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This week we resume our conversation on herbicide-resistant and other hard-to-control weeds by examining Palmer amaranth, a prolific, yield-robbing species of pigweed. Palmer amaranth is no stranger to most growers. Its aggressive and productive nature make it difficult to mitigate and highly prevalent throughout the country. Learn the strengths and weaknesses of this driver weed and recommended control tactics.

The Bad News (Strengths)

• Female Palmer amaranth plants can produce as many as 750,000 seed per plant.
• These plants are extremely prolific in growth habit, growing up to 2–3 inches per day.
• A dioecious weed, Palmer amaranth grows male and female flowers on separate plants, allowing for more genetic diversity in single populations and rapid development of herbicide-resistant genes, including cross resistance to multiple herbicides in the same population.
• Palmer seed emerges at soil temperatures from 65° to 95° Fahrenheit, so it has a longer emergence schedule than other weeds.
• The plants produce small seeds that are well adapted to minimum and no-till systems.
• If left uncontrolled, Palmer amaranth has been known to reduce yields by as much as 79 percent in soybeans. 

The Good News (Weaknesses)

• While seed can be viable in the soil for several years, research demonstrates that less than 12 percent of weeds will remain viable for longer than three years, and only two percent of seed will remain viable for longer than six years.
• Seed in the South is less persistent in the soil than in the North.
• Seed that is emerging or recently emerged is easier to control than plants that have had the opportunity to grow to 6 inches or greater.
• Shading, or the inability of growing seedlings to capture sunlight, can greatly inhibit Palmer amaranth’s prolific tendency.

Management Strategies

*Note: Current known herbicide resistance for Palmer amaranth includes Groups 2 (ALS), 3 (microtubule inhibitors), 4 (synthetic auxins), 9 (glyphosate),14 (PPO inhibitors) and 27 (HPPD inhibitors), There is some known cross resistance (single population resistant to both Groups 9 and 2).

• Know your resistance profile. Have your population tested to know what herbicides will be effective.
• Start clean. Consider tillage and/or an effective burndown to control and eliminate actively growing weeds. Consult your local agronomist or university extension office for specific burndown recommendations. Note that use of burndown products in multiple sequential applications is encouraged, where possible, to delay and diminish the germination and emergence of Palmer amaranth to allow soybeans a competitive advantage in row closure, canopy and plant density.
• Consider narrow rows. In some cases, narrow-row soybeans may be warranted to increase photosynthetic competitiveness and decrease Palmer amaranth growth habits.
• Cover crops. Cover crops have shown an increased ability to lessen weed seed bank populations and delay emergence.
• Manual eradication. In severe infestations, manual eradication may be necessary for a few years to get the population under control.
• Harvest eradication. In the South, some growers have turned to adding mechanical seed destroyers on combines to control weed seed escapes.
• Conventional herbicides. Using a conventional herbicide system that relies on multiple passes with soil-applied residuals and effective post-applied herbicides with current broad spectrum herbicides can be an effective strategy. This system is generally costly; however, the increase in yield from reducing weed competition generally outweighs the cost of treatment.

Trait System Usage

Use trait systems that provide the ability to apply multiple effective modes of action simultaneously to combat herbicide resistance.  For instance, the Enlist E3^® system allows the use of Enlist One^® with 2,4-D choline to be applied with both glyphosate and glufosinate as well as additional tank mix partners to eliminate and delay further Palmer amaranth emergence. This system combines many of the strategies discussed into a flexible platform.

To learn more about Palmer amaranth and how to manage this hard-to-control driver weed, contact your local Stine agronomist or university extension specialist. 

Resources and Citations:

Palmer Amaranth Management in Soybeans
https://extensiondata.missouri.edu/Pub/pdf/miscpubs/mx1125.pdf

Arkansas Field Crop Weed Control Resources
https://www.uaex.edu/farm-ranch/pest-management/weed/field-crops.aspx

Palmer Amaranth (Pigweed)
https://iwilltakeaction.com/weed/palmer-amaranth

Pigweeds
https://u.osu.edu/osuweeds/super-weeds/

Palmer Amaranth Biology, Identification, and Management
https://cpb-us-w2.wpmucdn.com/u.osu.edu/dist/7/3461/files/2016/01/WS-51-W-1cpo2fz.pdf

Marestail, otherwise known as horseweed, has been problematic for growers since the early 2000s, when the first glyphosate-resistant form was found in the United States. Because of this weed’s prevalence throughout the country, combined with the sheer size of the plant and number of seeds it produces, marestail is the focus for Part 3 of our series on herbicide-resistant and other hard-to-control driver weeds. This week we examine the strengths and weaknesses of marestail and recommended control tactics. 

The Bad News (Strengths)

• Marestail is a winter annual that emerges in the late summer through the fall and in early spring. Fall-emerging weeds typically overwinter and “bolt,” or develop a long stem with leafy growth that is difficult to control.
  • Bolted marestail is difficult to control because it can have hundreds of growing points.
  • Marestail can grow 6­–8 feet tall and develop 200,000+ seeds per plant that can be blown by the wind and carried by birds.
• Marestail has smaller leaves, making coverage difficult.
• Marestail can become hardened off in poor growing conditions, including hot and dry periods and extremely wet periods.
• If left uncontrolled, marestail can reduce yields by as much as 40 percent.

The Good News (Weaknesses)

• Marestail is relatively easy to control in the rosette stage, compared to the bolted phase. This happens in the 30–45 days after emergence in the fall and 21–30 days after emergence in the spring.
• Marestail responds well to soil-applied herbicides that inhibit root and shoot development.
• Fall cover crops such as cereal rye have proven effective at keeping marestail from germinating in the fall and provide a shading effect to emerging marestail in the spring if left in the field after termination.
• Corn and small grains are a good rotational option to fight marestail because there are more chemical options in these cropping systems.       

Management Strategies

• Start clean. For marestail, this means you will need to start in the fall. Apply a fall burndown to actively growing, emerged marestail. Consult your local agronomist or university extension office for specific recommendations.
• Control new and surviving weeds with a spring burndown.
• Use soil-applied residuals at planting to guard against late-emerging marestail.
• For post-emerge applications, target weeds less than 6” tall using multiple herbicides with adequate coverage.
• In some cases, narrow-row soybeans may be warranted to increase photosynthetic competitiveness and decrease marestail emergence.
• Cover crops have shown an increased ability to delay emergence and shade emerged seedlings.
• Cultivation in severe infestations may be necessary for a few years to get the population under control.
• Conventional tillage demonstrates a more effective control because weeds are uprooted in the spring. In severe cases of poor marestail control, conventional tillage in the spring may be warranted.

Trait System Usage
Use trait systems that provide the ability to apply multiple, effective modes of action simultaneously to combat herbicide resistance. For instance, the Enlist E3^® system allows Enlist One^® with 2,4-D choline to be applied with both glyphosate and glufosinate and additional tank mix partners to eliminate and delay further marestail emergence. This system combines many of the strategies discussed into a flexible platform.

To learn more about marestail and how to manage this hard-to-control driver weed on your farm, contact your local Stine agronomist or university extension specialist.

Resources and Citations
Management of Herbicide-Resistant Horseweed (Marestail) in No-Till Soybeans
https://iwilltakeaction.com/uploads/files/57229-7-ta-hrm-factsheet-horseweed-final.pdf

Plant of the Week
https://www.uaex.edu/yard-garden/resource-library/plant-week/horseweed-8-5-05.aspx

Marestail
https://u.osu.edu/osuweeds/super-weeds/marestail/

Marestail (Horseweed) Management
https://extension.psu.edu/marestail-horseweed-management

Adapting Soybean Burndown Programs for Large Marestail
https://agcrops.osu.edu/newsletter/corn-newsletter/2017-12/adapting-soybean-burndown-programs-large-marestail

For part two of our discussion on herbicide-resistant and other hard-to-control weeds (driver weeds), we’re examining waterhemp. This article highlights the strengths and weaknesses of waterhemp and the strategies available to control this driver weed.  

The Bad News (Strengths)

• Waterhemp typically grows in clumps or patches with large numbers of the plant concentrated in limited areas.
  
• The University of Missouri has demonstrated that more than 20 plants per square foot can reduce soybean yields by 44 percent. Even late-emerging waterhemp (after 5th trifoliate) can reduce yields significantly.
  
• Waterhemp is also an extremely productive weed, with the ability to grow 1.25 inches per day and produce between 250,000–500,000 seeds per plant.
  
• As a dioecious weed, waterhemp grows male and female flowers on separate plants, allowing for more genetic diversity in single populations and rapid development of herbicide-resistant genes.

The Good News (Weaknesses)

• Waterhemp seed may overwinter in the soil for a few years, but research demonstrates that less than 15 percent of that seed will remain viable for longer than four years.
  
• Seed that is emerging or recently emerged is easier to control than plants that have had the opportunity to grow and harden off in adverse weather conditions.
  
• Shading — the inability to capture sunlight — growing seedlings can greatly inhibit waterhemp’s prolific tendency.

Management Strategies

• Know your resistance profile. Test your waterhemp population to know which herbicides will be effective. Current known herbicide resistance includes Groups 5 (triazines), 2 (ALS), 14 (PPO inhibitors), 9 (glyphosate), 27 (HPPD inhibitors) and cross resistance (single population resistant to both Groups 9 and 2). 

• Start clean with tillage and/or effective burndown control that eliminates actively growing weeds.

• Use an effective soil residual herbicide. Consult your local agronomist or university extension office for specific recommendations. Note that use of soil residual herbicides in multiple sequential applications is encouraged, where possible, to delay and diminish the germination and emergence of waterhemp and to allow soybeans a competitive advantage in row closure, canopy and plant density.

• In some cases, narrow-row soybeans may be warranted to increase photosynthetic competitiveness and decrease waterhemp growth habits.

• Cover crops have also shown an increased ability to lessen weed seed bank populations and delay emergence.

• Manual eradication in severe infestations may be necessary for a few years to control the population.

• Using a conventional herbicide system that relies on multiple passes with soil-applied residuals and effective post-applied herbicides with current broad-spectrum herbicides can be an effective strategy. This system is generally costly; however, increase in yield from reducing weed competition generally outweighs the cost of treatment.

Trait System Usage
Use trait systems that provide the ability to apply multiple, effective modes of action simultaneously to combat herbicide resistance. For instance, the Enlist E3^® system allows the use of Enlist One^® with 2,4-D choline to be applied with both glyphosate and glufosinate and additional tank-mix partners to eliminate and delay further waterhemp emergence. This system combines many of the strategies discussed into a flexible platform.

To learn more about waterhemp and how to manage this hard-to-control driver weed on your farm, contact your local Stine agronomist or university extension specialist.

Resources and Citations
University of Arkansas Division of Agriculture Research & Extension https://www.uaex.edu/farm-ranch/pest-management/weed/field-crops.aspx

Take Action www.takeactiononweeds.com

Waterhemp Management in Soybeans https://weedscience.missouri.edu/publications/50737_3_TA_FactSheet_Waterhemp.pdf

Ohio State University Weed Management https://u.osu.edu/osuweeds/super-weeds/

Ohio State University Extension https://agcrops.osu.edu/newsletter/corn-newsletter/2014-20/postemergence-control-giant-ragweed-soybeans