Long-term herbicide-resistance management requires more than weed control aimed only at minimizing crop loss in any one season. It requires long-term strategies focused on delaying the evolution of herbicide resistance and reducing weed seed in your fields.

Effective herbicide-resistance management combines a variety of chemical and nonchemical management tactics to diversify selection pressure on weed populations and minimize spread of resistance genes.

It’s about giving your crops a competitive advantage against weeds. It’s about delaying the evolution of herbicide resistance and preserving herbicide technology.

    Crop Competitiveness

    What’s good for your crops can be bad for weeds. Maximizing crop competitiveness can slow weed emergence and growth and lessen the weed’s reproduction ability, which is critical to reducing the risks of herbicide resistance. Consider the following practices to give your crops the competitive advantage against weeds:

    Crop rotation – How does crop rotation make a difference? Rotating different crops allows for different herbicides, different planting dates and different production practices. These are all differences that add up. Crop rotation optimizes crop competitiveness at the expense of weed growth and reproduction, which can delay the evolution of herbicide-resistant weeds.

    Seed selection – Choosing the right crop seed can help suppress weeds. Your soybean plants’ growth rate, height, leaf angle and canopy formation all affect weed growth, so choosing varieties that will optimize those factors will be helpful.

    Planting date – Crop planting date can affect the severity of a weed infestation. Rapid and consistent emergence of the crop is critical to its success and competitive advantage over associated weeds.

    Row widths – Narrow row widths can accelerate canopy development, which slows annual weed emergence and diminishes their ability to compete with crops. Over time, this results in fewer weed seeds in your fields.

    Seeding rates – Like row widths, increased seeding rates can increase crop competitiveness and accelerate canopy development. For more information on this topic, see the Value of Residual Herbicide in Reduced Soybean Stands webcast provided by the Plant Management Network.

    Cover & Tillage

    Cover Crops

    Cover crops help suppress weed growth by providing year-round cover, and:

    • Help prevent soil erosion
    • Improve soil structure and, often, organic matter content
    • Enhances soil nutrients

    Tillage Practices

    Is tillage a viable option to control weeds? It can be. Tillage can reduce dependence on herbicides. It’s a viable option for some areas, but not for all. In areas susceptible to wind or water erosion, tillage would be a poor option.

    Tillage can be an effective means of weed control through burial of small weeds, disrupting roots leading to plant desiccation, and cutting or severely damaging broadleaf weeds below the apical region, leading to plant mortality.

    As soil disturbance decreases, there is generally an increase in the size and diversity of the soil seedbank. Perennial weeds, shallow-emerging annual grasses and small-seeded broadleaves are prominent in reduced-tillage systems. Large-seeded broadleaves are more prominent as tillage intensity increases.

    Like other production practices, tillage practices must be regularly changed to prevent buildup of any particular species or group of weeds in the soil seedbank.

    Field Management

    Long-term herbicide-resistance management requires more than weed control aimed only at minimizing crop loss in any one season. It requires long-term strategies focused on delaying the evolution of herbicide resistance and reducing weed seed in your fields.

    Effective herbicide-resistance management combines a variety of chemical and nonchemical management tactics to diversify selection pressure on weed populations and minimize spread of resistance genes.

    It’s about giving your crops a competitive advantage against weeds. It’s about delaying the evolution of herbicide resistance and preserving herbicide technology.

    Weed Seed-Free Fields

    Herbicide-resistance management plans must begin before planting because control options become limited once the crop is planted and crops and weeds begin to emerge. Planting into weed-free fields helps prevent challenges later in the season.

    • To keep crop fields as weed-free as possible, especially in conservation-tillage systems where preplant tillage is not feasible, residual herbicides can be used before or at planting (PRE).
    • Consider tillage and cover crops to prevent weed emergence.
    • Additional applications of residual herbicides in slow-growing or open-canopy crops, before the efficacy of the initial residual herbicide has dissipated, reduces selection pressure associated with sole reliance on POST herbicides.

    Crops planted into established weeds are risky because even herbicide mixtures with multiple sites of action (SOA) may not provide effective control. Failure of preplant measures to control emerged weeds is devastating to crop yield and even more devastating if resistance evolves.

    Once a dense crop canopy has formed, emergence of most weeds typically ceases. Once crops canopy, your focus should be on removing escapees and targeting weeds that emerge after crop harvest, if seed production is possible before a killing frost.

    After Harvest

    Weed management doesn’t end at harvest. Postharvest weed-seed production must be prevented to effectively manage soil seedbanks for the long term. In some weeds, such as Common Waterhemp, viable seed production can occur as early as seven days after pollination, meaning that these weeds are capable of producing seed after crop harvest, especially in southern climates.

    Seedbank Management

    For a production system to remain sustainable, the soil weed seedbank must be static or declining. An increasing seedbank is evidence of a weed that is escaping the current management regime through herbicide resistance or some other adaptation. Furthermore, the risk of resistance evolution is shown to be positively associated with the initial seedbank size; therefore, keeping the soil seedbank at low levels reduces the risk of future evolution of herbicide resistance.

    The concept of what constitutes an acceptable level of weed-seed production must be abandoned in favor of a zero or near-zero threshold to slow the rate of herbicide-resistance evolution. Weed management programs must aim to eliminate weed-seed production from the most competitive, resistance-prone weeds in a field.

    In particular, knowledge of the earliest time in the growing season when viable seed are produced is vital for timing late-season applications. Some nonherbicidal approaches to weed-seed prevention create additional management challenges. For example, weeds must be removed mechanically or physically before seed maturity.

    After Harvest

    After crop harvest, producers often allow weeds to grow uncontrolled. Such lapses in weed management can lead to increases in the soil seedbank if sufficient time elapses between harvest and a weed-killing frost, even if a high level of weed control was achieved during crop production. Post-harvest weed-seed production must be prevented to effectively manage soil seedbanks for the long term. In some weeds, such as Common Waterhemp, viable seed production can occur as early as seven days after pollination, meaning that these weeds are capable of producing seed after crop harvest, especially in southern climates.

    Field Borders, Fence Rows, Creek Beds and Ditches

    They won’t impact your yields, but left unmanaged, field borders, fence rows, creek beds and ditches provide breeding grounds for weeds, including herbicide-resistant weeds. Allowing weed-seed production in field borders can have long-term effects on your fields.

    Troublesome weeds like Waterhemp, Palmer amaranth and Giant Ragweed thrive in these areas.

    Perennial weeds often appear first in these areas. Left unmanaged, they can infest your fields.

    You can prevent weed infestations by planting and maintaining dense grass cover in field borders.

    Equipment Cleaning

    After all the work you’ve done to plant your crops and keep them weed-free all the way through harvest… why risk it all with contaminated equipment? Equipment can transport weed seeds between fields. This is especially a concern at harvest when mature seeds from one field can contaminate another.

    To prevent weeds from spreading, clean any equipment used in contaminated fields before using it in other fields.

    Scouting

    Scouting is all about early detection. And when it comes to weed detection, sooner is always better. The key to successful weed management is identifying resistance early, when the infestation can still be controlled.

    Timing is everything. Timely scouting is key to making appropriate management decisions. Scout fields early and often to keep weeds under control. In-season scouting should begin shortly after planting to evaluate weed control efficacy and to determine if additional control is needed. Continue to monitor weed sizes and populations throughout the season. Failure to scout and apply POST herbicides in a well-timed manner could reduce the efficacy and increase the risk of herbicide resistance.

    Scouting for Resistance

    Know the signs. Herbicide resistance can’t be confirmed with a visual inspection, but the following signs could indicate its presence.

    • Failure to control a weed species normally controlled by the herbicide at the dose applied, especially if control is achieved on adjacent weeds.
    • A spreading patch of non-controlled plants of a particular weed species.
    • Surviving plants mixed with controlled individuals of the same species.

    If herbicide resistance is suspected, samples should be collected for testing and escapees should be controlled before seed production.

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