Herbicide resistance levels and management of annual ryegrass in Tasmania

Take home messages

• Annual ryegrass resistant to Group 1 and 2 herbicides has been detected in at least 50% of fields in Tasmania.
• Resistance to pre-emergence herbicides and glyphosate in ryegrass is low.
• Understanding the properties of pre-emergence herbicides is important to optimise the fit into Tasmanian cropping rotations.
• Monitoring for resistance using herbicide resistance testing is important to maximise crop yields.

National herbicide resistance weed survey 2020–2023

A national GRDC funded weed survey commenced in 2020. In a national collaboration between universities, over 1500 paddocks were sampled across WA, SA, Vic, Tas, NSW and Qld in 2020 and 2021. Grower paddock details were supplied by agronomists and each university randomly selected a set number of paddocks in their respective state. After sampling, the national ryegrass collection was sent to the University of Adelaide for testing. The other species were tested by other collaborators. The results for ryegrass are presented in Table 1. Twenty paddocks were randomly selected in Tasmania. No resistance to any of the pre-emergent herbicides tested was detected (Table 1). This is most likely a consequence of the diverse rotational crops in Tasmania where herbicide residues are important. It follows that there has been a greater reliance on post-emergent's resulting in more resistance to Group 1 herbicides (Axial® and clethodim) than the national average (Table 2). No resistance to glyphosate or paraquat was detected and the incidence of resistance to the Group 2 herbicides Intervix® and Hussar® was lower than the national average, most likely due to less frequent use to minimise residues for following sensitive crops.

Table 1: Percentage of paddocks detected with resistant ryegrass treated with the recommended label rate of pre-emergence herbicides. Resistance is defined as a sample where 20% plant survival was detected in the 2021 pot trials.

*Clethodim (240 g/L), Paraquat (250 g/L), Glyphosate (540 g/L)

Table 2: Percentage survival of ryegrass to post-emergent herbicides from each state and the national average. Resistance was defined as any sample exhibiting 20% survival or greater in the pot trials. Adjuvants as per label recommendations were used.

Herbicide resistance testing has the benefit of providing information on resistance levels which can be used to plan future cropping rotations and weed control strategies. This autumn RM Consulting Group (RMCG) with funding from vegetable industry levy (Phase 3 of VegNET Program) have initiated testing of ryegrass plants collected from vegetable fields. Within 3 weeks of testing with the Quick-Test, the results were available (Table 3). Testing showed 12 of the 13 samples exhibiting strong resistance to Group1 herbicide Fusilade Forte®. The use of alternative Group 1 FOPs would not be effective. Over half the samples exhibited resistance to clethodim which supports the findings of the 2020 random weed survey. The presence of clethodim resistance indicates that these samples would be cross-resistant to Axial and Achieve®, herbicides used in cereals. Only three samples exhibited resistance to glyphosate, supporting the reduced use of glyphosate in Tasmania vs mainland cropping regions where at least 16% of ryegrass samples in the national 2020 survey were glyphosate resistant.

Table 3: Quick-Test resistance testing by Plant Science Consulting of 13 ryegrass samples in autumn 2023 from vegetable crops in Tasmania. This was funded by vegetable industry levy. The data presented is per cent survival with a resistance rating to indicate the damage observed on surviving plants. A ‘0 S’ indicated susceptibility, ‘R’ indicated weak resistance (survivors heavily damaged but reshooting), ‘RR’ indicates intermediate resistance (40–60% growth reduction) and ‘RRR’ indicates 0–30% growth reduction.

Improving ryegrass weed control – incorporating pre-emergent herbicides

In mainland cropping regions, resistance to in-crop post-emergence Group 1 and 2 herbicides for ryegrass control has increased the reliance on pre-emergent herbicides with differing modes of action. These herbicides include Sakura (Grp 15), Mateno® Complete (Grp 32, 12, 15), Luximax (Grp 30), Ultro® (Grp 23) and Overwatch (Grp 13). Additionally, mixtures of some of these herbicides such as Sakura with Avadex Xtra® has been shown to increase control of several grass weed species. In Tasmania, poppies and some vegetable crops are sensitive to some of these herbicides, therefore use of pre-emergence herbicides in the cereal or canola/ pulse phases has been limited. With the recent announcement of a global poppy oversupply, it is likely that fewer poppies will be grown that could lead to the growing of crops where residual herbicides such as the ones listed above can be used to control herbicide resistant ryegrass.

P= pulses, W= wheat, B = barley, V= vegetables, C= canola

Table 4: Properties of registered pre-emergent herbicides. Koc indicates the strength of binding to organic matter, the higher the value the greater the binding. A high solubility indicates the potential for movement with rainfall.

On the mainland, broadacre crops are predominantly sown using minimum tillage (knifepoints + press wheels) and to a lesser extent zero tillage (discs). This has resulted in weed seeds located close to the surface, thereby maximising the efficacy with pre-emergent herbicides. In contrast, in Tasmanian fields where crop tillage is required to till the soil for the sowing of crops such as onions, carrots and potatoes, weed seeds occur at greater depth. Controlling weed seeds that germinate from greater depths would therefore be more difficult, particularly with less soluble products (Table 4). The use of herbicides such as Boxer Gold and Mateno Complete that are registered for applying early post-emergent would be better suited. Some herbicides that bind readily to stubble (Table 4, high KOC value) should not be used in high stubble load situations, as herbicide binding will reduce efficacy on weeds (Table 4).

Conclusion

Higher reliance on in-crop herbicides in Tasmania has resulted in at least 50% of fields containing ryegrass with resistance to Group 1 and 2 herbicides. Resistance to pre-emergence herbicides remains low. With a diverse range of available pre-emergence herbicides, fitting them into Tasmanian cropping rotations will aid in controlling Group 1 and 2 resistant ryegrass.

Acknowledgements

The information for the random weed surveys was undertaken as part of GRDC project UCS00020 and UCS2008-001RTX. Naomi Palombi and Ossie Lang from RMCG for permission to present the data funded by the vegetable industry levy (Phase 3 of VegNET Program).

Useful resources

Herbicide resistance testing (https://www.plantscienceconsulting.com.au)

Contact details

Peter Boutsalis
Plant Science Consulting P/L and University of Adelaide
Waite Campus, Glen Osmond SA 5064
peter.boutsalis@adelaide.edu.au
@PBoutsalis