Thunder®: a new mechanism of action to expand the management of resistant weeds

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The increasing resistance of weeds to herbicides has significantly reduced the efficiency of important tools used in chemical management. Among the most concerning species in Brazil, goosegrass (Eleusine indica) stands out due to its wide distribution, high competitive capacity, and increasing occurrence of biotypes resistant to herbicides such as glyphosate and ACCase inhibitors. The severity of this plant is also associated with the architecture of its root system, which is dense, fasciculated, and highly aggressive, facilitating soil exploration and hindering the development of crop roots, drastically limiting its productive potential. In this scenario, the introduction of herbicides with new mechanisms of action becomes essential to expand control options and reduce the selection pressure exerted on weed populations.

Thunder® features Asulam as its active ingredient, a post-emergent, systemic herbicide and the only herbicide in Group 18 (I) of the HRAC, acting as an inhibitor of dihydropteroate synthase (DHPS), an enzyme essential for folate biosynthesis in plants. This mechanism of action expands the possibilities for diversifying management programs and represents an important tool for resistance management. 

Thunder® Mechanism of Action

Asulam works by blocking the enzyme dihydropteroate synthase (DHPS), which is responsible for a key step in folate biosynthesis. Folates participate in the synthesis of nucleotides, amino acids, and in carbon unit transfer reactions that are fundamental for cell division and meristematic activity. Therefore, the interruption of this metabolic pathway rapidly compromises essential physiological processes, initially resulting in the cessation of plant growth and, subsequently, in the development of characteristic visual symptoms, leading to plant death. (Figure 1).

In addition to its effects on growth, reduced folate availability can affect processes associated with energy metabolism and the maintenance of photosynthetic activity. This behavior helps explain why physiological changes can be detected shortly after herbicide application, even before the appearance of visible injury symptoms.

Thunder® acts quickly before visual symptoms appear.

 Inhibition of DHPS results in rapid suppression of plant growth, mainly in meristematic tissues, followed by the progressive appearance of chlorosis in young leaves and regions of intense metabolic activity (Figure 2). With continued herbicide action, symptoms progress to necrosis and plant death. Although visual manifestations occur a few days after application, critical physiological and metabolic changes are triggered soon after plant exposure to Thunder®.

To understand these early changes, researchers from the Weed and Pesticides in the Environment Research Group (PDPA/UFRRJ) evaluated goosegrass plants resistant to glyphosate, clethodim, and haloxyfop, using the transient chlorophyll a fluorescence technique, a tool capable of detecting changes in photosynthetic performance before the manifestation of visible symptoms.

 Just 24 hours after the application of Thunder®, a reduction in PIABS and PITOTAL indices was observed compared to the control (Figure 3), showing that both the initial photochemical efficiency and electron transport had already been compromised. In other words, even though the plant did not show symptoms of injury caused by Thunder® in the first hours after application, its energy metabolism was already compromised, reducing its growth and recovery capacity.

These results demonstrate that, even in the absence of visual symptoms, the plants' energy metabolism is already compromised, reducing their capacity for growth, recovery, and development. In other words, when the first signs of yellowing and necrosis become visible, the control process has already been underway for several days.

The effectiveness of Thunder® in the strategic control of goosegrass.

Thunder® is positioned for use in early and strategic management, acting on goosegrass plants with up to eight tillers. In weed management during pre-planting desiccation, Thunder® should be applied at least five days before sowing soybeans, corn, cotton, and beans, and at least ten days before sowing wheat. 

The application of Thunder® (2.000 g/ha of Asulam) for the management of goosegrass should be combined with glyphosate (1.440 g/ha) + 0,5% methylated soybean oil and sequential application, seven to 14 days after the first application, with glufosinate-ammonium salt (500 g/ha) + pre-emergent herbicide (s-metolachlor, clomazone, flumioxazin, etc.) + adjuvant. Control of goosegrass using Thunder® has shown excellent results (Figure 4).

In agricultural systems where glyphosate and ACCase-inhibiting graminicides are used repeatedly, incorporating distinct mechanisms of action is one of the main strategies recommended by resistance management programs. In this context, Thunder® contributes to the diversification of mechanisms of action, reducing selection pressure by acting on a biochemical target completely different from those present in the main tools currently used in grass management (Figure 5). To date, there are no confirmed cases of weed resistance to DHPS-inhibiting herbicides.

In this way, Asulam establishes itself as an important tool for managing difficult-to-control weeds. In addition to providing a distinct mechanism of action, the herbicide can be combined with molecules widely used in production systems, such as glyphosate, Protox inhibitors, PSII, auxins, or graminicides, contributing to diversifying management programs and increasing efficiency in controlling resistant weeds.

*Per Amanda de Moraes Azevedo Pereira, Jessica Ferreira Sabino e Camila Ferreira de Pinho, from the Weeds and Pesticides in the Environment Research Group - PDPA/UFRRJ; RRafael Jose Pereira Rovea e Jessica Ferreira Lourenço Leal, from UPL Brazil