Understanding resistance mechanisms in Group J and Group K resistant annual ryegrass (Lolium rigidum) populations

Take home messages

• There is increasing incidence of Group J herbicide resistance in annual ryegrass across southern Australia.
• Cross-resistance to the Group K herbicides, including Sakura®, has occurred.
• Glasshouse studies using phorate suggest P450 enzymes conferring resistance to Group J herbicides.
• Resistance to Group K herbicide pyroxasulfone is most likely conferred through glutathione S-transferases (GSTs).

Background

Annual ryegrass (Lolium rigidum) is a problematic weed species present in cropping fields across southern Australia with the capacity to germinate throughout autumn and winter and produce a large amount of seed. The evolution of resistance to commonly used post-emergent herbicides (Boutsalis et al. 2012) has resulted in increased reliance on pre-emergent herbicides for weed control. Resistance to trifluralin has been present in South Australia for many years and over the past 10 years has become widespread across southern Australia (Boutsalis et al. 2006). This resulted in the adoption of Group J herbicides, particularly Boxer Gold® after its release in 2008. This increased dependence on Group J herbicides in recent years has led to resistance, which has now been confirmed in South Australia, Victoria and New South Wales. Release of the Group K herbicide Sakura® in 2012 provided growers with an effective herbicide to manage annual ryegrass in wheat. No known resistance to pyroxasulfone (Sakura®) had been observed until its detection in early 2017 in a population of annual ryegrass from the Eyre Peninsula (Brunton et al. 2018). Furthermore, results from a recent herbicide resistance survey conducted in 2017 have shown an increase in the number of paddocks in south eastern South Australia that contain populations of annual ryegrass with resistance to both Group J and Group K herbicides (Boutsalis et al. 2017 unpublished data).

Most of these populations are resistant to trifluralin, suggesting that once trifluralin has failed the resulting increased selection pressure placed on other pre-emergent herbicides, inadvertently selects for a diverse resistance mechanism conferring resistance to these newer pre-emergent herbicides. The possible mechanisms conferring resistance within these populations can be described as complex metabolic cross-resistance in which the resistance mechanism causes multiple herbicides from different modes of action to fail nearly simultaneously. Two major plant enzymes, cytochrome P450s (P450s) and glutathione S-transferases (GSTs) are known to be involved in the metabolism and degradation of many xenobiotics, herbicides included. Group J herbicides require activation in order to exert herbicidal effects and this process involves conversion through P450s into a toxic metabolite. Research undertaken in a wild oat (Avena fatua) population in Canada found resistance to Group J herbicide triallate was due to reduced herbicide activation (Kern et al. 1996). GSTs are responsible for metabolising Group K herbicides, which do not require activation to exert herbicidal activity within the plant. Tolerance to the Group K herbicides is observed in wheat and it has been suggested that a similar mechanism exists in glasshouse selected annual ryegrass populations with resistance to pyroxasulfone. The specificity of P450 enzymes to bind a wide range of chemical compounds including organophosphate-based insecticides allows these to be used as an inhibitor of herbicide metabolism in studies. Understanding resistance mechanisms is important in better understanding the evolution of Group J and Group K resistance, but in addition provides opportunities to develop more effective management strategies to prolong the effectiveness of these herbicides.

Method

A series of dose response experiments were undertaken to characterise Group J and Group K resistant annual ryegrass populations collected from cropping paddocks across southern Australia. Additional experiments were undertaken in annual ryegrass populations with confirmed resistance to Group J and Group K herbicides to determine possible resistance mechanisms using the organophosphate insecticide, phorate. Included in the study was a population known as RAC1 (resistant biotype) which represents survivors from one million SLR4 (susceptible biotype) seeds exposed to 6000g ai/ha triallate in the field.

Results and discussion

Dose response characterisation of annual ryegrass populations

All resistant populations tested showed resistance to both Group J herbicides (Table 1). As expected, these populations also display resistance to trifluralin, except for RAC1 which was controlled at the field recommended rate. Cross-resistance was observed to the Group K herbicides metazachlor (Butisan®) and S-metolachlor (Dual Gold®), while RAC1 and the susceptible SLR4 were controlled with all herbicides at the recommended rates. Resistance to the Group K herbicide pyroxasulfone (Sakura®) was also present with four of five populations displaying moderate levels of resistance to this herbicide. The current ability to manage annual ryegrass with pre-emergent herbicides is threatened by the discovery of populations with resistance to pyroxasulfone (Sakura®) from different cropping regions across southern Australia. There is currently one effective pre-emergent herbicide available (propyzamide*) in pulses, however, no options remain currently available for cereal crops.

*Pulses are not included on all propyzamide labels, the chemical product that is registered is for use on pulses is Conquest Dargo 500 SC.

Table 1. Concentration of various pre-emergent herbicides required for 50% mortality (LD50) of example resistant (R) and susceptible (S) annual ryegrass populations.

*RAC1 represents survivors from one million SLR4 seeds exposed to 6000g/ha triallate.

Response of annual ryegrass populations treated with insecticide inhibitor

Annual ryegrass treated with the inhibitor phorate responded in two contrasting ways for the Group J herbicides, prosulfocarb and triallate (Table 2). Resistant population 198-15 treated with phorate plus herbicide showed a slight increase in LD₅₀ for prosulfocarb (18329 gai/ha) and triallate (8579 gai/ha) compared to herbicide only (15446 and 8089 gai/ha), respectively. In contrast, SLR4 treated with phorate resulted in a significantly higher LD₅₀ for both prosulfocarb (2006 gai/ha) and triallate (4710 gai/ha) compared to herbicide only 353 and 179 gai/ha, respectively. The same was seen in RAC1 (R) treated with both herbicides with and without phorate. Treatment with phorate had little impact on the efficacy of Group K herbicides (pyroxasulfone and S-metolachlor) suggesting limited involvement of P450s (Table 2). Populations 198-15 and RAC1 treated with trifluralin plus phorate showed 4% and 20% reduction in LD₅₀. Contrastingly, the population SLR4 displayed a 29% increase in LD50 similar to the response seen in the susceptible when treated with a Group J herbicide plus phorate.

Table 2. Annual ryegrasstreated with (+) or without (-) inhibitor (phorate) and concentration of various pre-emergent herbicides required for 50% mortality (LD50) of 198-15 and RAC1 (resistant) and SLR4 (susceptible) annual ryegrass populations.

Conclusion

Cross-resistance evolution in annual ryegrass threatens the sustainability of current pre-emergent herbicides available to growers and poses a significant challenge to its management in cereals. This study has highlighted two possible mechanisms conferring resistance in annual ryegrass populations exhibiting multiple resistances to a number of different pre-emergent herbicides. Resistance to Group J herbicides in annual ryegrass likely involves changes to a single or multiple P450s. Resistance to Group K herbicides was not affected by the P450 inhibitor and so GST involvement similar to that seen in wheat is most likely (Tanetani et al. 2013). This study has provided an understanding of the potential mechanisms involved in Group J and Group K herbicides, but highlights the strong possibility of resistance evolution to these modes of action if over reliance occurs. Populations with multiple resistances to pre-emergent herbicides have multiple mechanisms of resistance, making it easy to evolve resistance to new herbicides.

References

Boutsalis P, Gill GS, Preston C (2012). Incidence of herbicide resistance in rigid ryegrass (Lolium rigidum) across southeastern Australia. Weed Technology 26:391-398.

Boutsalis P, Preston C, Broster J (2016). Management of trifluralin resistance in annual ryegrass (Lolium rigidum Gaudin) in southern Australia. Pages 507-510.

Brunton DJ, Boutsalis P, Gill G, Preston C (2018). Resistance to multiple PRE herbicides in a field-evolved rigid ryegrass (Lolium rigidum) population. Weed Sci. 66:581-585.

Kern AJ, Peterson DM, Miller EK, Colliver CC, Dyer WE (1996). Triallate resistance in Avena fatua L. is due to reduced herbicide activation. Pest. Biochem. Physiol. 56:163-173.

Tanetani Y, Ikeda M, Kaku K, Shimizu T, Matsumoto H (2013). Role of metabolism in the selectivity of a herbicide, pyroxasulfone, between wheat and rigid ryegrass seedlings. J. Pestic. Sci. 38:152-156.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC – the author would like to thank them for their continued support. Additional thanks to members of the Weed Research Group at the University of Adelaide.

Contact details

David Brunton
The University of Adelaide
Waite Campus, PMB 1, Glen Osmond, South Australia, 5064
0439 978 133
david.brunton@adelaide.edu.au