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 2/5/2024

Herbicide System Effects on Waterhemp Emergence in Soybeans

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Tall waterhemp along the edge of a soybean field.

Agronomy Research Update
Written by Chris Olbach, Pioneer Area Agronomist & Maggie Durnin, Intern, Pioneer Agronomy Sciences

Key Findings

  • Aggressive, season-long emergence and resistance to multiple herbicides in waterhemp presents new challenges to weed management.
  • Soil-applied residual herbicides applied pre-emergence provided foundational value for control of waterhemp.
  • New flushes of waterhemp emergence were observed though August and September, demonstrating the need to utilize residual herbicides along with cultural management practices for control of waterhemp.

Related Articles

Waterhemp in Ontario

  • The number of herbicide tolerant weeds is increasing.
  • The development and spread of multiple-herbicide-resistant (MHR) waterhemp (Amaranthus tuberculatus var. rudis) is of particular concern to weed control specialists and farmers alike. 

Physiology of Waterhemp

  • Waterhemp – while hard to distinguish from many other Amaranthus species early in the season – bears unique physiological characteristics that contribute to its success as a weed.
  • Prolific seed production, high growth rate, and season-long emergence alone make waterhemp a very competitive weed
    to contend with (Costea et al. 2005).
    • A single female waterhemp will produce 35,000-1,200,000 seeds per plant – nearly 1.5x other pigweed species.
    • Waterhemp can grow at a rate of 1”-1¼” per day under optimal conditions (United Soybean Board, 2017).
    • The majority of waterhemp emergence occurs from June to August but has been observed to continue as late as September and October.

    High waterhemp density in soybeans

    Figure 1. High waterhemp density in soybeans.

    Waterhemp pressure just prior to pre-emergence herbicide application

    Figure 2. Waterhemp pressure as of June 6th, 2023 – just prior to pre-emergence herbicide application (Haldimand County, ON).

Waterhemp and Herbicide Resistance

  • The prevalence of herbicide resistance in waterhemp further complicates effective management.
  • Multi-herbicide resistance in waterhemp – while aggravated by selection pressures – can be quickly achieved and spread through its dioecious reproduction (Costea et al. 2005).
  • Unlike self-pollinating species of pigweed, waterhemp must reproduce by means of outcrossing pollen from male plants to receptive females nearby (Figure 3).
  • Mandatory outcrossing in waterhemp results in a high degree of recombinant genetic diversity (Montgomery et al. 2019).
  • Most waterhemp populations carry resistance to 2 or more herbicide groups, including: Group 2, 4, 5, 9, 14 and 27.

Objectives

  • In 2023, Pioneer agronomists commissioned a study involving a population of MHR waterhemp in soybeans in Ontario.
  • The objective of this project was twofold:
    • Study the physiology of the weed with respect to emergence over time.
    • Evaluate herbicide program effects on waterhemp emergence in soybeans.

Study Description

  • The experiment was conducted in a field in Haldimand County, Ontario planted to soybeans in 2023 that was known to contain a population of waterhemp resistant to multiple herbicide modes of action (Group 2, 5, 9 and 14).
    Planting Date:
    - June 5th, 2023    		 Soil Type:
    - Haldimand clay
    Variety/Brand:
    - P25A16E™ (E3)    		 Previous Crop:
    - Soybeans
    Seeding Rate:
    - 190,000 seeds/acre    		 Tillage System/Row Width:
    - No-till on 15” rows

Experimental Design:

  • Two herbicide programs were compared, a POST-only program and a PRE + POST program:

Treatments:

  • PRE POST
    Applied June 6th, 2023 Applied July 12th, 2023
    Metribuzin (5) @149g/ac M2,4-D Choline (4) @ 0.75L/ac
    Glyphosate (9) @ 1.25L/ac Glyphosate (9) @1.25L/ac
    Saflufenacil (14) @ 30ml/ac  
    Pyroxasulfone (15) @ 100ml/ac  
    S-Metolachlor (15) @ 628g/ac  
  • The two herbicide programs were applied to 6 by 8 ft plots with three replicates each.

    Layout of waterhemp treatment trial plots in soybeans

    Figure 4. Layout of quadrats within two 6 by 8-ft treatment plots.

  • Both PRE and POST treatments were applied by a John Deere® 4730 self-propelled sprayer with 100ft boom.
  • 6ft x 8ft heavy tarps were used to cover plots prior to the PRE herbicide application to create the POST only treatments.

Data Collection:

  • Three 1 ft² quadrats were randomly placed within each plot in which waterhemp emergence was recorded (Figure 4).
  • The positions of the quadrats were fixed for the duration of the experiment.
  • Waterhemp emergence was recorded before the PRE application and then weekly for the next 10 consecutive weeks.
  • Emerged waterhemp seedlings were counted within each quadrat and documented.
  • After newly emerged waterhemp seedlings were counted each week, they were eliminated with an application of Liberty® 200SN using a backpack sprayer.

Video - Controlling Difficult Weeds with the Enlist™ Weed Control System

Results

  • The PRE treatment was very effective in reducing waterhemp emergence – emergence of new waterhemp plants was very low during the time between PRE and POST applications.
  • In the POST-only program, waterhemp emergence was prolific during this time period, with 81% of the total seasonal emergence occurring between application timings.
  • Peak waterhemp emergence in the POST-only program was the week of July 5-11, with an average of 94 plants per ft².
  • The POST treatment largely prevented emergence of new waterhemp plants for two weeks following application in both herbicide programs.
  • A flush of new emergence was observed in both herbicide programs during the month of August, after residual activity of the herbicide treatments had faded.
  • Counts were discontinued after August 23, as the crop had achieved canopy closure and no new emergence had been observed for two weeks; however, another flush of emergence occurred in September that was not recorded in this study.
  • The PRE + POST program had much lower total waterhemp emergence over the season than the POST-only program:
    • An average of 18 plants per ft² of waterhemp was recorded for the duration of the study in the PRE + POST program.
    • An average of 154 plants per ft² was recorded over the study in the POST-only program.

Conclusions

  • Achieving full-season waterhemp control in the soybean crop is of growing interest to farmers as waterhemp moves and adapts.
  • Managing MHR populations of waterhemp effectively will require a more diligent approach to herbicide use – incorporating strong residual programs with effective post emergence treatments in crop.
  • Herbicides – while still an important tool in MHR waterhemp management in soybeans – should be coupled with other agronomic management strategies on farm where possible such as crop rotation with corn and winter wheat.

References

  • Costea, M., Weaver, S. E. and Tardif, F. J. 2005. The Biology of Invasive Alien Plants in Canada. 3. Amaranthus tuberculatus (Moq.) Sauer var. rudis (Sauer) Costea & Tardif. Canadian Journal of Plant Science 85: 507-522.
  • Montgomery J. S., Sadeque A., Giacomini D.A., Brown P.J. and Tranel P.J. 2019. Sex-specific markers for waterhemp (Amaranthus tuberculatus) and Palmer amaranth (Amaranthus palmeri). Weed Sci 67:412-418.
  • United Soybean Board. 2017. Take Action: Waterhemp Management in Soybeans. Accessed September 20, 2023.

Acknowledgements:
Grace Jones, Corteva Territory Manager; Southcoast Agronomy & Lenos Custom Farming Ltd.


Figure 3. A comparison of Amaranthus species from the same field showing the dioecious nature and differentiation of waterhemp (far left and left) in comparison to green (right) and redroot (far right) pigweed respectively. Smooth stems are a key differentiating characteristic of waterhemp. Click here or on the image below for a larger view.

Amaranthus species from the same field showing the dioecious nature and differentiation of waterhemp in comparison to green and redroot pigweed

Figure 5. Cumulative waterhemp emergence at the time of PRE herbicide application (recorded June 6, black bars in chart) and weekly emergence of new waterhemp seedings for the next 10 weeks in POST-only and PRE+POST herbicide programs. Click here or on the image below for a larger view.

Cumulative waterhemp emergence at the time of PRE herbicide application over the following 10 weeks

Enlist E3

The foregoing is provided for informational use only. Please contact your Pioneer sales professional for information and suggestions specific to your operation. 2023 data are based on average of all comparisons made in one location through August 23, 2023. Multi-year and multi-location is a better predictor of future performance. Do not use these or any other data from a limited number of trials as a significant factor in product selection. Product responses are variable and subject to a variety of environmental, disease, and pest pressures. Individual results may vary. Enlist Duo® and Enlist One® herbicides are not registered for sale or use in all states or counties. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your area. Enlist Duo and Enlist One are the only 2,4-D products authorized for use with Enlist crops. Consult Enlist herbicide labels for weed species controlled. Always read and follow label directions. Varieties with Enlist E3® technology (E3): The transgenic soybean event in Enlist E3® soybeans is jointly developed and owned by Corteva Agriscience and M.S. Technologies L.L.C. Plenish® (P) high oleic soybeans have an enhanced oil profile and are produced and channeled under contract to specific grain markets. Growers should refer to the Pioneer Product Use Guide for more information.