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Part 4: Management Strategies for Difficult-to-Control Weeds (Driver Weeds) in Soybeans

November 2019 

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