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Corn+Soybean Digest

Weed Revolt Marches On | Glyphosate-Resistant Weed Population Grows Rapidly


Your herbicide plans for 2012 should not include glyphosate use alone, because the weeds are winning. Weed scientists continue to see glyphosate’s effectiveness falter.

“2011 was a breakout year for glyphosate weed resistance expansion. I don’t see that train wreck slowing down at all," says Mike Owen, Iowa State University Extension weed specialist. "So far, three glyphosate-resistant weeds (waterhemp, horseweed and giant ragweed) have been confirmed in Iowa. In the U.S., 13 weeds are currently resistant to glyphosate, and there are 21 weeds resistant to glyphosate worldwide.”

Yet, glyphosate isn’t the only herbicide for which weeds are now evolving resistance, says Aaron Hager, University of Illinois Extension weed scientist. “The herbicides to be careful with for the future include HPPD-inhibitors in corn,” he says. “We first found a waterhemp population resistant to it in a seed-corn field in 2009. HPPD-inhibitor weed resistance is not yet widespread in Illinois, but it might be sometime soon if farmers aren’t careful in their management of it.”

Iowa confirmed a case of HPPD-resistant waterhemp in 2009, and Nebraska found HPPD-resistant waterhemp in December, Owen says. “We’re also seeing more PPO-resistant weed populations in Iowa, but not at the rate we’re seeing with glyphosate and HPPD- and ALS-inhibitor herbicides,” he adds.

Whatever herbicides farmers decide to use, they need to be more careful in how much they rely on them, Hager adds. “There is really no one product that has to be preserved more than others, because we’re going to need them all,” he emphasizes. “The products we have right now are what we’re going to have for the near future, if not longer, and one herbicide family (ALS inhibitors) is already completely gone for use on waterhemp control, due to resistance following overuse.”

Ultimately, the best way to manage herbicide-resistant weeds is to never get them. “Unfortunately, that horse has already left the barn, whether farmers admit it or not,” Hager adds. “Some areas are already going back to using tillage as the solution to herbicide resistant weeds. This is certainly the case for some areas of the south that need to do this to manage palmer amaranth.”

The key thing farmers should keep in mind about herbicide resistance is its inevitability. “Seed movement can take place and – with palmer amaranth and waterhemp – many types of resistance can move with the pollen,” he points out. “So, don’t assume it’s not going to happen on your farm, and do all you can now to prolong the effectiveness of the current chemistry that’s available to you.”

Herbicide resistance is nothing new. In Illinois, problems for row crops began with triazine-resistance in common lambsquarters, notes Hager. “The most widespread resistance that we have now is to ALS-inhibitors, but we also have three weeds that are resistant to glyphosate in Illinois (waterhemp, horseweed and palmer amaranth),” he adds.

Herbicide-resistant weeds like waterhemp first began appearing in southern states and have been gradually making their way north, notes Jeff Gunsolus, University of Minnesota Extension weed scientist. “During the 1970s and 1980s, waterhemp was not yet a significant weed anywhere in the Midwest,” he says. “Now it’s one of our worst weeds.”

In the early 1990s, new ALS-inhibitor postemergence herbicides like Pursuit and Accent helped farmers to move away from soil-applied residual products like atrazine and Treflan and to adequately manage weed control after planting, says Gunsolus. “With these ALS-inhibitor products, farmers could plant first and worry about weed control later,” he says.

However, too much reliance on the ALS-inhibitor herbicides allowed weeds to evolve resistance, explains Gunsolus. Herbicide resistance evolved again after glyphosate was introduced to the market and overused, he adds. “The reason farmers liked glyphosate was that it was effective enough as a postemergence product that they could decouple their planting date from their spray date, which lead to greater efficiency and the ability to farm more acres.”

Many Iowa fields already have weed populations that are resistant to multiple herbicides and herbicide types, points out Owen. “In southeast Iowa, waterhemp populations have been confirmed to be resistant to ALS- and HPPD-inhibitor herbicides, glyphosate and atrazine,” he says. “In central Iowa, waterhemp populations have been confirmed to be resistant to ALS- and HPPD-inhibitor herbicides and glyphosate.”


Limit future problems


To limit future problems with herbicide-resistant weeds, Gunsolus advises farmers to return to using a pre-emergence herbicide with residual chemistry to control early weed growth.

“The pre-emergence application will make your postemergence weed control better, by giving you a little wider window of application,” he says. “Plus, the weed density will be less when you apply postemergence products, which helps provide better herbicide contact to the weeds that are up. Particularly in soybeans, you can get better chemical diversity by using a pre-emergence herbicide with soil residual chemistry than you can by solely using postemergence products.”

However, the biggest strategy with herbicide resistance management is to use crop rotation to provide greater diversity in weed management chemistry, advises Gunsolus. “You really need to take full advantage of the different herbicides available to you for each crop, and don’t keep using the same product on crop after crop, year after year,” he says. “I also recommend that farmers try to limit their glyphosate use for any crop where an effective herbicide alternative to glyphosate exists. It’s best to target glyphosate use in those crops where its weed control is of greatest value to you.

“Also, it’s crucial that your postemergence timing occurs on 2-3 in. weeds,” he adds. “Larger weeds are more apt to survive a postemergence application and develop resistance.”



Top 5 Strategies to Slow Resistant Weeds

Like most potential problems, it’s best to be pro-active rather than reactive to herbicide resistant weed threats, says Mike Owen, Iowa State University Extension weed specialist.

“The increase in the number of herbicide-resistant weeds is partly a function of the grower not believing it’s a problem until it’s already a train wreck,” he says. “By then, it’s too late.”

Rather than waiting for a train wreck to occur with resistant weeds overrunning your fields, Owen provides the following five strategies to head off a future crisis:

1. Admit there is a potential for disaster from herbicide overuse and take steps now to avert it.

2. Whatever you’ve done previously, don’t repeat again any time soon.

3. Use a pre-emergence residual herbicide to control early-emerging weeds.

4. Inclusion of some form of mechanical weed control is now absolutely necessary.

5. How you use postemergence and pre-emergence herbicides has to change. “In other words, don’t use a single mode of action (MOA) as your only weed control tool,” he says. “Don’t repeat the same herbicide MOA when applying a follow-up application.”

Although rotating the herbicide MOA is a good strategy, “rotation of the MOA will only delay the inevitable,” says Owen. “Every year you rotate a MOA, you get one more year’s use from it.”

Rotating MOA is just one strategy that helps, not the ultimate solution, he adds. “You don’t want to use any one tactic exclusively in a crop,” says Owen. “Instead, you need a diversity of tactics in every crop.” 

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