Combatting weed resistance – best practice management for tackling stubborn resistance with ryegrass, radish, fleabane plus others in the Vic Wimmera

Take home messages

• Herbicide resistance is most prevalent in ryegrass, sowthistle (milk thistle) and prickly lettuce (whip thistle). It is less common in brome, barley grass, wild oats, wild radish, Indian hedge mustard and fleabane.
• The most common resistance is to Group 1 and 2 herbicides.
• The highest incidence of resistance is in ryegrass and sowthistle in Victoria.
• Including herbicides with alternative modes of action such as Group 13 (Q), 15 (J/K), 22 (L), 23 (E) and 30 (T) for ryegrass and Group 13 (Q), 14 (G) and 27 (H) for broadleaf weeds can help combat resistance.

Broadleaf weed species in broadacre cropping

Herbicide resistance has been detected in several broadleaf weed species in cropping paddocks (Table 1). Wild radish, Indian hedge mustard, sowthistle (milk thistle), prickly lettuce (whip thistle) and fleabane are the most prevalent broadleaf weeds with resistance. Herbicide resistance has also been confirmed in another six broadleaf weed species (Table 1).

Table 1: Occurrence of broadleaf weed species confirmed herbicide resistant from southern Australia. Source of data: CropLife Australia.

Resistance has been quantified from random weed surveys (GRDC investment UCS2008-001RTX) by testing plants from weed seeds collected prior to harvest (Table 2).

Wild radish

In random weed survey samples, resistance to Group 2 sulfonylurea (SU) herbicides was more prevalent than resistance to imidazolinone (IMI) herbicides. A significant substantial percentage of samples from SA exhibited resistance to Group 4 (2,4-D) (30%), with fewer from Victoria (5%). No resistance to Group 5 (atrazine) or Group 12 (diflufenican) was detected.

Indian hedge mustard (IHM)

Similar to wild radish, greater resistance was detected to SU than to IMI herbicides. The incidence of resistance to Group 2 herbicides was higher in SA than Vic. In contrast, greater resistance to Group 12 (diflufenican), Group 5 (atrazine) and Group 4 (2,4-D) was detected in Vic.

Sowthistle

Sowthistle is the most frequently encountered broadleaf weed species in random weed surveys. In samples collected during the random weed surveys, over three quarters of the samples from both states exhibited resistance to SU herbicides. Although IMI herbicides were not tested, other trials have indicated that there is a strong correlation between SU and IMI herbicide resistance in sowthistle. A recent survey identified 78% of populations resistant to SU and 68% resistant to IMI herbicides (Merriam et al. 2018). A low percentage of samples from SA (4%) and Victoria (3%) exhibited resistance to 2,4-D. No resistance to Group 9 (glyphosate) was detected.

Table 2: Extent of resistance to various herbicides in broadleaf weed species from surveys conducted 2013–2020. The number of samples for each species is in brackets ( ) next to the species name. Samples were considered to be resistant if >20% of individuals within that population survived application of the herbicide in pot trials.

A dash indicates herbicide was not used on this species.

Prickly lettuce

Prickly lettuce is not collected in the pre-harvest surveys as it has not set seed. Directed surveys were conducted in SA in 1999 and 2004 that showed SU resistance in prickly lettuce was already high, with 66% of populations from 1999 and 82% of populations from 2004 resistant to SU (Lu et al. 2007). In 2018, 30 populations were collected from four regions in South Australia. Every population exhibited resistance to both SU and IMI herbicides, showing a continued increase in Group 2 herbicide resistance.

Fleabane

Fleabane has not been included in the species to collect list during the random weed surveys as the seed is not mature at this time. Seeds of fleabane from a survey of 89 agricultural locations in 2014 across north-east Victoria were collected in a survey targeting only fleabane (Aves et al. 2020). Of these, 40% exhibited resistance to glyphosate, 100% resistant to SU, with no resistance to Group 4 herbicides or paraquat (Group 22) detected. Testing of suspect samples sent by agronomists to PSC confirmed resistance to paraquat particularly from southern NSW.

Combatting herbicide resistance in broadleaf weeds

Where there are several herbicides within a chemical group (such as SU and IMI), testing for resistance is important to identify specific herbicides that are still effective. It is also useful to test for resistance to other mode of action (MoA) herbicides such as Group 4 (2,4-D), Group 5 (atrazine) and Group 12 (diflufenican), to identify herbicides that may still work.

Resistance to one MoA, such as to Group 4 herbicides, may be controlled using products containing multiple herbicides with different MoA, such as Group 6 (bromoxynil) or Group 12 (diflufenican, picolinafen). These multiple herbicide products sometimes contain Group 4 herbicides also.

In the recent few years, newer herbicides with alternative modes of action have become available such as Group 14 (G) and Group 27 (H). The option to use pre-emergent herbicides at sowing, such as Voraxor® (Group 14) and Callisto® (Group 27), can increase the flexibility of broadleaf weed control in cereals. A continuing challenge has been in controlling resistant broadleaf weeds in broadleaf crops where IMI herbicides have been widely used. Herbicides including Overwatch® (Group 13, selective in canola) and Reflex® (Group 14, selective in pulses) enable the use of diverse MoA thereby extending the life of the more commonly used products. Pre-harvest control can be effective with glyphosate (Group 9), paraquat (Group 22) and Sharpen®(Group 14). Sharpen is registered in wheat and pulse crops. Seed capture techniques, such as the Seed-Terminator, can be effective for weeds that do not shed readily, such as Indian hedge mustard and in certain cases, wild radish.

Ryegrass

The latest (2020) random weed surveys have identified that resistance has increased to post-emergent herbicides and to trifluralin and Boxer Gold® but not to propyzamide or Sakura®.

No resistance to Rustler® (propyzamide), Overwatch (Group 13) or Luximax® (Group 30), has been detected from the random weed surveys, indicating the resistance levels are very low. Propyzamide has been used for over a decade, with no cases of resistance reported in ryegrass.

Clethodim is arguably the most important herbicide in break crops. The increase in resistance to clethodim, and more so to glyphosate, is of particular concern. Testing for herbicide resistance is important to check if plants that are present after a herbicide application are actually resistant. The use of paraquat (Group 22) as part of the double knock is important to reduce the selection pressure of both herbicides.

Table 3: Percentage of paddocks containing herbicide resistant ryegrass in western Victoria. Paddocks were scored as resistant if the seeds collected exhibited ³20% survival in a pot test conducted the following autumn–winter. In the 2020 survey, paraquat, Luximax and Overwatch were also included with no resistance detected.

For each post-emergent herbicide, recommended field rates plus recommended adjuvants.

nt= not tested

Control of resistant ryegrass using alternative mode of action pre-emergent herbicides

Several herbicides, most of which have diverse modes of action that differ to currently registered herbicides, such as Luximax (Group 30), Overwatch (Group 13) and Ultro® (Group 23), are currently available. Use of these herbicides, particularly in combination with older chemistry such as trifluralin or Avadex, can control multiple resistant ryegrass and other important weed species. If used as part of an integrated weed control strategy, these alternative MoA herbicides are likely to reduce selection pressure to any single MoA herbicide.

Other grass weed species important in Victoria

Brome, barley grass and wild oats are less prevalent than ryegrass, with resistance not increasing in the last decade. Nevertheless, they can be problematic where resistance occurs. Group 1 herbicides remain effective in most cases, as are Group 2 IMI herbicides. Control with most pre-emergent herbicides that are used to control ryegrass is variable. Ultro (Group 23), registered in pulse crops, has shown good activity on barley grass and brome.

Table 4: Resistance detected to key weed species from the last two random weed surveys conducted in 2015 in Victoria.

Herbicide resistance testing

A weed control failure is not always due to herbicide resistance. By identifying which herbicides are effective and which are not, an optimised weed control strategy can be implemented. In Victoria in the past seven years, resistance in a number of species has been detected (Table 5). With the exception of ryegrass in some cases, where resistance was detected in the other species, the resistance testing detected other herbicides that were effective.

Table 5: Summary of weed species tested for herbicide resistance from Victoria. Samples sent to Plant Science Consulting by growers or their agronomists 2016–2023. Seed sent at harvest time or plants (Quick-Test) during the growing season were tested.

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.

References

Aves C, Broster J, Weston L, Gill GS, Preston C (2020) Conyza bonariensis (flax-leaf fleabane) resistant to both glyphosate and ALS inhibiting herbicides in north-eastern Victoria. Crop and Pasture Science 71, 864-871.

Lu Y-Q, Baker J, Preston, C (2007) The spread of resistance to acetolactate synthase inhibiting herbicides in a wind borne, self-pollinated weed species, Lactuca serriola L. Theoretical and Applied Genetics 115, 443-450.

Merriam AB, Boutsalis P, Malone J, Gill G, Preston C (2018) Extent of herbicide resistant common sowthistle (Sonchus oleraceus) in southern Australia. 21st Australasian Weeds Conference. Sydney, Australia, September 2018, pp. 16-19.

Additional resources

Tranel PJ, Wright TR (2002) Resistance of weeds to ALS-inhibiting herbicides: what have we learned? Weed Science 50, 700-712.

Contact details

Peter Boutsalis
Plant Science Consulting and The University of Adelaide
Waite Campus, Glen Osmond SA 5064
info@plantscienceconsulting.com.au