Take home messages

• Glyphosate resistance in annual ryegrass continues to increase as detected from random weed surveys and from commercial resistance testing.
• Resistance to paraquat has not been detected in the Wimmera-Mallee.
• Reduced control occurs when herbicides are applied to large or stressed weeds.
• Improving herbicide efficacy by good application can reduce selection for herbicide resistance.
• Minimise reliance on glyphosate to control ryegrass on fencelines.
• Test to check or confirm glyphosate or paraquat resistance.

Incidence of resistance in western Victoria

The GRDC continues to fund random weed surveys in cropping regions to monitor for changes in resistance levels in key weed species. The methodology involves collecting weed seeds from paddocks chosen randomly. Plants are tested in outdoor pot trials during the growing season.

Figure 1. Extent of herbicide resistance to various herbicide actives as detected by random weed surveys in western Victoria in annual ryegrass by the University of Adelaide. (Samples with ≥20% survival classified as resistant).

Extent of glyphosate resistance in western Victoria

The most recent GRDC funded random weed survey in 2015 revealed 3% of the ryegrass samples from the Mallee exhibiting resistance to glyphosate and 9% from the Wimmera. Data from a 2020 random weed survey will be available end 2022.

Resistance to glyphosate in the Wimmera-Mallee has been detected in many samples sent to Plant Science Consulting with approximately 30% of the samples tested from this region resistant to glyphosate (Table 1). It is important to note that for at least half the samples, glyphosate resistance was not suspected. No resistance to paraquat has been detected in this region. The addition of clethodim to glyphosate is an effective tool in glyphosate tolerant canola, although a few resistant samples (3%) have been confirmed (Table 1).

Table 1: Percent of resistant and developing resistant samples to knock-down herbicides that have been confirmed by Plant Science Consulting since 2016. ‘Resistant’ classified where ≥20% survival was detected in a pot trial and ‘Developing resistant’ classified 1-19.9% survival was detected. Numbers in brackets are the number of ryegrass samples tested.

Evolution of glyphosate resistance

Glyphosate was first registered in the 1970s and rapidly became the benchmark herbicide for non-selective weed control. Resistance was not detected until 1996 in annual ryegrass in an orchard in southern NSW (Powles et al. 1998). Only a few cases of resistance were detected in the following decade. The fact that it required decades of repeated use before resistance was confirmed indicated that the natural frequency of glyphosate resistance was initially extremely low.

There are several contributing factors for the increasing glyphosate resistance in ryegrass with generally more than one contributing factor. Reducing rates increases the selection for resistance, particularly in an obligate outcrossing species such as ryegrass, resulting in the accumulation of multiple weak resistance mechanisms creating individuals capable of surviving higher rates. This has been confirmed by Dr Chris Preston where ryegrass hybrids possessing multiple resistance mechanisms were generated by crossing parent plants with different resistance mechanisms (Figure 2).

Figure 2. Crossing of two glyphosate resistant ryegrass biotypes, one with a translocation resistance mechanism and the other with a target site resistant mechanism to create a hybrid containing both mechanisms.

There are many factors that reduce the efficacy of glyphosate and therefore increase the selection for resistance.

• Using low quality glyphosate products and surfactants. There are at least 500 glyphosate products registered in Australia. The quality of the inbuilt surfactants determines the effectiveness of the product.
• Mixing glyphosate with several active ingredients can become antagonistic, particularly where low water rates are used.
• Using fast acting products in the warmer months, for example, mixtures with Group 14 (G) herbicides in summer.
• Using low quality water, particularly hard water. Glyphosate is a weak acid and binds to positive cations (such as magnesium, calcium and bicarbonate) that are in high concentration in hard water (>200 ppm).
• Applying glyphosate during periods of high temperature and low humidity, resulting in the rapid loss of glyphosate from solution on leaf surfaces, thereby reducing absorption.
• Translocation of glyphosate in stressed plants can be reduced. Optimising glyphosate performance requires the translocation to the root and shoot tips. This occurs rapidly in small seedlings and takes longer in larger plants because glyphosate must translocate further to reach the root and shoot tips.
• Shading effects reducing leaf coverage resulting in sub-lethal effects.
• Applying glyphosate onto plants covered with dust can result in reduced available product for absorption as glyphosate strongly binds to soil particles.
• Application factors such as speed, nozzle selection and boom height can reduce the amount of glyphosate coverage.
• A combination of the above factors can reduce control and increase selection for resistance.

Optimising glyphosate performance

The selection of glyphosate resistance can be minimised by considering the points above. There are a number of important pathways to improve glyphosate performance.

• Avoid applying glyphosate under hot conditions. A trial spraying ryegrass during the end of a hot period and following a cool change was conducted in October 2019. Ryegrass growing in pots were sprayed at 8am, 1pm and 8pm with temperature and Delta T recorded prior to each application. Control of well hydrated plants ranged between 0% and 40% when glyphosate was applied during hot weather (30°C to 32.5°C) and high Delta T (14 to 16.7), with the lowest control when glyphosate was applied at midday (Figure 3). In contrast, glyphosate applied under cool conditions just after a hot spell resulted in significantly greater control (65%-80%), indicating that plants can rapidly recover from temperature stress provided moisture is not limiting.

Figure 3. Effect of temperature and Delta T on glyphosate for ryegrass control (Plant Science Consulting).

• Improving water quality and glyphosate activity by using ammonium sulfate (AMS). The addition of AMS has several functions. One is to soften water by combining with positively charged ions such as magnesium and calcium common in hard water. The negative charged sulfate ions combine with the positive cations preventing them from interacting with glyphosate and reducing its solubility and leaf penetration. Additionally, AMS has been shown to independently improve glyphosate performance, as the ammonium ions can work with glyphosate to assist leaf entry, increasing uptake and activity. In a pot trial conducted with soft water, ammonium sulfate significantly improved control of ryegrass with 222mL/ha (100g ai/ha) of glyphosate 450 (Figure 4). As a general rule, growers using rainwater (soft) should consider 1% AMS, if using hard water (such as bore, dam) 2% AMS is recommended. The addition of a wetter resulted in a further improvement of herbicide efficacy.

Figure 4. Effect of ammonium sulfate and wetter on glyphosate for ryegrass control.

• Herbicide activity can vary at different growth stages. In a pot trial investigating the effect of glyphosate at four ryegrass growth stages (1-leaf to 4-tiller), good control was achieved at the three older growth stages but not on small 1-leaf ryegrass (Figure 5). Most glyphosate labels do not recommend application of glyphosate on 1-leaf ryegrass seedlings. Very small seedlings (namely, 1-leaf) are still growing on seed reserves and have not yet commenced sugar production via photosynthesis. As a consequence, little glyphosate is translocated downwards with the sugars to the growing point of shoots and roots (meristem), reducing efficacy.

Figure 5. Effect of ryegrass growth stage on glyphosate activity.

• Relying on glyphosate only to control ryegrass on fencelines. Two trials were conducted, one in Hilltown, north of Adelaide and another in Ungarra on the Eyre Peninsula. Glyphosate resistance was highest closest to the fenceline and diminished further away from the fenceline. The level of glyphosate resistance in individuals closer to the fenceline was greater indicating that glyphosate resistance genes were more concentrated. Movement of glyphosate resistance within a paddock from a fenceline occurs due to pollen movement and mechanical processes (including sowing, harvesting, movement by vehicles, stock).

Figure 6. Survival of annual ryegrass plants collected at various distances into the crop to different rates of glyphosate.

• Fenceline control of ryegrass. It is a common practice to spray fencelines in spring. It is generally accepted that controlling weeds at younger growth stages is more effective than larger plants. Therefore, controlling multi-tillered ryegrass in spring is less effective, particularly as temperatures increase. Resistance develops quicker along fencelines treated regularly with glyphosate due to the lack of crop competition enabling survivors to recover and produce thousands of seeds per plant. Several fenceline trials were conducted between 2007 and 2013 to investigate the control of glyphosate resistant ryegrass. Mixtures of Uragan® with paraquat or glufosinate were very effective (Table 2).

Table 2: Effect of herbicides on annual ryegrass in a fenceline at Hilltown, South Australia.

¹Roundup Attack™(540 g ai/L glyphosate)

²Spray.Seed® (135 g ai/L paraquat + 115 g ai/L diquat)

³Basta® (200 g ai/L glufosinate-ammonium)

⁴Uragan® (800 g ai/kg bromacil)

Rather than relying on glyphosate to control ryegrass growing along fencelines, switching to diverse herbicide and non-herbicide tactics including double-knock, chipping survivors, slashing, burning, livestock, cultivation and spraying different herbicide modes of action including residuals is recommended. Residual herbicides registered for fencelines include Uragan (bromacil - Group 5 mode of action) and Terrain™ (flumioxazin - Group 14). These should be applied in autumn or winter for maximum effect.

• A double-knock strategy is defined as the sequential application of two weed control tactics to combat the same weed population. The most common double-knock strategy is glyphosate followed by paraquat. It has been widely adopted for over a decade to prevent or combat glyphosate resistance particularly in ryegrass. The first ‘knock’ with glyphosate controls the majority of the population with the second ‘knock’ (paraquat) intended to kill any individuals that have survived glyphosate. On glyphosate resistant ryegrass, paraquat applied 1-5 days after glyphosate provided optimum control in trial work conducted by Dr Christopher Preston (Figure 7). The timing depends on weed size and growing conditions, with 2-5 days the optimum timing to maximise glyphosate uptake and translocation. After 7 days, glyphosate resistant plants treated with glyphosate can stress resulting in the absorption of less paraquat, reducing control with the second tactic. If growing conditions are poor or plants are large, the stress imposed by glyphosate may be further delayed. The use of glyphosate first in a double-knock situation, even where glyphosate resistance exists in a fallow situation, is very important to reduce the selection pressure on paraquat resistance. Exposing large populations to paraquat increases the risk of paraquat resistance.

Figure 7. Double-knock timing. Glyphosate applied onto a susceptible (S) and two glyphosate resistant ryegrass biotypes (R1 and R2) followed by paraquat 1, 3, 5, 7 and 10 DAA. Trial work conducted by Dr Christopher Preston (The University of Adelaide).

Incidence of paraquat resistance

Resistance to paraquat has been detected in a few ryegrass populations from WA, SA and Victoria. They have originated along fencelines, non-cropped farm areas, lucerne/clover seed production paddocks and vineyards where multiple applications in a single season are common (Figure 8). While the number remains low, it is important to use paraquat according to label recommendations with emphasis on using medium nozzles, rate, growth stage and population size. The first case of paraquat resistance in ryegrass detected globally was in South African orchards after decades of use on advanced growth stages resulting in sub-lethal effects (Cairns and Eksteen, 2002). More recently, a few populations have been confirmed paraquat resistant from paddocks in south-western Victoria indicating that selection for paraquat resistance is occurring. Including other fast acting herbicides in combination with paraquat, such as the Group 14 (G) herbicides Terrad’or® or Voraxor®, can reduce the selection pressure on paraquat.

Figure 8. Efficacy of the first confirmed cases of paraquat resistance in annual ryegrass from SA, Victoria and WA. Paraquat formulation (250 g ai/L). Error bars indicate variation. Trial reference no. 2045 conducted by Plant Science Consulting.

Herbicide resistance testing

Plants can survive a herbicide application for several reasons, one being resistance. In the case of a herbicide failure, a resistance test can confirm if resistance contributed to the reduced control. In some cases, increasing the rate of glyphosate or paraquat can provide complete control. For more information, visit Plant Science Consulting.

Conclusion

Glyphosate resistance continues to increase in annual ryegrass in the southern zone. Decades of strong selection pressure resulting from repeated use have contributed to the concerning resistance levels recently detected. Optimising the use of glyphosate and paraquat will slow down the onset of resistance. Although paraquat resistance is rare, its reliance has increased due to glyphosate resistance. More efficient use of glyphosate combined with effective IWM strategies is required to minimise further increases in resistance.

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. The information for the random weed surveys was undertaken as part of GRDC projects UCS00020 and UCS00024.

References

Powles SB, Lorraine-Colwill DF, Dellow JJ, Preston C (1998) Evolved resistance to glyphosate in rigid ryegrass (Lolium rigidum) in Australia. Weed Science 46, 604-607.

Contact details

Sam Kleemann
Plant Science Consulting P/L

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

Chris Preston
University of Adelaide, Waite Campus, Glen Osmond SA 5064
christopher.preston@adelaide.edu.au