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Purdue News
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January 30, 2004 Purdue expert advises on strategy for subduing superweedsWEST LAFAYETTE, Ind. - A Purdue University weed expert said the best way to combat so-called "superweeds" is to get at them before they get super tall. Bill Johnson, Purdue Extension weed specialist, said there are some 300 varieties of herbicide-resistant weeds worldwide. On the top 10 list of worst varieties, all but two are found in Indiana. "There are many reasons why herbicide resistance is a growing concern," Johnson said. "Among them the widespread adoption of Roundup Ready soybeans, increased use of no-till practices and the lack of cheap herbicide alternatives to glyphosate (aka Roundup)." While a few herbicide-resistant weeds can be found in any field, Johnson said farmers in southeast Indiana are facing the most problems with "superweeds." "We've found glyphosate-resistant horseweed in Bartholomew, Clark, Jackson and Washington counties," he said. Resistant horseweed also is suspected in the nearby counties of Jefferson, Jennings, Scott, Shelby, Decatur and Ripley. Johnson said using more than one herbicide will help control and minimize spread of glyphosate-resistant weeds. "For no-till fields, we suggest combining the herbicide 2,4-D in your initial burndown application of Roundup. As of yet, no horseweed has been found with resistance to 2,4-D," he said. "Farmers should also spray weeds before they get more than 4 to 6 inches tall." Johnson said tilling the soil also helps wipe out stubborn weeds. However, the disadvantages to tilling are increased soil runoff and the added expense of plowing the field. "It's cheaper to run a sprayer over the land, then to drag a plow through it," he said. Purdue weed experts are studying glyphosate resistance as the problem seems to move across the state. Johnson said as more data is collected, the goal is to develop a model to predict the risk of glyphosate resistance spreading into new areas. In the meantime, farmers should check their fields in early spring for weed development. "Look for emerging weeds in March, and then plan to spray in April," Johnson said. "If the weeds get too big, or re-emerge after planting soybeans, then it's very difficult to control them." Johnson said not all weeds that survive the first application of herbicide should be considered resistant. "It could be that the application rate was too low, the spray coverage was spotty or the environmental conditions were poor," he said. One of the reasons why horseweed is such a difficult weed - and number nine on the top 10 list of worst weeds - is because of the timing of its emergence. "Horseweed traditionally sprouts in the fall, overwinters, and then produces seed in the spring and summer," he said. "The wind can widely disseminate the seed, and any seed coming from already herbicide-resistant plants will produce more resistant weeds." Johnson said a field that had only minor problems with glyphosate-resistant horseweed one year can quickly be overcome with the troublesome weed the next. More information about weed management can be found in the 2004 Weed Control Guide for Ohio and Indiana. Writer: Beth Forbes, (765) 494-2722; forbes@purdue.edu
Source: Bill Johnson, (765) 494-4656; wgjohnso@purdue.edu Ag Communications: (765) 494-2722; Beth Forbes, bforbes@aes.purdue.edu
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