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Glyphosate-resistant weeds cause management challengesBy Jennifer Bremer
With new advances in crop hybrids and weed control methods, also comes the problem of weeds becoming resistant to chemicals. University of Missouri weed scientist Kevin Bradley said the first glyphosate-resistant weeds were seen in 1998. Glyphosate is a broad-spectrum, non-selective systemic herbicide. It is useful on essentially all annual and perennial plants including grasses, sedges, broad-leaved weeds and woody plants. It can be used on non-cropland and among a great variety of crops. Since the first glyphosate-resistant weeds were discovered in California, different resistant weeds have been found all across the nation. "Some of the most common problems are with horseweed, ragweed and waterhemp, but there continues to be more and more weeds becoming glyphosate-resistant," said Bradley. He said weed resistance exists within a natural population and repeated applications of herbicides with the same mode of action may influence the natural selection for the resistant weed biotype. Eventually those weeds then become homogenous for resistance, passing it on to the future generations. "With 500,000 seeds per plant on some weeds, it doesn't take long to replicate the problem," he said. "The continuous use of glyphosate-resistant crops in a glyphosate system has led to these weed problems," he continued. Bradley has done several research trials on these resistant weeds, applying glyphosate at extreme levels--up to six pounds per acre. He found no matter what the level of dosage is, the weeds are still resistant. He said the only way to deal with these resistant weeds may be by going back to using conventional cultivation methods. Another approach is the use of pre-emergence herbicides. "Pre-emergence programs seem to help increase net income and control weeds better," he said. "Even though it might not kill off all the resistant weeds, it still controls weeds better." Pollen has a great effect on the transfer of a resistant trait, especially in waterhemp. Waterhemp pollen can travel as far as a half mile, according to Bradley. Since nearly all farmers are now using a glyphosate system, he said, there is no way to go back. "We have to figure out how to manage these weeds which have become glyphosate-resistant," Bradley said. "Farmers want to farm more acres with less equipment and less labor and have been spoiled by the simplicity of the system. We won't be able to do things the same, if we are going to get rid of the resistance." There is currently a cooperative effort among 16 different universities and 24 weed scientists to learn how to manage glyphosate-resistant weeds. For more information, visit www.glyphosateweedscrops.org. Jennifer Bremer can be reached by phone at 515-833-2120 or by e-mail at jbremermaj@hotmail.com. 10/27/08 Date: 10/22/08 Advertisement
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