What does it cost to manage herbicide resistance?
Without question, if you must manage a herbicide-resistant weed or weeds, it will require more time and money. With fewer effective herbicide options, you will likely need to use additional herbicides in your tank mixes or make more trips over the field to control the resistant biotypes. One of the most common recommendations for preventing selection of herbicide-resistant weeds is to add a second or even third effective mode of action to the spray program. This can prevent or delay selecting for resistant biotypes and is usually necessary after resistant weeds become established. However, rotating to another herbicide mode of action to control an already resistant population can lead to additional resistance. You will need to integrate more non-chemical control tactics such as mechanical control, changing cultural practices and using other strategies such as cover crops and harvest weed seed control. This multi-pronged approach is known as integrated weed management (Figure 5). If you are not successful, weed control will be inadequate and you could lose crop yield and quality.
Currently, managing a herbicide-resistant weed population may require use of a less effective herbicide, treating weeds when they are smaller, and ensuring the herbicide is applied at the right time. According to 2022-23 estimates of custom ag operator rates from university Extension publications in Illinois, Iowa, Ohio, Maryland, and New York, each spray pass (not including the individual cost of herbicides) can cost between $5 and $15 per acre for application. Custom rates for primary tillage average $17 per acre, while custom rates for row cultivation average almost $15 per acre across the Central and Eastern Corn Belt. Depending on the resistant target weed, a study conducted in Ontario, Canada showed that glyphosate-resistant weeds increased herbicide costs on average by $38 per acre for corn and $27 per acre for soybeans. The study reported that glyphosate-resistant weeds cost the province $28 million more annually in management, while causing $15 million in yield loss.
Figure 5. Various integrated weed management (IWM) tactics are employed to manage herbicide-resistant weeds. They can include using several types of preventive tactics, such as crop rotation, tillage, hand removal, integrating cover crops, using alternative herbicides, and adopting various harvest weed seed control tactics. (Photo credits: Claudio Rubione, GROW).
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For many years, weed scientists studied how many weeds it takes to impact crop yield, a concept known as “weed-crop competition.” They developed economic thresholds for individual species and the overall weed community. An economic threshold for weeds is defined as the weed population at which the cost of control is equal to the value of crop yield attributable to that control. In other words, that’s the point when the yield you would lose costs as much as the control option you would use. The action threshold is the weed population at which the decision is made to institute a control tactic and generally occurs just prior to reaching the economic threshold, which usually translates into the time to apply herbicides or other control measures.
These concepts were delivered by scientists to ag professionals and farmers for more than 20 years, but they were never widely adopted in the U.S. This was mostly because of the complexity and variability in developing reliable thresholds for multiple weed species at multiple locations, and the belief by many that “the only good weed is a dead weed.” With the evolution of herbicide resistance, the no-seed or zero threshold concept has been adopted as a goal by many growers whereby all weeds should be controlled to prevent the production of weed seeds returning to the soil to create greater problems in the future. For problematic herbicide-resistant species like Palmer amaranth, a no-seed threshold is a worthy goal. However, for weeds that are easier to manage in more diverse cropping systems and where there is less concern for resistance, such as giant foxtail and common lambsquarters, the economic threshold concept still holds true.