The biology and management of Palmer amaranth (Amaranthus palmeri S. Wats) in Indiana

Douglas J Spaunhorst, Purdue University


Palmer amaranth is a pernicious summer annual weed that has evolved resistance to six herbicide sites of action in the U.S. and threatens agriculture production. In 2011, Palmer amaranth was identified in flood plains in two southern Indiana counties (Posey and Vanderburgh). Determining if Palmer amaranth can survive and reproduce in northern Indiana may provide insight if this weed will be problematic for northern row crop producers. The objectives of our research were to identify fields containing Palmer amaranth and determine the distribution of herbicide resistance traits in Indiana; determine the influence of tillage frequency and tillage intervals on Palmer amaranth emergence throughout the growing season; evaluate biological responses of five Palmer amaranth accessions collected from the Midwest and south US that were established in Indiana; evaluate future herbicide programs (Enlist®, Extend®, and Balance Bean®) for control of glyphosate-resistant Palmer amaranth; determine the effect of cover crops on Palmer amaranth control in glyphosate- and glufosinate-resistant soybean; and evaluate the effect of single and multiple herbicide combinations for control of multiple-resistant Palmer amaranth. Results from these experiments suggest that Palmer amaranth has infested numerous Indiana fields throughout the state and >80% of accessions containing resistance to glyphosate. Reducing the frequency of tillage lowered seed recruitment to the germination zone where Palmer amaranth readily emerges. Palmer amaranth seed introduced to Indiana from other geographies that germinate will complete their life cycle if emergence occurs by mid-July. Future herbicide-resistant traited technologies mixed with one or more additional preemergence herbicides were more effective than applying herbicides with a single active ingredient. Soybean yield is compromised by Palmer amaranth competition without the use of residual preemergence and overlapping preemergence residual plus postemergence herbicides. Cereal rye and annual ryegrass cover crops were not detrimental or beneficial for control of Palmer amaranth in Indiana. Palmer amaranth biotypes with resistance to multiple herbicide sites of action will require alternative control methods to herbicides and threatens future use of herbicide chemistries to which resistance has been selected for. Overall, a multifaceted approach is needed for long-term management of Palmer amaranth in Indiana.




Johnson, Purdue University.

Subject Area

Agronomy|Plant biology|Agriculture

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