Detailed mode of action research by Rothamsted Research is helping to pinpoint and explain the important role that Avadex (tri-allate) retains in herbicide programmes for grass weed management.

The research was initiated after the Herbicide Resistance Action Committee (HRAC) placed tri-allate in the same Group 15 mode of action class as flufenacet, prosulfocarb, and ethofumesate as part of a classification overhaul in 2020.

The move called into question whether such actives should be used together in herbicide programmes, particularly when considering resistance management, with best practice advice suggesting growers apply combinations or sequences from different modes of action groups.

While Group 15 herbicides are suspected of inhibiting very long chain fatty acid synthesis, tri-allate manufacturer Gowan believed there were differences among actives within Group 15, commissioning Rothamsted Research scientist Dr Dana MacGregor and her team to investigate.

Initial experiments in a controlled environment compared the effects of flufenacet, tri-allate and a third active, EPTC, which is not registered in the UK, on blackgrass growth. It found that tri-allate and EPTC impacted shoot growth in contrast to flufenacet, which alters both root and shoot growth.

Members of the team, Dr Hannah Blyth, Dr Frederick Beaudoin and Dr Alex Borg, then used various biochemical techniques to understand what was happening at the tissue and cellular levels in the plant, providing a clearer understanding of the mode of action.

They found that tri-allate inhibits fatty acid synthesis – crucial for building cellular structures and enabling plant growth – at two different places and stages, early in the chloroplast and later in the endoplasmic reticulum. In contrast, flufenacet and EPTC only alter the later part of the pathway that happens in the endoplasmic reticulum.

“We can say this because when we measure different components within the pathway in our controlled environment experiments, the early ones are altered in response to tri-allate but not by the other Group 15s we tested.” Dana explained.

This suggests that tri-allate is acting at multiple sites along the pathway, which helps to explain why resistance has not developed to tri-allate after 60 years of use.

The research has three main implications for current and future herbicide programmes, according to Hank King, Gowan UK country manager.

First, tri-allate has a long history of significant and consistent uplift in efficacy over base programmes across a wide range of key grass weeds. “Add that to this research, and we have confirmation that Avadex is not only extremely efficacious but is also a proven tool for use as a resistant management partner for existing chemistry. Additionally, there is no reason to avoid tank mixing or sequencing Avadex with other Group 15 herbicides,” he said.

“Secondly, we see growers in many situations in the UK increasingly relying more heavily on a single active in cinmethylin to do the bulk of the heavy lifting for autumn grass weed control.

“Where that is the case, we need to be sure the product is robustly protected and supported not only in delivering additional weed control, but also for resistance management.

“Avadex is a robust partner in this instance as it is delivering a mode of action that acts at multiple sites in the target weed,” he said.

Thirdly, by investing in this type of understanding of its chemistry, Gowan has provided the science-led evidence necessary to support growers in dealing with the real challenges they face on farm, Hank suggested.

“This research is ongoing and will be deepened, which for growers should give them proven and genuine justification for using Avadex to control grass weeds as well as helping protect existing chemistry,” he concluded.