Resistance continues to increase to key herbicides across the wheat belt of Southern Australia. Of the samples (seed & “Quick-Test”) presented to Plant Science Consulting for resistance testing, annual ryegrass remains number 1, followed by wild oats, predominantly from Nth NSW, and wild radish from WA. Resistance testing is also undertaken, but on significantly fewer samples of phalaris, brome, barley grass, Indian hedge mustard (IHM) and sowthistle.
Resistance in ryegrass to key MoA herbicides continues to increase in each of the states (Table 1). For the first time, a ryegrass sample was recently received from Queensland and confirmed resistant, showing the spread of ryegrass North. Most of the samples, however, are from SA, Victoria, NSW and WA, and to a lesser extent, Tasmania.
Table 1. Percentage of ryegrass samples tested by Plant Science Consulting and confirmed resistant from each of the states for the past 4 years (2020-2024). Samples were scored as resistant if the seeds collected exhibited ≥20% survival; value in brackets represents % of samples developing resistance (1-19% survival). Herbicide tests were undertaken following commercial label rates and with recommended adjuvants where required.
Post-emergent | National | SA | VIC | NSW | WA | |
Group 1 | Axial | 78 (11) | 63 | 83 | 80 | 73 |
Clethodim | 43 (23) | 37 | 44 | 47 | 40 | |
Group 2 | Hussar | 85 (6) | *55 | 91 | 86 | *100 |
Intervix | 72 (15) | 65 | 75 | 74 | 75 | |
Group 9 | Glyphosate | 36 (21) | 30 | 44 | 45 | 28 |
Group 22 | Paraquat | 4 (2) | 7 | 5 | 1 | 4 |
Pre-emergent | ||||||
Group 3 | Trifluralin | 44 (22) | 67 | 37 | 18 | 30 |
Propyzamide | 0 (0) | 0 | 0 | 0 | 0 | |
Group 15 | Boxer Gold | 2 (11) | 3 | 1 | 0 | 0 |
Sakura | 1 (9) | 2 | 0 | 0 | 0 | |
Group 13 | Overwatch | 0 (0) | 0 | 0 | 0 | 0 |
Group 30 | Luximax | 0 (0) | 0 | 0 | 0 | 0 |
*Insufficient sample number (<20 samples tested)
Ryegrass
Resistance remains high to Group 2 herbicides, particularly the SU’s, with 85% of samples confirmed resistant to Hussar® nationally. Cross-resistance to the imi-chemistry (ie. Intervix®) within the same MoA group has also continued to increase (72% nationally) and is in step with greater uptake of Clearfield technology. Most samples are showing resistance to both fop and den Group 1 herbicides. Axial® resistance ranged from 63% in SA to as high as 83% in Victoria, respectively. Increased selection pressure in pulses and canola has also seen a steady increase in Group 1 dim resistance, with nationally 43% of samples confirmed resistant to the registered field rate of clethodim (0.5 L/ha), with a further 23% developing resistance (1-19% survival). In some of these cases, testing to mixtures of clethodim + butroxydim (Factor®) has proven to be effective. Like clethodim, a significant number of samples are showing resistance to glyphosat,e with 30%, 44%, 45% and 28% of samples confirmed resistant from SA, Victoria, NSW and WA. Resistance to glyphosate continues to rise across all state,s with many growers relying on the Double-knock approach with robust rates of paraquat. Fortunately, only a small percentage of samples have been detected as resistant to paraquat, with resistance highest in SA (7%).
Apart from trifluralin, resistance remains low for other pre-emergent herbicides. Only 2% of samples were confirmed resistant to Boxer Gold® and 1% to Sakura®, with no resistance detected to propyzamide or newer herbicides Overwatch® and Luximax®. Resistance to trifluralin, as has been the case for several years, remains highest in SA (67%), followed by Victoria (37%), WA (30%) and NSW (18%).
Wild oats
Resistance to Group 1 herbicides continues to increase, particularly for samples from Nth NSW. As many as 66% of samples from this region showed resistance to Topik® (clodinafop) and were cross-resistant to Axial (pinoxaden) and Verdict® (haloxyfop). Many of these populations were controlled with clethodim; however, an increasing number are starting to show resistance (21%). Fortunately, resistance remains low to Group 2 herbicides (8% to Atlantis® and 11% to Rexade®), with resistance not yet detected to Intervix.
Wild radish
Most samples tested are from WA, where resistance to Group 2 (Eclipse®, Intervix) and 4 herbicides (MCPA, 2,4-D amine and Ester) remains high. As many as 68% of WA samples were resistant to 2,4-D Ester, with an increasing number of samples (36%) showing resistance to Group 12 herbicide Brodal (diflufenican). Resistance to Group 5 herbicides, atrazine and metribuzin, remains low (9%), with most samples presenting from the northern wheatbelt of WA where lupins and TT-canola are extensively grown. Fortunately, no resistance has been detected to bromoxynil, and only 1 sample from several hundred tested was confirmed resistant to Velocity (Group 27 + 6). Whilst HPPD resistance is rare (ie. Velocity), over-reliance on this important chemistry should be avoided.
Other spp.
An increasing number of phalaris samples from Nth NSW have recently been confirmed resistant to Group 1 and 2 herbicides (ie. Axial and Rexade). This trend is expected to continue with both MoA, which is heavily relied upon in the absence of effective alternate chemistries. Despite the growing adoption of Clearfield, resistance in both brome and barley grass to Intervix remains low, however, more samples of IHM are presenting as resistant. Resistance to 2,4-D amine has also been detected in IHM, fortunately, only a few populations have been identified.
Apart from ryegrass, resistance to glyphosate remains low for most other spp., including brome, barley grass and sowthistle, with resistance yet to be detected for wild oats or wild radish.
For further information on herbicide resistance testing please visit the Plant Science Consulting webpage: