Waterhemp and Palmer amaranth: Concepts for control

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What will your weed control program be for 2013 soybeans? My Dad would say, “You and a weed hook.” However, today there are many alternatives that can be used before manual labor, but for some fields, a crew with hoes and weed hooks may be necessary, where weeds have developed resistance to many herbicides.  

As you attend agronomy meetings this winter, weed specialists will likely have the attention of everyone in the room when they suggest alternatives for controlling waterhemp and Palmer amaranth. Their ability to escape an herbicide shower and reproduce their resistant genes has been a vexing problem for either you or at least someone you know well.


Iowa State University weed specialist Bob Hartzler says late in 2012 farmers responded to a survey at his agronomy meetings about resistance to 5 groups of common herbicides. He says, “In the survey, 60 percent of farmers reported that Group 2 (ALS inhibitors, such as Classic and Pursuit) resistant waterhemp was not present in fields they managed or they were unsure of its presence.

Industry representatives were somewhat more aware of Group 2 (ALS inhibitor) resistance, with 38 percent saying resistance was widespread and 42 percent reporting it was isolated in their territories. Both groups reported that glyphosate resistance was more common than Group 2 resistance.”

However, Hartzler says over 90 percent of waterhemp populations are resistant to ALS inhibitors, where possibly less than 20 percent of waterhemp populations are resistant to glyphosate, but that number is increasing rapidly. His point is that Group 2 herbicides were introduced in the 1980’s, became quite popular and by the mid-1990s Pursuit was used on more that 75 percent of soybeans in Iowa. That resulted in the widespread development of resistance to ALS inhibitors, and was one of the reasons that farmers adopted Roundup Ready beans in nearly wholesale fashion when they were introduced in the mid-1990s. Hartzler says most farmers have forgotten that waterhemp was becoming resistant to Pursuit, and “The loss of this Herbicide Group to manage a major weed problem shows us that similar things could happen with other herbicides if we do not manage weeds appropriately.”

Palmer amaranth

Today’s growing problem is with the spread of Palmer amaranth into the Cornbelt. It has been a problem for farmers in southern states and seeds that are being germinated in Purdue laboratories have found the ability to survive 20lb ae/acre of glyphosate, or the equivalent of 7 gallons per acre of generic glyphosate. 

Purdue weed specialist Bill Johnson says “The initial transport of Palmer amaranth to Northern Indiana is proposed to have been in manure spread on fields from dairy or beef operations that feed cotton seed hulls from the Southern U.S. that were contaminated with Palmer seed. The exact timing of the initial event is unknown, but is estimated to have happened at least two to three years ago due to the severity of infestation in multiple fields. Further spread of Palmer seed has and will occur through farm equipment, specifically combines. Wildlife can also spread the seed into new, previously uninfested fields.”

Johnson says it is typically called pigweed by many who believe it is a red-root pigweed. However, he says red-root pigweed has tiny hairs on the stem and leaves that are not present on either waterhemp or Palmer amaranth. Along with his on-line factsheet about identification, Johnson offers many tips for control:

  • Crop Rotation: Rotation of fields to corn offers additional modes of action that will control Palmer amaranth, and in highly infested fields, producers may want to grow corn for at least two years to maximize Palmer control. Although, caution should be used in order to prevent resistance to corn herbicides.
  • Tillage: Deep tillage with a moldboard plow will not provide complete control, but will reduce the number of seeds that emerge from the top 1” of soil. This should be used in extreme cases, as one pass of a moldboard plow will essentially delete the benefits of years of no-till/minimum tillage practices.
  • Crimped Cereal Rye Cover Crop: A properly managed and crimped cereal rye cover crop can provide a mulch that will suppress Palmer amaranth emergence. No other cover crops have been extensively studied for use as a Palmer amaranth suppressor.
  • Hand Weeding: In severe infestations in Southern U.S. cotton fields, producers have gone to the lengths of hiring hand weeding crews to remove Palmer amaranth. Producers should be willing to scout for late season escapes and pull these plants to avoid any further spread of seed or pollen.
  •  Drainage Ditches and Field Borders: Take care to control Palmer amaranth plants that may exist in ditches and field edges.

Johnson offer extensive suggestions for herbicide programs that will provide pre and post emergence control of Palmer amaranth in corn and soybeans.


Waterhemp has been, and continues to be, a growing problem for Cornbelt farmers due to its resistance to popular herbicides. And moving into the region from the southern states is a cousin, Palmer amaranth that has the same ability to quickly generate resistance. 

Control must be employed as soon as problem populations exist in a field, and that control can be much more than just application of another herbicide. Use of rotations, tillage, cover crops, and hand weeding of fields may be necessary. While there are herbicide programs that can provide control, they must be carefully managed and applied at early growth stages for such weeds.

Source: FarmGate blog

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