Palmer Amaranth

Palmer amaranth (pigweed) is the bellwether or index weed for herbicide-resistance. Biotypes that are resistant to four Mechanisms/Modes/Sites of Action (MOA) have been identified. Glyphosate-resistant (GR) Palmer amaranth is of particular concern, and is found throughout the southern soybean production region of the US.

GR Palmer amaranth has created a conundrum for producers. Using glyphosate for weed control in GR soybean has been and continues to be a preferred management tool, but the advent of GR weeds such as Palmer amaranth has reduced its utility. So even though glyphosate is still an effective herbicide for weed management in GR soybeans, its use must be tempered to either prevent or delay the selection for GR weeds resulting from its overuse.

This is a problem that must be managed if profitable soybean production is to continue. Control options depend on using

●          Older non-glyphosate herbicides,

●          Premixes of non-glyphosate herbicides,

●          Premixes of non-glyphosate herbicides and glyphosate, and

●          Liberty herbicide with Liberty Link soybean varieties.

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The below compilation of the best measures for control of GR pigweed in Roundup Ready, Liberty Link, and conventional soybean production systems is from the below sources.

●          A recent article in Crop Management by Ahmed and Holshouser (designated CM in this article; available with free subscription provided by the USB);

●          MAFES Information Sheet 1352 (designated MSU in this article) published by Dr. Tom Eubank;

●          Dr. Eubank’s Annual Report of Results from MSPB-funded weed management research (designated MSPB in this article);

●          The University of Tennessee by Dr. Larry Steckel (designated UT in this article);

●          The University of Arkansas by Drs. Scott and Smith (designated UA in this article); and

●          North Carolina State University by Drs. Everman and York (designated NC in this article).

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From the above sources, the following tenets apply to management systems for control of GR pigweed.

{Click here for a more detailed presentation that includes herbicide programs for the three production systems}.

●          Rotate herbicide chemistry as much as possible. Use the Group numbers shown in parenthesis in the table in the above-linked article to determine the MOA of the listed herbicides and premixes. Rotation of Roundup Ready, Liberty Link, and Conventional herbicide systems will delay the onset of HR weeds if not present, or will help manage populations of HR weeds that are present (MSU & NC).

●          Rotate crops where possible. Including corn in a crop rotation with soybeans is an advantage since atrazine (MOA Group 5)-based weed control programs, which are an effective management tool for controlling pigweed, can be used (NC).

●          A residual herbicide should be part of any weed management program for GR pigweed in all three systems–Roundup Ready, Liberty Link, and Conventioinal soybeans. This reduces the pressure on postemergence (POST)-only management (MSU, NC, & UT).

●          Herbicide combinations such as Authority XL, Authority MTZ, Boundary, Canopy, Envive, Prefix, and Valor XLT that contain at least two MOA’s provide the most consistent and effective control of pigweed (MSPB).

●          POST options in the table in the above-linked article–Flexstar, Flexstar GT, Prefix, Pursuit, Sequence, and Warrant–have residual activity on Palmer amaranth, and their use will enhance in-season control of pigweed (MSU, NC, UA, & UT).

●          Palmer amaranth should be treated POST when no taller than 4 in. Preferably, POST treatments should be applied when pigweed is 2-3 in. tall (MSU and NC).

●          POST applications of herbicide mixes that include glyphosate should be applied to weeds at the recommended time/size for the non-glyphosate herbicide in the mix since that is the component that will control the GR weed (NC).

●          Contact POST herbicides such as Cobra, Flexstar, Flexstar GT, Liberty, and Prefix that are applied to kill pigweed should be applied using flat-fan or twin-jet nozzles and a spray volume of at least 15 gal./acre (MSU and NC).

●          Management tenets for control of pigweed in Liberty Link soybeans are basically the same as those for Roundup Ready soybeans with the following exceptions: Liberty herbicide is used instead of glyphosate (NC).

●          Soybean yield (MSPB) and control of pigweed (MSPB & CM) resulting from use of recommended POST applications of Liberty alone on Liberty Link soybean varieties were as good as those resulting from the use of preemergence (PRE) residual herbicide applications followed by recommended POST applications of Liberty. See next item for resistance management caveat.

●          Do not overuse Liberty since this will increase selection pressure for resistance to this herbicide; i.e., do not depend entirely on Liberty for weed control. Do not exceed two applications of Liberty per year, do not use reduced rates, and treat weeds when they are small. Apply residual herbicides preplant, PRE, or POST as part of the total weed management program with Liberty Link soybeans to prevent possible resistance development to the Liberty class of chemistry (MSPB & NC).

●          There is no economic threshold for pigweed management; thus, near-zero tolerance is required each year. Mechanical removal of pigweed escapes should be done immediately to prevent pollen and/or seed production (UA).

Composed by Larry G. Heatherly, Oct. 2012, larryheatherly@bellsouth.net