20/20 Vision: How to Tell if Weeds Really are Glyphosate-Resistan

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20/20 Vision: How to Tell if Weeds Really are Glyphosate-Resistant

Greensboro, NC (June 2003) - Corn and soybean growers are wise not to 'fly blind' when it comes to glyphosate-resistant weeds. Resistant horseweed (marestail) impacted at least six states in 2002 and, in Tennessee, infested acreage jumped fivefold.

Plant physiologist Dale Shaner of the USDA, weed scientist Tom Bauman of Purdue University and other experts say that those resistant horseweed populations were probably already prone and pre-existing in many fields. That would mean growers in other locales who overuse glyphosate-based herbicides could easily push resistant-prone species out of the normal control zone for glyphosate in their fields, too. "It's just a matter of time," Bauman says.

Although growers have seen herbicide resistance before, this time is different. Overuse of glyphosate-based herbicides threatens the productive value of Roundup Ready* crops - the most successful crop system in the history of American agriculture. Here's what to do to help identify potential tolerance or resistant weeds - before they become a major economic problem.

1. Closely monitor weed species that glyphosate-based herbicides are already variable on beginning with horseweed and waterhemp. Populations of those two species have already been identified as resistant or tolerant to glyphosate in the U.S. Waterhemp is already prevalent across much of the Corn Belt where it has shown variable tolerance to glyphosate, and it can produce 10 times more seed than horseweed. If excessive selection pressure continues, other species could follow, Bauman says. Some other species to watch include eastern black nightshade, ivyleaf morningglory, lambsquarters and giant ragweed.

2. Look for stubborn weeds, unexpected flourishes and species that were not a serious problem before. Glyphosate resistance usually develops gradually, after repeated use over several generations in a plant. At some point, the full-rate is no longer effective on a species - that's when a species is defined as 'tolerant' according to the Weed Science Society of America. If those traits are passed to the next plant generation, then the species is 'resistant.' "Growers should already be scouting at least two or three times a season, regardless of crop," Shaner says.

3. If resistance is indeed developing, applying more glyphosate only accelerates the problem. Shaner says that applying glyphosate-based herbicides alone three to four times a season simply does not make sense. Instead, use one or more alternative herbicides or herbicide mixtures that have different mode of action to establish control - at least until the facts are clear. Syngenta Crop Protection now recommends that corn and soybean growers use no more than two applications of Touchdown* herbicide, Roundup* herbicide or any other glyphosate-based herbicide in a given field during any two-year period.

4. You can't tell if a weed is glyphosate-resistant just by looking at it. Glyphosate failed to control some giant ragweed in southern Illinois last season, but that failure was due to stalk borers in the plants, which interfered with the translocation of glyphosate. So if a weed persists, spend a few minutes to get it checked out. Call the county agent or crop consultant about sampling and testing. Before eradicating questionable weeds, collect a few random samples. Keep samples from each weed population and each field in separate pots with their own soil. Then keep them cool and watered until they are sent for further study.

5. Test results, either way, can help keep control costs (and frustration) in check. Shaner says that controlled greenhouse studies can verify if a problem exists, measure the degree of any tolerance, and indicate whether traits persist into successive generations. The feedback is invaluable for tweaking next year's weed control and resistance management programs. If the problem is not glyphosate resistance, the scientists can often identify what needs addressing. Pinpointing the issue can avoid wasting money on the wrong rate - or the wrong herbicide - with every extra pass next season.