Manage resistant weeds in soybeans

LAWRENCE, Kan. – A study featured in the most recent issue of the journal “Weed Technology” shows that properly timed post-emergence herbicides can play an important role in the control of glyphosate-resistant Palmer amaranth and waterhemp in new soybean cropping systems.

Twenty-five states have confirmed populations of glyphosate-resistant Palmer amaranth, while 16 have confirmed populations of glyphosate-resistant waterhemp. Next-generation herbicide-resistant soybeans hitting the market soon will offer growers broader options for managing these resistant weeds, using combinations of glyphosate, glufosinate, dicamba, 2,4-D, isoxaflutole and mesotrione.

Scientists at seven universities decided to explore the effectiveness of various pre-emergence and post-emergence herbicide programs compatible with the new soybean traits. They conducted field experiments in Arkansas, Illinois, Indiana, Missouri, Nebraska and Tennessee. Five of the experiments involved pre-emergence herbicides only. Ten involved pre-emergence treatments followed by a post-emergence treatment applied three to four weeks later or six to seven weeks later.

Results showed pre-emergence herbicide treatments combined with a post-emergence herbicide applied three to four weeks afterward delivered the best results, producing 94 percent or greater control of Palmer amaranth and waterhemp. Scientists also determined that precipitation matters. In situations of little or no rainfall in the weeks following application of residual herbicides, a second application may be needed for season-long control.

Jason Norsworthy, a professor at the University of Arkansas and one of the authors of the study, said that despite the success of a combined preemergence/postemergence herbicide program, growers should adopt an integrated best-practices-based approach to weed management.

“There are already Amaranthus species weed populations resistant to many of the herbicides used in the new soybean herbicide programs,” he said. “For sustainable results over the long haul, it is vital that we rotate herbicide mechanisms of action and incorporate appropriate nonchemical controls as well.”

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