and Prevention Control of Palmer Amaranth in Soybean
David Holshouser, Extension Agronomist Henry Wilson, Extension Weed Scientist Scott Hagood, Extension Weed Scientist
Palmer amaranth (Amaranthus palmeri), a member of the "pigweed" family, is one of the most troublesome weeds in many southern row crops. Seed can germinate all season and plants can grow to over 6 feet in height. Plants have either male flowers that shed pollen or female flowers that can produce up to 600,000 seed per plant. Four Palmer amaranth plants per 100 ft2 of row can reduce soybean yield by 12 to 17%.
In Virginia, the weed has been documented in seven southeastern counties, but will likely enlarge its territory. Its spread can be rapid because of custom harvesting, failing to clean vehicles and equipment after exiting infested fields, and failing to hand remove escapes. More troublesome is that extensive use of herbicides has led to resistance to glyphosate and ALS-inhibiting herbicides. We currently have populations of ALS-resistant Palmer amaranth in Virginia; glyphosate resistance to the weed was observed as close as Gates County, North Carolina. Palmer amaranth is threatening Virginia's cropping systems. Aggressive steps are necessary to minimize its spread and delay the development of herbicide resistance. No longer can Virginia growers settle for the easiest, least expensive, or short-term (single-season) control strategies. Soybean weed management must quickly become a carefully planned and integrated control program that preserves herbicide and genetic technologies. For effective and long-lasting Palmer amaranth control, one needs to look 3 to 5 years into the future and not focus solely on herbicides and varieties. Long-term management of Palmer amaranth requires a multiple tactic approach that includes: integrating crop and herbicide rotation, diversifying in-season herbicides, closely monitoring fields, completely controlling the weed in rotational crops, cleaning harvest and tillage equipment, and removing escapes before seed production. Of the tactics listed, integrating herbicides of different mechanism of actions will have the greatest benefit within the soybean growing season (see table 1). This can be accomplished by including a soil residual herbicide with the burndown application, using soil-applied residual herbicides at planting, and/or tank-mixing postemergence herbicides. Suggestions for controlling Palmer amaranth in Roundup Ready and conventional soybean are listed in Tables 1 and 2, respectively.
2008
Virginia Polytechnic Institute and State University
2808-1006
Virginia Cooperative Extension programs and employment are open to all, regardless of race, color, national origin, sex, religion, age, disability, political beliefs, sexual orientation, or marital or family status. An equal opportunity/affirmative action employer. Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Mark A. McCann, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; Alma C. Hobbs, Administrator, 1890 Extension Program, Virginia State, Petersburg.
Table 1. Herbicide programs for Palmer amaranth control in Roundup Ready soybean varieties1
Glyphosate ALS Infestation Resistant Resistant Level No Light Preplant Burndown2 Glyphosate or Gramoxone Inteon + 2,4-D or Clarity Preemergence3 Postemergence Application #14 Glyphosate + Dual Magnum or Intrro Glyphosate + Synchrony XP (1/2 rate), Classic, Pursuit, Raptor, or Scepter Glyphosate + Cobra, Reflex, Flexstar, Ultra Blazer, or Storm Sequence (glyphosate + Dual Magnum) Extreme (glyphosate + Pursuit) Glyphosate Postemergence Application #24 Glyphosate
Heavy
Glyphosate or Gramoxone Inteon + 2,4-D or Clarity
Single MOA Cinch, Dual Magnum, Intrro, Outlook, or Prowl Linex or Sencor FirstRate, Pursuit, Scepter, or Synchrony XP Spartan or Valor SX Multiple MOA Authority MTZ, Authority Assist, Authority First, or Sonic Canopy Prefix Squadron Valor XLT or Envive Single MOA Glyphosate + Synchrony XP (1/2 rate), Classic, Cinch, Dual Magnum, Intrro, Outlook, or Prowl Pursuit, Raptor, or Scepter Glyphosate + Cobra, Multiple MOA Reflex, Flexstar, Ultra Prefix Blazer, or Storm Extreme (glyphosate + Pursuit) Single MOA Glyphosate + Dual Magnum or Intrro Prowl Sequence (glyphosate + Linex or Sencor Dual Magnum) FirstRate, Pursuit, Scepter, or Synchrony XP Spartan or Valor SX Multiple MOA Authority MTZ, Authority Assist, Authority First, or Sonic Canopy Squadron Valor XLT or Envive Glyphosate + Dual Magnum or Intrro Glyphosate + Cobra, Reflex/Flexstar, Ultra Blazer, or Storm Sequence (glyphosate + Dual Magnum) Glyphosate
No
Yes
Light
Glyphosate or Gramoxone Inteon + 2,4-D or Clarity
Table 1. Herbicide programs for Palmer amaranth control in Roundup Ready soybean varieties1
Glyphosate ALS Infestation Resistant Resistant Level Preplant Burndown2 Preemergence3 Single MOA Cinch, Dual Magnum, Intrro, Outlook, or Prowl Linex or Sencor Spartan or Valor SX Multiple MOA Authority MTZ Prefix Single MOA Cinch, Dual Magnum, Intrro, Outlook, or Prowl Multiple MOA Prefix Single MOA Prowl Linex or Sencor Spartan or Valor SX Multiple MOA Authority MTZ Single MOA Cinch, Dual Magnum, Intrro, Outlook, Prowl or Linex or Sencor Spartan or Valor SX Multiple MOA Authority MTZ Prefix Postemergence Application #14 Glyphosate Postemergence Application #24
Heavy
Glyphosate or Gramoxone Inteon + 2,4-D or Clarity
Glyphosate + Cobra, Reflex, Flexstar, Ultra Blazer, or Storm Glyphosate + Dual Magnum or Intrro Sequence (glyphosate + Dual Magnum) Glyphosate + Cobra, Reflex/Flexstar, Ultra Blazer, or Storm Classic, Pursuit, Raptor, Scepter, or Synchrony (STS varieties only) Reflex, Flexstar, Ultra Blazer, or Storm
Yes
No
Light or Heavy
Glyphosate or Gramoxone Inteon + 2,4-D or Clarity
Yes
Yes
Light or Heavy
Glyphosate or Gramoxone Inteon + 2,4-D or Clarity
Glyphosate + Synchrony XP (1/2 rate) , Classic, Pursuit, Raptor, or Scepter Extreme (glyphosate + Pursuit) Cinch, Dual Magnum, Glyphosate + Cobra, Intrro, Outlook, or Prowl Reflex/Flexstar, Ultra + Linex or Sencor Blazer, or Storm Prowl + Linex, Sencor, No emerged Palmer Spartan, Valor SX, or Glyphosate + Dual Authority MTZ Magnum or Intrro Sequence (glyphosate + Dual Magnum)
Reflex, Flexstar, Ultra Blazer, or Storm Reflex, Flexstar, Ultra Blazer, or Storm
1
Following label instructions for rates, adjuvants, timing, and rotational restrictions. Choice of herbicide program may depend on other weeds present in the field. Herbicide options are generally grouped by mechanism of action. Make only one application of ALS-inhibiting containing herbicide and no more than two applications of PPO-inhibiting herbicides per season. For heavy infestation levels, conservation tillage with heavy residue cover crop has shown to improve control of Palmer amaranth. 2 For full-season plantings; 2,4-D 1.0 pt/A, 15 days prior to planting or 1 qt/A, 30 days prior to planting; Clarity 8 oz/A, 1 inch of rainfall + 14 days prior to planting; Valor may be added under heavy infestation levels and substituted for a preemergence PPO herbicide. 3 MOA mechanism of action. Include glyphosate or Gramoxone Inteon if weeds are present. 4 Make postemergence herbicide applications before Palmer amaranth reaches 3 inches. Make a 2nd postemergence application only if Palmer amaranth continues to emerge.
3
Table 2. Herbicide programs for Palmer amaranth control in conventional (non-Roundup Ready) varieties1.
Glyphosate ALS Infestation Resistant Resistant Level Yes or No No Light Preplant Burndown2 Glyphosate or Gramoxone Inteon + 2,4-D or Clarity Preemergence3 Single MOA Cinch, Dual Magnum, Intrro, Outlook, or Prowl Linex or Sencor Spartan or Valor SX Multiple MOA Authority MTZ Postemergence Application #14 Cobra, Flexstar, Reflex, Ultra Blazer, or Storm Postemergence Application #24 Classic, Pursuit, Raptor, Scepter, or Synchrony (STS varieties only)
Single MOA Cobra, Flexstar, FirstRate, Pursuit, Scepter, or Reflex, Ultra Blazer, Synchrony XP or Storm Multiple MOA Authority First, Authority Assist, Envive, or Sonic Canopy Heavy Glyphosate or Gramoxone Inteon + 2,4-D or Clarity Cinch, Dual Magnum, Cobra, Flexstar, Intrro, Outlook, or Prowl + Reflex, Ultra Blazer, Linex, Sencor, Spartan, Valor or Storm SX, or Authority MTZ Prefix Cinch, Dual Magnum, Intrro, Outlook, or Prowl + FirstRate, Pursuit, Scepter, Synchrony XP, or Canopy S...