Monsanto reports that there's a glyphosate resistant population of common agweed in south central South Dakota.
Tests have been underway since the fall of 2007, the company announced in a press release.
Field and greenhouse studies confirm that the population is resistant. "The population of common ragweed that has been identified as resistant is a case where we are working with academics in evaluating and recommending the most effective methods to control weeds in Roundup Ready cropping systems, including the no-till systems used in South Dakota and other states," says Jennifer Ralston, Roundup technical manager.
"We want to make sure farmers have the information to choose the weed control options that will minimize the risk of selecting for glyphosate resistance and also control this common ragweed population," says Ralston. "One option farmers should consider is the use of a pre-emergence residual herbicide in both corn and soybean fields to help manage early season weed control and reduce the selection pressure brought on when you use a single over the top product."
If you are experiencing poor control of common ragweed in corn or soybeans with Roundup Ready technology, Monsanto recommends:
In Roundup Ready® soybeans – Use a pre-emergence residual product at planting such as FirstRate® or Gangster®. Tank mix Roundup WeatherMAX or Roundup PowerMAX with Cobra®, Flexstar®, or FirstRate® if post-emergence common ragweed control is needed.
In Roundup Ready Corn® 2 – Use a pre-emergence residual product containing atrazine, such as Harness Xtra or Degree Xtra at planting. Tank mix Roundup WeatherMAX or Roundup PowerMAX with 2,4-D or dicamba (Clarity, Status, Banvel or Distinct) if post-emergence common ragweed control is needed.
Follow all label directions for appropriate weed height limits, use rates and other restrictions. Ralston also recommends considering the benefits of a corn-soybean rotation, which include the ability to use different herbicide programs, practicing alternative cultural practices and disruption of disease and insect pressure.
"Knowing your fields and the weeds present helps growers adopt the optimal weed control program. It allows you to confirm that you are using the right herbicides and rates to get control," she adds. "We find that making changes in chemistry by crop rotation is a good way to achieve effective control of many weed species."
For more information on weed management recommendations for Roundup Ready crops, please visit www.weedresistancemanagement.com. You can also visit www.weedtool.com to evaluate the risk of selecting for resistant weeds on a field-by-field basis, where you'll receive best practices and tips to minimize the risk based on your individual field practices.