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Overcoming Herbicide Resistant Weed Pressure

Over the past few decades, producers have begun to continually face herbicide resistant weed pressure throughout their cropping systems. Continuous use of the same herbicide program can quickly develop resistant weed biotypes, especially if there are no other weed management tactics being used.

Integrated Pest Management (IPM) can be defined as “a science-based decision-making process that combines tools and strategies to identify and manage pests” (USDA, n.d.) Integrated weed management (IWM) is a category of IPM that combines multiple weed management practices to ensure that weeds are exposed to many different avenues of control rather than one. Repeated exposure to the same control method can result in weeds developing resistance, becoming even more problematic to control.

This resistance may go unnoticed at first due to low resistance frequencies and weed populations that develop more slowly than other pests such as insects and diseases, but it can quickly gain traction as the same herbicides are repeatedly applied and the resistant weed biotypes continue to reproduce (Gunsolus, 2021). While herbicides will remain a backbone of weed management programs for most farmers, it is important to take action now to preserve their utility. To successfully combat these herbicide resistant weed populations, growers can implement recommended mechanical or cultural weed management practices into their own operations to reduce the pressure from herbicide resistant weeds.

Recommendation #1: plant cover crops

Recommendation #2: adopt longer cropping rotations

Recommendation #3: incorporate occasional tillage passes

Recommendation #4: use harvest weed seed control (HWSC)

It is important to keep in mind that only adapting one practice will likely not be able to successfully delay the development of herbicide resistant weed populations. Rather, multiple practices as a part of an effective IWM program should be implemented together to control these difficult weeds.

Plant Cover Crops

Some cover crop species can produce sufficient biomass during the usual fallow periods of crop fields and even into the growing season of the following cash crop to suppress winter annual or summer annual weeds. Fallow fields during the winter months following crop harvest can give weed populations a great advantage as they can germinate in the fall months and overwinter or germinate and establish in the early spring before growers can get into the fields (Wallace, 2020). By planting winter-hardy cover crops in the fall, the cover crop can begin growth in the fall and continue that growth the following spring. This creates an environment of competition for the weeds, potentially reducing weed stand establishment and slowing growth rates (SARE, 2020).

Spring termination of these cover crops may look different for each grower’s operation. Growers may choose to use a roller-crimper, mower, tillage, or herbicide application. Once terminated via a practice that leaves the majority of the plant matter in the field, the cover crop plant matter will create a mulch effect on the soil, continuing to suppress weed seed germination and growth (SARE, 2020).

Adopt Longer Cropping Rotations

Monoculture cropping practices allow specific weed species to quickly become out of control due to their ability to thrive in a cropping system that presents minimal competition to its success (University, n.d.). To receive weed management benefits from cropping rotations, the rotations must become dynamic in responding to the current and adapting environmental and pest forces, not just to the market demand (University, n.d.). Implementing multiple crops in a cropping rotation allows an opportunity to better manage herbicide resistant weeds with the different modes of action that can be applied to the crop species and varieties (SARE, 2020). Some trait platforms are available that allow for multiple modes of action to be applied at once. This tool can be highly effective alongside the cropping rotation diversity, but over-use of this system will only lead to herbicide resistant weed biotypes that are unaffected by multiple modes of action (Beckie, 2019).

Rotation systems that embrace resource competition, such as allelopathic interference, soil disturbance, and mechanical damage, are able to combat herbicide resistant weed species that have become dominant in cropping systems that do not implement effective cropping rotations (Liebman, 1993). This is due to the system creating an unstable and inhospitable environment toward weed populations (Liebman, 1993).

Incorporate Occasional Tillage Passes

No-till operations heavily rely on herbicide applications to control weed pressure (SARE, 2020). After many years of no-till, plant residue can build significantly on the soil surface. These high levels of plant residue can create less effective herbicide applications as soil or plant surface contact becomes lost (SARE, 2020). However, when herbicide applications and tillage practices are used in a complementary system, weed management becomes more effective against herbicide resistant weed populations (SARE, 2020).

Studies have shown that if a grower implements occasional tillage passes, 1 pass in 5 to 10 years, it is able to aid in weed control when used as a piece of an integrated weed management plan without affecting any soil health ecosystem services (Lyon, 2021). Small tillage changes in a cropping system can help to keep one weed species from thriving and creating a resistant biotype to be challenged by (Lyon, 2021).

Use Harvest Weed Seed Control (HWSC)

New technologies have been created to be used during crop harvest time to either destroy seed in the combine or concentrate weed seed to make it easier to manage in future years. While these methods are not a primary weed management tactic, the technologies can bring farmers closer to the ultimate goal of preventing weed seed from entering the seedbank. There are two methods that are becoming increasingly popular for growers to use in their operations. These are chaff lining and weed seed destruction.

Chaff Lining: A specialized chaff-lining chute is attached to the rear of the combine that separates the straw material from the chaff and lays the chaff in a 12” band behind the combine (Rubione, 2020). The chaff line may then be burned or left alone to compost the weed seeds.

Weed Seed Destruction: A chaff mill is a device attached to the rear of the combine that runs the chaff material through a rolling cage mill. This mill damages the weed seed to an extent in which it is rendered non-viable (Hartzler, 2018).

The costs associated with purchasing these additional pieces of equipment has significantly lowered in the past few years. Growers are now adopting these HWSC practices more now than ever (Beckie, 2019) as they become more feasible investments. HWSC practices work best on weeds that effectively retain their seeds until the cash crop is harvested (Beckie, 2019). If this method becomes heavily relied upon, weeds may begin to physically adapt by setting earlier flowers to reach an earlier seed maturity, lower their seed retention rates, or reproduce seed at levels located below the combine cutting height (Beckie, 2019).

Considerations When Incorporating These Practices

Factors that are beyond a grower’s control can also play a role in the success of these practices. Weather events can impact the success of cover crop growth and herbicide applications. Cooler regions may see less benefit due to decreased growth and lower herbicide uptake by plants. Tillage practices can have varying impacts on soil conditions based on the soil types that they are applied to. Even with varying soil challenges, occasional tillage rather than no tillage can effectively manage herbicide resistant weeds as long as growers adapt the type of tillage and how frequently they use it to fit their operation (Lyon, 2021). Weed seed destruction methods may not be compatible with all machinery currently used, posing financial challenges to some growers for equipment upgrades (Hartzler, 2018). Certain systems may be implemented more readily than others, but long-term plans should be created to implement multiple management tactics.

If you have any additional questions or concerns over management tactics that are available to combat herbicide resistant weeds, please reach out to your local field agronomist. A list of the Iowa State University Extension and Outreach field agronomists can be found here. Many of the resources listed throughout this post are referenced below and can provide additional insight on weed management tactics.

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