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.”