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Managing metabolic resistance in weeds

Dealing with herbicide-resistant weeds is nothing new for row-crop producers in MFA’s trade territory. Most of us in the farming community, myself included, have faced these rebellious weeds one way or another for an entire career—or even a lifetime, if you were born after the mid-1980s. Waterhemp, for example, has shown evolving tolerance to many major herbicide sites of action that we use to manage weeds in row crops.

If this is a familiar issue, why write about it? Simply put, we’re seeing resistance become more complex, requiring us to take weed management up a notch. Many herbicides that were effective are now less or not effective at all when applied after waterhemp has emerged. In most reported cases, resistance is linked to a single, specific mutation at the target site in the plant’s DNA. As a result, herbicides such as Pursuit, atrazine, glyphosate and Flexstar are no longer as effective on waterhemp as they once were.

“Producers need to be aware of metabolic resistance and what it could mean for their weed management program.”


Farmers have been managing this resistance by returning to soil residual herbicides. Mixing multiple modes of action in the spray tank and overlapping those residuals have been excellent strategies. However, some waterhemp populations in the U.S. are breaking through certain residual herbicides with a different type of resistance—metabolic resistance.

Like human metabolism breaks down food, plant metabolism also processes substances, in this case, herbicides. In weeds that have developed metabolic resistance, the herbicide’s active ingredients are broken down into nontoxic byproducts that don’t kill the plant.

Metabolic resistance in weeds is not widespread, but the true extent is unclear. Researching herbicide resistance is a labor-intensive process that involves sampling and collecting seeds from suspected fields and evaluating them under greenhouse conditions. Regardless, producers and agronomic advisors need to be aware of metabolic resistance and what it could mean for their weed management program when they encounter it.

Weed size appears to be a factor in controlling weeds with metabolic resistance. Even though some waterhemp plants may exhibit metabolic resistance, control of small seedlings less than 2 inches tall can be achieved with mesotrione (Callisto or Explorer). However, this is likely a short-term solution if practices are not altered or soil residuals are not activated by sufficient rainfall. Another concern is that other herbicides may not work as effectively. Some data shows that metolachlor and 2,4-D may not control plants with metabolism-based resistance.

How does metabolic resistance change your herbicide programs? So far, the best tactic is to prevent weeds from producing seeds. Integrated weed management and early action on escaped weeds are crucial. Removing escapes during the growing season is much easier than letting one female waterhemp plant release 100,000 seeds or more, worsening the problem in following seasons. Chemical recommendations need to be evaluated field by field, but limiting weed emergence is a great first step.

Soil residuals are an economical and effective way to control most weeds, so do not steer away from them. See-and-spray technologies, which offer precision chemical application, are tools for robust post-emergence applications with multiple effective modes of action when applied to small areas. This method is likely more cost-effective than broadcasting a multiple mode-of-action herbicide mixture on an entire field when weeds are absent. Precision tillage tools can also help spot and mechanically remove weeds but may require additional trips across the field.

Basic agronomic fundamentals also play a role and cannot be ignored. Don’t underestimate the power that proper seed placement and good crop establishment have on reducing weed competition. Selecting the right corn hybrid, soybean variety and planting population for your acre could help by limiting the amount of sunlight that reaches the soil. Light quality can impact weed seed germination, which is why covering the soil with crop leaf foliage is important.

Metabolic-based resistance in weeds is a significant challenge, but producers can minimize its impact with planning and proactive, integrated strategies. Consult your MFA agronomist for more information.

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