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Take a proactive approach to managing Palmer amaranth in Minnesota crop production fields

Jeff Gunsolus, Extension weed scientist

Palmer amaranth in Yellow Medicine
Early detection and eradication of Palmer amaranth will pay you dividends in reduced management costs. Differentiating Palmer amaranth from the other common amaranth species is challenging. However, because Palmer amaranth and tall waterhemp are biologically similar, you can approach this challenge with the weed management tactics that you would use for effective tall waterhemp control.

Steps to successful management of Palmer amaranth and tall waterhemp:
  1. Identify and report Palmer amaranth to ensure proper management.
  2. Determine if they are herbicide-resistant.
  3. Prevent seed production.
  4. Use residual herbicides in both corn and soybean for effective control. This is essential.

Identification and reporting process for Palmer amaranth

Prevention of Palmer amaranth infestations is the goal of the partnership between the U of MN Extension, Minnesota Department of Agriculture and the farmer and crop consultant relationship. Early detection and eradication of Palmer amaranth will reduce farmer management costs, help to determine primary means of seed movement and prevent the rapid spread of this challenging weed.

Detecting and reporting Palmer amaranth in Minnesota: Timeliness is key outlines identification and reporting procedures.

Are the Palmer amaranth plants found in Minnesota expressing herbicide resistance?

Figure 1. Percentage of 86 tall waterhemp
plants resistant or sensitive to PPO
and/or glyphosate herbicides.
In the six counties where Palmer amaranth has been detected only plants from Jackson and Redwood counties tested positive for glyphosate resistance (Group 9). None of the populations tested positive for PPO resistance (Group 14 – e.g. Flexstar and Cobra).

Tall waterhemp populations are more likely to express resistance to multiple herbicides. Figure 1 shows the results of PPO and glyphosate resistance testing for 86 plants from west central and southern Minnesota. The largest portion of tall waterhemp plants was resistant to both PPO and glyphosate herbicides (43 percent), reinforcing the idea that this is a serious and prevalent issue.

In addition, the majority of tall waterhemp populations are historically resistant to group 2 herbicides (ALS – e.g. Pursuit).

Do not let these plants go to seed

Plants can adapt quickly to herbicides

Palmer amaranth and tall waterhemp resistance to various groups of herbicides (e.g. glyphosate –group 9, Pursuit – group 2 and Flexstar – group 14) brings uncertainty about which herbicides will be most effective in any given field. Molecular tests for group 9 and 14 herbicides are readily available through the University of Illinois Plant Clinic for $50 per sample.

Pollen can transfer herbicide resistance traits

Pollen from these weeds has been shown to transfer herbicide resistant traits to nearby plants of the same species. Therefore your weed management goals should not only focus on reducing weed impact on crop yield but just as importantly, to reduce weed seed production that increases the amount of seed in the soil seed bank.

Plants produce large amounts of seed

Palmer amaranth and tall waterhemp are capable of producing hundreds of thousands of small seeds that can be easily transported via field equipment, water, wildlife and manure. Fortunately if Palmer amaranth and tall waterhemp seed production is eliminated, seed already present in the soil can be reduced to much more manageable levels in just four to five years. The bottom line is, annual herbicide costs can easily double and multiple passes through a field will be necessary once either of these weed species becomes established. Setting a goal of zero weed seed production for the next four to five years can reap long term dividends. Prevention of establishment is still the best option.

Influence of weed germination, duration of emergence and growth rate on management tactics

Use effective soil-residual herbicides

Both Palmer amaranth and tall waterhemp have high germination rates and emergence patterns that extend well into July. Application of a full rate (based on label recommendations for soil texture and organic matter) of effective soil-residual herbicides applied preemergence is essential. Note that after Palmer amaranth emerges from the soil it has a rapid growth rate of up to 2 to 3 inches per day. This growth rate makes timely postemergence applications challenging, especially when application should occur before weeds reach 3 inches in height. The odds of effectively controlling either amaranth species by relying solely on postemergence weed control is not in your favor.

An effective preemergence residual herbicide program followed by an effective postemergence herbicide program is essential for control of both Palmer amaranth and tall waterhemp.

Include effective postemergence herbicides in your program

High germination rates create the potential for a large weed flush that can overwhelm even a highly effective preemergence or postemergence herbicide. A long duration of weed emergence means that multiple passes in the field will be necessary to control fields with a high weed density. Including a soil-residual group 15 herbicide in your postemergence program can extend the control of late-emerging weeds and allow the crop to gain a competitive advantage. Fortunately these two species are effectively controlled by the same herbicides, unless the biotype is resistant to a particular class of herbicide (i.e. glyphosate resistance is common).

Addressing the uncertainties created by herbicide resistant weed populations

Management in soybean

Regardless of which amaranth species is present, herbicide resistance limits your herbicide choices, especially in soybean. Cultural practices that shift the competitive advantage to the crop are fundamental. Early canopy closure is beneficial in limiting the impact of late-emerging weeds. In Minnesota, the impact of iron chlorosis (IDC) and soybean cyst nematodes (SCN) on slowing soybean canopy closure is significant as an open canopy results in more late-emerging weeds. Please consider IDC and SCN tolerance traits during your seed selection process.

Effective soil-applied residual herbicides are essential. Group 14 herbicides such as Valor and Fierce (Group 14 & 15) or Authority (Group 14)-based package mixtures (e.g Authority MTZ (Group 14 & 5), Authority First and Sonic (Group 14 & 2) provide a good start to an effective management strategy. Group 15 herbicides (e.g. Dual II Magnum, Warrant, Outlook, or Zidua) also provide good initial control but may be best utilized as tank mixtures with the postemergence application (i.e. herbicide layering) for extended control late into the growing season – Got waterhemp? Layer residual herbicides to maintain control.

Soybean postemergence options are limiting due to herbicide resistance, inconsistency of control, and label restrictions. As glyphosate resistance becomes more prevalent Liberty (Group 10 for use only in LibertyLink soybeans) and Group 14 PPO herbicides (e.g. Cobra, Flexstar, Ultra Blazer) offer effective alternatives when applied to small, 3 inches or less, weeds. However, adjustments such as increased gallon per acre rate and finer spray droplets are necessary for complete coverage with these contact herbicides.

Xtendimax with VaporGrip technology (Group 4 – dicamba) applied to Roundup Ready 2 Xtend soybean is best suited for early emerging weeds. For effective control weeds must not exceed 3-inches in height at the time of application. The rapid growth rate and extended emergence period of Palmer amaranth and tall waterhemp creates challenges for the Xtendimax technology as you near the end of June, a timeframe when tall waterhemp and Palmer amaranth can have large flushes of emergence.

Management in corn

A sequential preemergence followed by postemergence herbicide approach is recommended for effective management of Palmer amaranth and tall waterhemp in corn. Effective preemergence herbicides are in the following groups:
  • Group 4 (e.g. dicamba)
  • Group 5 (e.g. atrazine)
  • Group 14 (e.g. Sharpen)
  • Group 15 (e.g. Dual, Harness/Surpass, Outlook, Zidua)
  • Group 27 (Callisto, Balance)

Postemergence control of Palmer amaranth and tall waterhemp requires an effective herbicide to be applied to small (3-inches or shorter) plants and often requires an herbicide that provides residual control. Emerged Palmer amaranth control in corn can be achieved with a Group 27 herbicide plus atrazine. These herbicides include:
  • Mesotrione (active ingredient in Callisto and Halex GT)
  • Topramezone (Impact or Armezon)
  • Tembotrione (Laudis, Capreno, DiFlexx Duo)
Including atrazine with the Group 27 herbicides requires these applications be applied before the corn is 12 inches tall.

Liberty is an option with Liberty Link corn. Dicamba products such as Status or Diflexx will control small Palmer amaranth. Liberty and dicamba do not provide residual control and should be tank mixed with atrazine or a Group 15 product such as Dual, Harness/Surpass, or Zidua to provide residual control. Be sure to check the label for maximum corn size with any herbicide applied postemergence.

For more information about diversification strategies and crop rotation restrictions, please go to:

PRE and POST Herbicide Diversification Options for Glyphosate-Resistant Corn and Soybean

Key Palmer amaranth reporting contacts

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