Understanding the Threat of Herbicide Resistance

As vegetation management professionals, we must overcome a number of hurdles in our efforts to obtain results for our customers and the industries we serve. Unpredictable weather and operational challenges vary from season to season, and, most recently, a growing concern for vegetation managers is herbicide resistance in weed populations. 


Herbicide resistance refers to the inherited ability of weeds to survive and reproduce after being exposed to herbicide at a rate that would normally control or kill them. It is believed that herbicide resistant weeds occur naturally, at a very low frequency, within plant populations, as random genetic mutations. When a single herbicide, or a small group of herbicides with the same mode of action, are used repeatedly, susceptible weeds are killed. However, resistant individuals will survive and reproduce, thus passing on their resistant traits to the next generation. Over time, these resistant plants will eventually dominate the weed population. Another factor contributing to herbicide resistance is the ability of weeds to adapt and evolve. Plants have a remarkable ability to develop defense mechanisms against threats, including herbicides. They can change their metabolic processes or develop thicker cuticles to prevent herbicide absorption. Some weeds can even detoxify the herbicide inside their cells, rendering it ineffective. This adaptability can allow weeds to survive and reproduce despite herbicide treatments. 


The key strategy to combatting resistance is diversifying herbicide use. By rotating herbicides with varying modes of action, the selection pressure on weed populations can be reduced, making it less likely for resistance to develop. Products like Alligare’s Mainline and Ballast accomplish this by combining multiple active ingredients, with different modes of action, in a single herbicide solution. Another strategy is the use of herbicide mixtures, or tank-mixes, and rotating these combinations between applications. 

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It is important to note that the failure of an herbicide to control a specific weed does not indicate resistance. More likely, there are underlying factors such as environmental conditions, growth stage of the target species, improper application, or poor herbicide selection that can be attributed to the lack of control. However, understanding how herbicide resistance develops is important when planning and implementing effective management strategies. As more weeds become resistant to commonly used herbicides, spray programs can become more costly and complex. 

Before implementing any herbicide management plan, it is essential to accurately identify the targeted species, stage of growth, and level of weed pressure. Different species may require different control methods at varying stages of growth, and misidentification can lead to ineffective treatment. Having a firm understanding of the application site, and the desired results, is key to herbicide selection and application technique.  

Finally, accurate recordkeeping is an important element of herbicide resistance management. Knowing the site history, weed populations and prior herbicide usage allows for proper planning when it comes to future management efforts. Many applicators have a favorite herbicide or tank-mix combination that is their go-to solution, which is great, until it stops working. Now is a good time to look back at what’s been done, plan, and diversify our herbicide management strategies for the future. As vegetation management professionals, and stewards of the environment, we all have an opportunity and obligation to combat herbicide resistance, which will keep our favorite solutions effective going forward. 

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