Recently Dr. Phil Stahlman, Kansas State University weed scientist at the university research center in Hays, Kan., confirmed that there are kochia populations resistant to glyphosate in Kansas, demonstrated in both greenhouse and in-field studies.
Over the winter Owen began advising growers to implement resistance management practices as early as 2000, cautioning about the possible development of glyphosate-resistant waterhemp. University experts in Iowa were some of the first to discuss glyphosate-resistant weeds and their potential impact.
“Unfortunately, predictions about glyphosate resistance have become reality,” says Chuck Foresman, manager of weed resistance strategies for Syngenta Crop Protection. “Glyphosate-resistant weeds currently affect an estimated 8.8 million U.S. row crop acres.”
All three of these recently confirmed glyphosate-resistant weeds can dramatically cut yields. Dr. Mike Owen, Iowa State University weed scientist, confirmed that Iowa growers have to deal with glyphosate-resistant common waterhemp and giant ragweed. In-field trials showed that resistance to glyphosate has developed in these weeds, although resistance isn’t yet widespread in the state.
* Giant Ragweed: According to Glyphosate, Weeds, and Crops Group research, season-long competition from just two giant ragweed plants per 110 ft2 can reduce corn yield by 13%, and in soybeans, just one plant in that same area can reduce yield by 50%.
* Kochia: In corn, just one uncontrolled kochia per square yard (about the area of a hula-hoop) can cut yield nearly 10%. In soybean, that same weed can cost 33% of your yield, according to www.WeedSOFT.org.
* Waterhemp: Early-season competition from heavy infestations of waterhemp can reduce corn yields 15% by the time weeds are 6 inches tall, and season-long competition can reduce soybean yields 44% in 30-inch rows.
But there are residual herbicide options for control. “Residual herbicides with multiple modes of action such as Prefix® pre-emergence or Flexstar® GT post-emergence in soybeans and Lumax®, Lexar® or Halex® GT in corn are great options to manage giant ragweed and waterhemp,” says Foresman. “To control kochia in corn, a pre-emergence application of a full rate of Lumax or a post-emergence application of Halex GT plus AAtrex® herbicide is effective. Note that if emergence kochia is present for the pre-emergence Lumax application, we recommend adding an effective burndown to the tank.”
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