Database Research Summaries
2018 Investigation of Multiple Herbicide Resistant Palmer Amaranth and Waterhemp

calendar_today Year of Research: 2018
update Posted On: 12/04/2019
group J.D Green (Principal Investigator, University of Kentucky), Erin Haramoto (Co-Investigator, University of Kentucky), Travis Legleiter (Co-Investigator, University of Kentucky)
bookmark Kentucky Soybean Promotion Board

Research Focus

The focus of this project is to enable soybean producers and crop consultants to make better decisions when employing weed control programs for managing these and other herbicide resistant weeds.


  • Conduct field research on multiple herbicide resistant populations of Palmer amaranth/waterhemp for developing best management strategies.
  • Conduct greenhouse studies to evaluate the level of herbicide resistance in each population.
  • Conduct plant genetic analysis and laboratory procedures to characterize the heritability of the mechanism for resistance to glyphosate and PPO inhibitor herbicides within and between a mixed population of Palmer amaranth and waterhemp.


  1. Soil residual (PRE) herbicide treatments applied at planting that contained three-way mixtures of pyroxasulfone + flumioxazin + chlorimuron (ie. Fierce XLT) or S-metolachlor + fomesafen + metribuzin (ie. Intimidator) provided the best early season control (>80%) of Palmer amaranth and/or waterhemp.
  2. Soil-applied herbicide products with S-metolachlor + metribuzin (ie. Boundary) and S-metolachlor + sulfrentrazone (ie. Broadaxe XC) were less effective.
  3. When an effective soil residual herbicide such as Fierce XLT or Intimidator was followed by a timely postemergence (POST) herbicide treatment >80% control was maintained through mid season.
  4. Postemergence treatments containing dicamba (ie. Xtendimax) + glyphosate, 2,4-D + glyphosate (ie. Enlist Duo), and glufosinate (ie. Liberty) following a soil-applied treatment at planting achieved the most effective season-long control.
  5. These results indicate that herbicide treatments with PRE/POST combinations with 3 to 4 sites of herbicidal activity are best for combating herbicide resistant populations of Palmer amaranth and waterhemp.


  • Soybean growers and crop advisors will become more aware of the impact of herbicide resistant weeds, particularly weeds expressing resistance to multiple herbicidal sites of action.
  • As a result they should be in a better position to make informed decisions for developing and implementing weed control programs to effectively mitigate and manage herbicide resistant weeds.

For more information about this research project, please visit the National Soybean Checkoff Research Database.

Funded in part by the soybean checkoff.