The search for synergists to 2,4-D

Written by: Peter Newman

When I was at university, I was discussing my fourth-year project with a colleague in the corridor telling him that my experiment wasn’t going to plan. An old, wise, and respected Prof. Moir overhearing me, ducked into his office and presented me with this cartoon:

This edition of AHRI Insight is a report on a collaboration between AHRI researchers and Nufarm, and unfortunately, it is an example of the dragon having a win. Science doesn’t always go to plan, but it’s important to keep trying.

There are two ways to discover a new herbicide.

  1. Make hundreds of thousands of chemical compounds and test them for herbicidal activity. This is what we currently mostly do.
  2. Understand the biochemical pathways in plants, then go looking for ways to interrupt them. This hasn’t been common in the past, could it be the next frontier?

2,4-D was discovered in the 1940s using method 1 above.

Sixty years later, scientists finally discovered how it works.

This discovery involved understanding plant hormones in great detail, as 2,4-D and other related auxinic herbicides mimic naturally occurring hormones.

The main group of plant hormones that control plant growth are called auxins. Plants maintain auxin levels in the goldilocks range (just right) through many intricate adjustments including regulating gene expression, controlling the amount of auxin building blocks, and controlling the transport of auxin between cells.

This got our AHRI researchers, Dr Danica Goggin and others, along with their collaborators at Nufarm thinking, “If we understand what is controlling auxin levels, can we use some of these compounds to synergise 2,4-D to help control 2,4-D resistant wild radish?”. This AHRI Insight is about using method 2 above.

Unfortunately, they didn’t find the holy grail, a 2,4-D synergist, so they only know what didn’t work. Now they just need to keep trying other compounds to see if they can find a synergist.

They also found that despite some resistance tests suggesting that in some cases MCPA can control 2,4-D resistant wild radish, there was no indication that MCPA can control any of the 11 resistant populations studied.

Wild radish in flower

Wild radish in flower

Wild radish arrived in Australia in the 1860s as a grain contaminant. 2,4-D was one of our first herbicides in modern agriculture, and we have used it extensively to the point where the 2015 random herbicide resistance survey in WA found that 61% of wild radish populations collected showed resistance to 2,4-D.

A lot of research has been undertaken by Dr Danica Goggin and others to understand the resistance mechanism but, at this point, it remains unclear.

The research to understand how 2,4-D works, and what is causing resistance, has given us a greater understanding of how plants regulate their growth using the auxin hormones. This research was used to develop a number of hypotheses.

Hypothesis 1

Given that auxin levels are maintained at the goldilocks level by adjusting the amount of building blocks (precursors), as well as compounds that affect signalling and transport, these compounds could be candidates as possible synergists.

Some of these compounds include abscisic acid (ABA – the germination hormone), cyclanilide and tryptamine.

Some mild synergism was observed, but not enough to get excited.

Strike one.

Hypothesis 2

Given that ABA is a hormone involved in germination and dormancy, and we know that ABA and auxin hormones talk to each other (the researchers call it crosstalk – sounds like a term a teenager would use!), the researchers speculated that perhaps 2,4-D would be more effective when applied to the seeds rather than young plants.

Strike two! This hypothesis was also rejected.

Hypothesis 3

In the past, some resistance tests have shown differences in susceptibility of wild radish to 2,4-D versus a similar auxin herbicide, MCPA. In some cases, the wild radish demonstrates high levels of resistance to 2,4-D, yet MCPA gives higher levels of control, and vice versa. There were also some early indications from field trials that a mixture of MCPA and mecoprop could control 2,4-D-resistant wild radish.

The researchers did dose-response experiments across 11 populations of 2,4-D resistant wild radish with the auxinic herbicides 2,4-D, MCPA, mecoprop and MCPA + mecoprop.

They found some differences between the herbicides, but essentially, if wild radish is resistant to 2,4-D, it is also resistant to other auxinic herbicides.

Strike three! Bummer.


Unfortunately in science, we don’t always win, but it’s the ongoing quest for knowledge that eventually gets results. The team didn’t find a synergist, or a new application window for 2,4-D, but they did confirm that 2,4-D and MCPA resistance go hand in hand.

Next time around maybe we’ll get John Snow on the research team, and the dragon won’t stand a chance!


Posted in: AHRI Insight, Herbicide resistance mechanisms

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