The Catchers of the Ryegrass:
The Weedy Tendencies of Australia’s Most Troublesome Grass

By Ana Krsteska,
Master of Biosciences student, The University of Melbourne

Weeds are any plants that are unwanted in a particular location. Most often non-native species, they pose a serious threat to Australia’s environment. The cost of weeds to the agricultural industries alone due to reduced farm productivity is estimated at about $4 billion per year.¹

Herbicides were initially seen as an easy remedy to manage agricultural weeds, but over time they have unintentionally fuelled the rise of ‘superweeds’—plant species that quickly evolve herbicide resistance, transforming a solution into a growing problem.²

Among the most noxious of herbicide-resistant weeds is annual ryegrass (Lolium rigidum), a persistent adversary for farmers. Decades of battling this resilient weed have raised pressing questions about whether poor herbicide stewardship is doing more damage than good.

Our research group, the Adaptive Evolution Lab at the University of Melbourne, is dedicated to uncovering the evolutionary mechanisms that allow ryegrass to adapt to agricultural interventions. By studying the physical and genetic traits behind its ecological responses, we aim to identify its strengths and weaknesses, providing farmers with valuable insights for more effective management.

A pain in the ryegrass

The grains industry is under increasing pressure to feed a growing global population, all while facing the challenges of a rapidly changing climate. Ryegrass, a persistent weed, is only adding to the burden. Competing fiercely with essential crops like wheat and barley for vital nutrients, it chokes yields and threatens crop profitability.² This, in turn, strains food supply chains, and makes it harder to meet the rising demand for staple foods.

The ability of ryegrass to adapt rapidly means that even well-rounded control programs might face diminishing effectiveness over time. In addition to developing resistance to multiple herbicides, it can also adapt to alternative weed control methods, such as tilling (soil turnover) and early crop sowing.³ Even with the use of integrated weed management (IWM), which combines diverse and complementary strategies, the immense potential for ryegrass to adapt raises concerns about whether current methods can prevent it from becoming a larger problem in the future.

Historically, weed research has prioritised improving crop yields, often viewing weed control strategies solely through their impact on crops, while their direct effects on weeds like ryegrass were treated as an afterthought. Yet, it is equally important to investigate how these interventions impact ryegrass dynamics. Our understanding of ryegrass ‘biotypes’ — sub-types of plants that have adapted to specific conditions — is still limited, particularly in terms of how they evade or resist control efforts.

Adaptations stem from genetic changes that lead to distinct physical traits, raising an important question: how can we link ryegrass traits and their variants to specific conditions, across different environments?

Weeding out the truth

This is where our research comes in. At the heart of our approach is Grime’s CSR triangle, a cornerstone of plant evolutionary ecology.⁴ This powerful framework categorises plant species into three key survival strategies: ‘competitors’ (C), ‘stress-tolerators’ (S), and ‘ruderals’ or ‘disturbance-tolerators’ (R).

While this framework has traditionally been used to classify differences between plant species, we are extending its application to characterise variation within a single species. In the context of agriculture, it provides a unique lens through which we can explore how ryegrass populations adapt their survival tactics in response to different weed control methods.

For example, ‘competitors’ excel in resource-rich environments, outcompeting crops for nutrients, while ‘stress-tolerators’ thrive in harsh conditions, like herbicide-treated soils. Meanwhile, ‘ruderals’, are quick to colonise disturbed areas, making them resilient to practices like tilling.⁵

By understanding how these strategies impact the weed’s response in various agricultural settings, we can help farmers manage ryegrass more effectively and, quite literally, root out the problem.

We hypothesise that the potential rise of ryegrass ‘superweeds’ will be supported by one of two possible mechanisms. First, some individual plants might possess exceptional plasticity in their traits, making them what we would call our ‘super biotype’. These plants could adapt and thrive in a variety of environments – whether it is in fields with competitive crops, under herbicide pressure, or in areas disturbed by tilling.^6,7

Alternatively, survival could depend on diversity within the ryegrass population. Instead of relying on a single highly adaptable plant, the population might be made up of various biotypes, each specialised for different management challenges. Some plants might excel in competitive environments, while others are better equipped to handle herbicide stress or physical disruption from tilling. This diversity would ensure that, no matter what control method is used, a portion of the weed population would survive, continue to spread, and persist.⁸

We are conducting a field experiment that simulates different combinations of management strategies likely to elicit responses in every corner of the CSR triangle. In different growing plots, we observe how ryegrass responds to these conditions, measuring plant growth and biomass.

We are also taking genetic ‘snapshots’ of ryegrass populations at different stages, tracking how their genetic makeup changes over time. This helps us see whether genetic diversity changes in response to these management techniques. By doing so, we aim to pinpoint specific genes that may play a role in ryegrass’ ability to resist these interventions.

Ultimately, by studying both the genetic (inherited changes in DNA) and plastic (flexible, non-genetic adaptations) responses, we aim to uncover just how adaptable ryegrass is—and answer a crucial question: is there a risk of annual ryegrass gaining a ‘superweed’ status?

Sowing the seeds of victory

So far, our research has revealed that ryegrass populations tend to favour certain survival strategies, particularly those that align with stress tolerance (S) and competitiveness (C). But adaptation is not simple — while a plant might develop resistance to a specific herbicide, it often comes with a biological cost, like slower growth or smaller biomass.⁹ This means that it is tough for a single plant to thrive across all strategies, such as being both a top competitor and herbicide survivor at the same time. Based on these preliminary findings, we are leaning towards the second hypothesis of superweed evolution: that ryegrass populations are made up of different biotypes, enabling the population to persist under varying conditions.

The CSR framework may serve as a valuable lens in linking specific physiological traits to survival strategies, allowing us to categorise ryegrass biotypes based on their traits and genetic makeup. This deeper understanding helps us pinpoint the exact type of ‘superweed’ we are dealing with. In agriculture, this could be game-changing. We could better assess weed populations’ potential in crop fields —considering the land’s management history — and provide farmers with a more individualised strategy for more sustainable weed control. However, we are still in the early stages of this research, and there is much more to explore. Future studies could push our hypothesis further, testing it in different climates or with other weed species.

Much like the ryegrass growing in our plots, the field of experimental evolution is gradually taking root. It could become a powerful tool in building more resilient food systems, ensuring we keep food on our tables.

Ana Krsteska is a Master of Biosciences student in the Adaptive Evolution Lab at the University of Melbourne.

References:

1. Department of Agriculture, Fisheries and Forestry. (2023). Weeds Management. Vegetation. www.agriculture.gov.au/agriculture-land/farm-food-drought/natural-resources/vegetation/weeds2
   
2. Matzrafi, M., et al. (2021). Review: Evolutionary drivers of agricultural adaptation in Lolium spp. Pest Management Science, 77(5), 2209–2218. doi.org/10.1002/ps.6219
   
3. Bajwa, A. A., et al. (2021). The Remarkable Journey of a Weed: Biology and Management of Annual Ryegrass (Lolium rigidum) in Conservation Cropping Systems of Australia. Plants, 10(8), Article 8. doi.org/10.3390/plants10081505
   
4. Grime, J. P. (1977). Evidence for the Existence of Three Primary Strategies in Plants and Its Relevance to Ecological and Evolutionary Theory. The American Naturalist, 111(982), 1169–1194.
5. MacLaren, C., et al. (2020). An ecological future for weed science to sustain crop production and the environment. A review. Agronomy for Sustainable Development, 40(4), 24. doi.org/10.1007/s13593-020-00631-6
   
6. Escobedo, V. M., et al. (2023). Adaptive plasticity to drought of Grime’s CSR strategies. Oikos, 2023(11), e09754. doi.org/10.1111/oik.09754
   
7. Richards, C. L., et al. (2006). Jack of all trades, master of some? On the role of phenotypic plasticity in plant invasions. Ecology Letters, 9(8), 981–993. doi.org/10.1111/j.1461-0248.2006.00950.x
   
8. Drenovsky, R. E., et al. (2012). A functional trait perspective on plant invasion. Annals of Botany, 110(1), 141–153. doi.org/10.1093/aob/mcs100
   
9. Vila-Aiub, M. M., et al. (2009). Fitness costs associated with evolved herbicide resistance alleles in plants. New Phytologist, 184(4), 751–767. doi.org/10.1111/j.1469-8137.2009.03055.x