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The Pigweeds Prone to Escape

May 23, 2017

Sound the alarm: The University of Illinois urges growers in the Midwest to rethink their waterhemp and Palmer amaranth management plans. If waterhemp and Palmer amaranth aren’t controlled early in the season, they risk going to seed and passing their herbicide-resistance genes to new generations. Waterhemp has already demonstrated resistance to 6 different herbicide groups.

This agronomic image shows waterhemp.


In 2016, the University of Missouri identified a Palmer amaranth population with resistance to both glyphosate and PPO inhibitors. A major weed of concern in the South, Palmer amaranth is quickly spreading across the Midwest and was identified in Minnesota for the first time in 2016.

This agronomic photo shows palmer amaranth.

Palmer amaranth

If left unchecked, Palmer amaranth has the potential to completely wipe out yield, and its relative, waterhemp, can rob growers of between 42 – 48% of their yield. Because these pigweeds can germinate throughout the season, managing them requires:

  • Extensive scouting
  • A two-pass pre-emergence and post-emergence herbicide program
  • Multiple effective modes of action
  • Mechanical weed removal practices

To tell the two apart, examine the shapes of their seedling leaves and the length of the petioles. The seedling leaves of Palmer amaranth are egg-shaped, with the petiole often longer than the leaf blade, while the waterhemp plants have narrow, lanceolate-shaped leaves.

The first step in managing Palmer amaranth and waterhemp and delaying resistance in your field is boosting your resistant weed knowledge: Take the Weed IQ Quiz and see how you score as a Resistance Fighter®.

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