At Moa, our aim is to develop new herbicides for farmers that are safe, effective, affordable and – crucially- that function via a novel mode of action ( i.e. work within a plant in a completely different way to any other known herbicide).
But why are new herbicides needed – and why are novel modes of action so important?
Put simply, weeds have always been tough to control, because they continually develop new survival strategies to evade the control methods used by farmers.
Weeds have learned to dodge mechanical controls such as weed seed crushing mills attached to harvesters, by dropping their seeds early before harvest or by growing closer to the ground to pass under the mill. They are also evolving to avoid cultural controls like shifting drilling dates or reverting to tillage – tactics which also have environmental and financial consequences. Even the herbicides used by farmers for decades to control their toughest weeds are less effective than they used to be, with more weed species becoming resistant to their chemistries, and many regulators worried about the impact of increasing use-rates in attempts to overcome resistance.
Doing nothing, however, is not an option, if we want to ensure our future food security. Weeds can reduce yields by as much as 70%, and Rothamsted Research has estimated that resistant blackgrass – one of the most problematic weeds in Northern Europe – could cost the UK economy alone £1 billion a year if it is not tackled.[1]
The good news is that a new generation of herbicides, with novel modes of action, has the potential to overcome existing resistance mechanisms and provide weed control at lower application rates to minimise environmental impacts. They will form a crucial part of integrated weed management strategies that combine chemical treatments with cultural, mechanical and biological approaches to achieve sustainable weed control in the future [2][3].
The problems that need to be solved
Weeds can develop resistance via two primary mechanisms: Target Site Resistance (TSR) and Non-Target Site Resistance (NTSR).
Herbicides function by binding to and inhibiting important proteins within plant cells. Target Site Resistance occurs when a naturally occurring DNA mutation in an individual plant leads to increased expression of one of these target proteins or or a change in the protein’s structure [4]. Structural changes can prevent herbicide molecules binding to their target as effectively, allowing the plant to survive a chemical treatment. If the plant is allowed to set seed, a new resistant population of weeds carrying the same mutation is born!
Non-Target Site Resistance is the result of genetic changes that reduce the amount of herbicide that reaches the target protein, rather than altering the target protein itself. Plants can achieve this in several ways such as enhanced metabolism (i.e. faster breakdown) of the herbicide molecule, reduced uptake and transport of the molecule or storage of the molecule away from its target protein (sequestration) [4].
The most difficult weeds to manage are those that have developed resistance via both these mechanisms. Blackgrass (Alopecurus myosuroides) populations in the UK are a good example of this. Mutations in the acetyl-CoA carboxylase (ACCase) and acetolactate synthase (ALS) proteins have resulted in TSR to the most common post-emergence graminicides (grass herbicides). Meanwhile, increased activity of metabolic enzymes like cytochrome P450s (CYPs) and glutathione S-transferases (GSTs) has provided NTSR to many herbicides, including microtubule assembly inhibitors like pendamethlin [5]. Other “superweeds” with significant levels of both TSR and NTSR include species in North America, such as Amaranthus palmeri (pigweed), and Lolium rigidum (annual ryegrass) in Australia.
Overcoming resistance in weeds
Resistance is the reason that finding novel modes of action is so important. Weeds like annual ryegrass have, over the years, developed TSR and NTSR resistance to as many as 12 known modes of action, rendering many herbicides ineffective. However, because novel modes of action herbicides work in different ways to any existing products, current resistance mechanisms can be bypassed. This means farmers can eliminate out-of-control weed populations, reclaiming their fields for crop use and increasing their yields.
At Moa, we find mode of action herbicides in our GALAXY platform, but this is just the first step. The new herbicides then undergo a range of tests at our labs in Oxford and our glasshouses in Yorkshire. Here, and in collaboration with international research institutions, we test our molecules against some of the most problematic resistant weed species in the world.
Our resistance trials aim to find out the dose of Moa novel mode of action herbicide needed to control herbicide-sensitive and herbicide-resistant populations of the same species. We then compare this to the performance of existing commercial herbicides. Excitingly, many of our novel compounds can overcome weed resistance mechanisms and achieve full control of both populations at an equivalent rate, such as the examples with waterhemp and blackgrass below.
We also use modern sequencing technologies to determine the specific mechanisms each resistant weed population uses. This helps us to confirm that our Moa novel herbicides can overcome a range of mechanisms and provide broad and robust resistance breaking.
The exciting results that we’ve seen against resistant weeds so far give us real hope that our compounds will soon be an important weapon in the fight against herbicide resistant weeds and a key part of sustainable weed-free food production.
Illustrations:
Above: Waterhemp (Amaranthus tuberculatus) with Target Site Resistance is strongly resistant to a commercial herbicide in comparison to a sensitive population (left), but a Moa compound is equally effective against both the sensitive and resistant populations.
Above: Blackgrass (Alopecurus myosuroides) collected from fields in the UK is strongly resistant to a commercial herbicide in comparison to a sensitive population from Germany (left), but a Moa compound is equally effective against both the sensitive and resistant populations.
1. https://www.rothamsted.ac.uk/news/herbicide-resistant-weed-could-cost-uk-ps1-billion-year
2. Riemens, M. et al. (2022). An Integrated Weed Management framework: A pan-European perspective. European Journal of Agronomy., 133, 126443.
3. Parven, A. et al. (2025). Herbicides in modern sustainable agriculture: environmental fate, ecological implications, and human health concerns. International Journal of Environmental Science and Technology., 22, 1181-1202
4. Gaines, T. et al. (2020). Mechanisms of evolved herbicide resistance. Journal of Biological Chemistry., 295(30):10307–10330
5. Cook, S. et al. (2023). Herbicide resistance in Alopecurus myosuroides: The value of routine testing of seed samples submitted by farmers since 1985. Weed Research., 63(6), 339–347
