Herbicide resistance isn’t breaking news for growers, but is resistance always the culprit when herbicides don’t achieve complete control? Researchers put tons of hours into researching suspected herbicide-resistant weed populations. However, results do not always reveal resistance as the cause of poor control. Recent cases of HPPD-inhibitor (group 27) herbicide failures have shown that late application, inappropriate herbicide rate, adjuvant misuse, and perhaps other application errors are more common culprits than resistance. Understanding these caveats is essential to achieving effective weed control and postponing the evolution of herbicide resistance.
According to the Herbicide Resistance Action Committee (HRAC), herbicide resistance is “the ability of a weed biotype to survive an herbicide application, where under normal circumstances that herbicide applied at the recommended rate would kill the weed.”³ Nearly 300 weed species have evolved resistance to commonly used herbicides. Majority of weed resistance is in the following herbicide groups: ALS-inhibitors, (group 2), glutamine synthetase inhibitors (group 10), glycines (group 9), HPPD-inhibitors (group 27), photosystem II inhibitors (group 5), and synthetic auxins (group 4).²
Figure 1: The global number of HPPD-resistant weeds has increased over the years.²
For HPPD-inhibitor herbicides, a few confirmed resistance cases have been reported, but cases are not widespread in the U.S., yet. While the number of resistant species is still relatively low, further spread of resistance will leave growers with fewer tools for post-emergence control of key troublesome weeds like waterhemp (amaranthus tuberculatus) and palmer amaranth (amaranthus palmeri). This is where research steps in to help growers understand how to optimize their tank mix and fend off resistance.
The research process begins when public or private weed scientists receive a complaint about an herbicide application with poor or reduced control. Researchers then collect seeds from the complainant and test them for resistance in the greenhouse via preliminary discriminatory screenings and dose-response experiments. Depending on the results, the process may continue with tests in a molecular lab. Without this intensive research process, we would not be able to determine the reason for poor herbicide performance.
In 2022, researchers from the University of Missouri investigated several complaints of failed HPPD-inhibitor control of waterhemp in corn. The group screened waterhemp populations from 10 Missouri counties to determine the likelihood of HPPD resistance. These populations were sprayed with several tank mixes containing a mesotrione-based herbicide and evaluated three weeks after application (percent visual control relative to untreated plants, figure 2).
Figure 2: Response of select Missouri waterhemp populations to HPPD-inhibiting herbicides.¹
It was expected that all populations would be classified as resistant to HPPD-inhibiting herbicides, however, only three populations were resistant (figure 2, red) and just a portion of these populations were exhibiting signs of early resistance (figure 2, yellow). Nearly half of the locations had no signs of HPPD-inhibiting herbicide resistance.
So, what could have happened with these waterhemp populations that were not confirmed as resistant? The possibilities could be endless, but according to the researchers the most common reasons for poor waterhemp control are: 1) late application, weed height is past label recommendation, and/or 2) adjuvants were not optimal for the herbicide chemistry.
At Exacto, we strive to resolve these issues by optimizing the existing chemistry of tank mixes with adjuvants. Using optimized adjuvants can ensure pesticides reach their target and are absorbed by the target plants, prolonging the active ingredient’s life expectancy.
In 2023, Exacto specially designed and replicated several field trials to investigate adjuvant chemistries for optimal mesotrione performance. The studies were conducted in collaboration with public institutions in Wisconsin and Indiana, using suboptimal rates (half of the labeled rate) of Callisto (mesotrione). Using suboptimal rates of pesticides is a common practice in adjuvant research that enables researchers to properly identify the value of individual adjuvant formulations.
Across all plot locations, the percent weed control with Callisto and ammonium sulfate (AMS) was substantially lower than applications made with an optimized adjuvant. In Arlington, WI, lambsquarters control with Callisto and AMS was practically non-existent whereas the addition of an oil-based adjuvant increased control by over 30%. A similar finding was noted with common ragweed (figure 4).
Figure 3: Callisto control (percent visual control, 0-100 scale) of common ragweed and lambsquarters at 18 days after herbicide application in Arlington, WI.
Figure 4 represents the actual control scenario in the field, but to untrained eyes, the control difference between the two treatments could be misrepresented. Using an image analysis app (Canopeo app), these images were saturated (figure 5) and summarized based on the percent of green cover. This visual distinction makes it easier to separate differences to highlight the role of the adjuvant in these applications.
Figure 4: Corn plots infested with common ragweed and common lambsquarters 8 days after Callisto (mesotrione) application. Callisto + ammonium sulfate (left) and Callisto + ammonium sulfate + TOLEX 6058 (0.5 % v/v) (right) show limited visual differences.
Figure 5: Images of the study plots that were processed to determine the percent of green coverage. Callisto + AMS was rated at 59.52% cover, whereas Callisto + AMS + TOLEX 6058 (0.5 % v/v) was rated 36.72%.
Very similar numerical differences were observed in our replicated trial conducted in Indiana (figure 6). When adding an oil + water conditioner (TOLEX 6058) to a mesotrione tank-mix, we enhanced control of both broad leaves and grass weeds such as Palmer amaranth, common ragweed, common lambsquarters, and large crabgrass (figure 7).
Figure 6: Callisto control (percent visual control, 0-100 scale) of palmer amaranth and crabgrass at 14 days after herbicide application in Winamac, IN.
Figure 7: Corn plots 14 days after Callisto (mesotrione) application. Callisto + ammonium sulfate (left) and Callisto + ammonium sulfate + TOLEX 6058 (0.5 % v/v) (right).
Adjuvants can make a tremendous difference in pesticide application effectiveness. Misapplication or application without label-required adjuvants or suboptimal adjuvants most often results in poor herbicide efficacy, which may resemble herbicide resistance. Here we showed the importance of TOLEX 6058, an oil-based adjuvant, that can take mesotrione performance to the next level. Continuous research on optimizing pesticide applications, close monitoring of resistance, and proper use of agronomic tools play a crucial role in the economic sustainability of agriculture.
Resources:
1. Bradley, K. (2023) Watch Out for HPPD-Resistance in Waterhemp. ipm.missouri.edu. Accessed February 2024.
2. Heap, I. (2024) The International Herbicide-Resistant Weed Database www.weedscience.org. Accessed February 2024.
3. Herbicide Resistance Action Committee. (2024) Herbicide Resistance. www.hracglobal.com. Accessed February 2024.
Dicamba, 2,4-D, and glufosinate tolerance for soybeans and cotton have opened a new world of opportunity for using these herbicides for post-emergence weed control. However, these opportunities also introduce challenges with antagonism when multiple herbicides are tank-mixed.
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