphosate on Roundup Ready crops remedied weed worries. Currently, though, the spray-and-go-away approach isn't working.

“In a majority of fields today, simplicity and convenience aren't working,” says Mike Owen, Iowa State University Extension weed specialist.

The Roundup Ready system changed agriculture's dynamics. “It has supported conservation and no-till,” Owen says. “It has cut down on a number of trips across the field, it has allowed us to effectively manage time, and it's allowed the expansion of enterprises that farmers have.”

Coming to an end

Over time, though, certain weeds developed resistance to glyphosate. Weed resistance isn't just limited to glyphosate. A good example is waterhemp. Thus far, waterhemp has developed resistance to six herbicide modes of action, and it isn't done yet. Repeated use of any herbicide results in weed resistance.

That's something to keep in mind the next time you read about new herbicide-tolerant systems being developed. True, new systems coming on the market that tolerate 2,4-D and HPPD inhibitors will give farmers new weed-management tools to join those technologies that resist glyphosate (Roundup) and glufosinate (Liberty).

“People will say, ‘We have these new tools coming,’ ” says Pat Tranel, a University of Illinois (U of I) weed scientist. “By the time they arrive on the market, waterhemp will already have developed resistance to these tools and to more modes of action.”

It's also important to recognize that herbicide-tolerant systems work differently. Take glufosinate and glyphosate-tolerant systems. Other than the fact that both herbicides used in these systems start with G and they're nonselective, glufosinate and glyphosate differ.

Glufosinate is a contact herbicide; glyphosate is systemic. Although not recommended, glyphosate can be applied later to taller weeds. Not so with glufosinate (now back to the brand name Liberty after a few years of being called Ignite).

“Coverage with glufosinate is crucial,” says Aaron Hager, U of I Extension weed specialist. “You have to apply the first application when weeds are at the 3- to 5-inch stage.”

He says including a residual preemergence is a good way to set weeds up for a later postemergence application of Liberty.

What to do?

Weed specialists like Owen, Tranel, and Hager recommend a diverse strategy including:

● Rotating herbicide modes of action.

● Including a preemergence herbicide in front of postemergence applications.

● Rotating crops.

● Tilling, including cultivation on severe herbicide-resistant infestations.

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