While many factors affect herbicide performance, the ingredients in tank mix have the power to make or break an application. If active ingredients antagonize or don’t get along in the tank, weed control will not be optimized, leading to additional applications and accelerating the process of herbicide resistance selection. Although antagonism can be a challenge, it can be managed with strategic tank mixing. Adjuvants are an effective tool to maximize active ingredient effectiveness in multi-active pesticide tank mixes.
In post-emerge applications, it’s quite common for growers to mix multiple herbicides for broadleaf weed control in their spray tank. This practice saves time and labor while reducing application costs in farming operations. Antagonism occurs when a tank mix of multiple herbicides results in weaker weed control than expected by each herbicide alone. It often occurs when mixing grass herbicides and broadleaf herbicides such as combinations of 2,4-D, glufosinate, clethodim, and/or quizalofop-p-ethyl (QPE).
An ineffective tank mix can devalue an application, forcing growers to return to the field for additional herbicide applications to avoid catastrophic yield losses due to crop-weed competition. If left untreated, fields with surviving weeds exposed to suboptimal herbicide applications are subject to accelerated selection for herbicide resistance.
An Illinois soybean field with a large patch of volunteer corn remaining days after a third 2,4-D + clethodim application.
It’s essential to identify and address antagonism before the application process. Successfully managing antagonism saves both time and costs by reducing trips across the field, but also contributes to sustainability by keeping the amount of active ingredient applied to a minimum.
While there is no definitive answer on the cause of herbicide antagonism, a few culprits have been cited: competitive binding in grass herbicides, reduced uptake, reduced translocation, and weed-enhanced metabolism (enhanced herbicide detoxification) when applying ACCase inhibitors herbicides.
Continuous research on antagonism is necessary to maintain farm profitability, especially where novel herbicide chemistries are rare and herbicide-resistant weeds continue to emerge. The studies below are examples of research that offer solutions to overcome the challenge of antagonism.
Following complaints from growers experiencing difficulties controlling volunteer corn, weed scientists from the University of Nebraska-Lincoln published a study examining quizalofop-p-ethyl (QPE), 2,4-D, and glufosinate applications on volunteer corn. QPE is a commonly used grass herbicide, but its efficacy when mixed with other herbicides was not well understood, particularly in the context of volunteer corn.
This study identified that 2,4-D choline antagonized volunteer corn control for at least the first two weeks after application. The results of this study indicate that increasing the rate of QPE overcame antagonism caused by 2,4-D. If volunteer corn control is unacceptable, a second application of QPE can be made more than 7 days after the first application. The study recommended that QPE should be applied alone or sequentially with broadleaf herbicides to control volunteer corn. While necessary in the absence of a tool to mitigate herbicide antagonism, these recommended strategies are not ideal from an economic or environmental standpoint.
Exacto also conducted research that shows adjuvants can mitigate antagonism. Research conducted in Sharon, WI and Winamac, IN demonstrated that while increasing clethodim rates can overcome antagonism, adding adjuvants is a more cost-effective and environmentally friendly approach, emphasizing their critical role in complex tank mixes.
While increasing the active ingredient may seem like the obvious solution to overcome antagonism, this approach may not always be feasible or sustainable. Avoiding higher herbicide rates results in more economical and environmentally friendly applications and reduced selection pressure on the weeds.
Understanding the tank mix and selecting the right adjuvant or combination of adjuvants is essential in combating antagonism. Whether the active ingredients are known to cause antagonism or not, following the label is key to a successful application. If there is a known antagonism, find the right adjuvant to mitigate the antagonism from the start.
A thorough understanding of the dynamics within your tank mix is essential for a successful herbicide application. When dealing with herbicides known to antagonize each other, incorporating a high-performing adjuvant of top performance is not just a recommendation; it is a necessity for ensuring the efficacy and value of your application.
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