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.
- 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