While glyphosate resistance in weeds is on the rise, research has demonstrated control options are available.
Professor Chris Preston, from the University of Adelaide, told the 2024 GRDC Updates that experiments have explored the effects of different management strategies on glyphosate-resistant weeds.
For example, he said preliminary results for common sowthistle (Table 1) and feathertop Rhodes grass
(Table 2) showed that double knocks were better than using glyphosate alone for sowthistle but not for feathertop Rhodes grass, where a double knock of just glyphosate followed by paraquat was ineffective.
Table 1: Survival percentage of two glyphosate-resistant common sowthistle survival after herbicide treatment in the second year of a trial at Hermitage Research Facility, Warwick, Queensland.
| Survival percentage | Survival percentage | |
|---|---|---|
| Herbicide strategy | Sowthistle white | Sowthistle yellow |
| Double knock alternative - 2,4-D followed by paraquat + diquat (Spray.Seed®) | 1.1 | 0 |
| Double knock - glyphosate followed by paraquat + diquat (Spray.Seed®) | 0.1 | 0.6 |
| Single knock - glyphosate applied morning | 8 | 7 |
| Single knock - glyphosate applied midday | 20 | 13 |
| Residual herbicide - isoxaflutole (Balance®) | 0 | 0 |
Note: Populations with 30 per cent resistant individuals were sown and treated over two consecutive seasons with the same
herbicide strategies.
Source: Michael Widderick, Queensland Government Department of Agriculture, Fisheries and Forestry
Table 2: Feathertop Rhodes grass survival percentage with different target site gene mutations after herbicide treatment in the second year of a summer fallow trial at Hermitage Research Facility, Warwick, Queensland.
| Survival percentage | Survival percentage | Survival percentage | |
|---|---|---|---|
| Herbicide strategy | Pro 106 Leu1 | Pro 106 Ser1 | Pro 106 Thr1 |
| Double knock alternative - haloxyfop followed by paraquat | 16 | 55 | 0 |
| Double knock - glyphosate followed by paraquat | 92 | 59 | 51 |
| Single knock - glyphosate | 80 | 54 | 71 |
| Residual herbicide - s-metalachlor (Dual Gold®) | 0 | 0 | 0 |
Note: Populations with 30 per cent resistant individuals were sown and treated over two consecutive seasons with the same herbicide strategies. 1. Pro 106 Leu, Pro 106 Ser, Pro 106 Thr are the names of different gene mutations.
Source: Michael Widderick, Queensland Government Department of Agriculture, Fisheries and Forestry.
Furthermore, he said, herbicides other than glyphosate were better at keeping glyphosate-resistant populations low in problem species.
“For barley grass, research showed a double knock is better than glyphosate mixtures with Group 14 herbicides.”
Professor Preston said a challenge for managing glyphosate and paraquat-resistant annual ryegrass was that neither herbicide in the double knock would be effective.
“An alternative for managing glyphosate-resistant annual ryegrass when seasonal conditions are appropriate is to dry sow and use pre-emergent herbicides and crop competition.
“However, with dry sowing, choose your pre-emergent herbicides wisely.”
He said more persistent herbicides, such as s-metolachlor + prosulfocarb (Boxer Gold®), were better for dry sowing than less persistent herbicides (Table 3).
Table 3: Annual ryegrass control in a dry-sown wheat trial at Concordia, SA, 2023.
| Herbicide active(s) | Trade name | Formulation(s) | Rate(s) | Annual ryegrass (plants/m2) |
|---|---|---|---|---|
| Nil | Nil | 76.8a | ||
| Trifluralin | TriflurX® | 480g/L | 2L/ha | 24.9b |
| Pyroxasulfone | Sakura® Flow | 480g/L | 210mL/ha | 13.2bc |
| Prosulfocarb + s-metolachlor | Boxer Gold® | 800g/L +120g/L | 2.5L/ha | 37.6ab |
| Cinmethylin | Luximax® | 750g/L | 0.5L/ha | 15.2bc |
| Alconifen + pyroxasulfone + diflufenican | Mateno® Complete | 400g/L + 100g/L + 66g/L | 0.75L/ha | 24.0b |
| Alconifen + pyroxasulfone + diflufenican | Mateno® Complete | 400g/L + 100g/L + 66g/L | 1L/ha | 15.2bc |
| Bixlozone | Overwatch® | 400g/L | 1.25L/ha | 14.2bc |
| Trifluralin followed by aclonifen + pyroxasulfone + diflufenican | TriflurX® followed by Mateno® Complete | 480g/L followed by 400g/L, 100g/L, 66g/L | 2L/ha | 14.7bc |
| Trifluralin followed by aclonifen + pyroxasulfone + diflufenican | TriflurX® followed by Mateno® Complete | 480g/L followed by 400g/L, 100g/L, 66g/L | 2/ha followed by 1L/ha | 6.8bc |
| Bixlozone followed by aclonifen + pyroxasulfone + diflufenican | Overwatch® followed by Mateno® Complete | 400g/L followed by 400g/L, 100g/L, 66g/L | 1.25L/ha followed by 1L/ha | 0.5c |
| Trifluralin followed by prosulfocarb + s-metolachlor | TriflurX® followed by Boxer Gold® | 480g/L followed by 800g/L + 120g/L | 2L/ha followed by 3L/ha | 8.3bc |
Note: P-value = 0.0004. The second herbicide applications (where applied) were early post-emergent herbicide products put out 21 days after sowing. Weed counts were made 49 days after sowing. Treatments with different superscript numbers are significantly different to others.
Source: Chris Preston, the University of Adelaide
Including an early post-emergent application of s-metolachlor + prosulfocarb (Boxer Gold®), prosulfocarb (Arcade®) or aclonifen + diflufenican + pyroxasulfone (Mateno® Complete), he said, could provide better control of annual ryegrass and insurance against seasonal conditions causing poor weed control by pre-emergent herbicides.
Improved control
Professor Preston said pre-emergent herbicides failed to control weeds when:
- weeds were herbicide resistant;
- the herbicide did not persist;
- rainfall moved the herbicide below
- the root zone; or
- there was insufficient rainfall to activate herbicides.
He said too little persistence was a problem for products such as s-metolachlor + prosulfocarb (Boxer Gold®), prosulfocarb (Arcade®) and metazachlor (Tenet®), where the efficacy of the herbicide declined rapidly after application.
“This allows later-emerging weeds to avoid the herbicide, which is more likely to be a problem in higher-rainfall zones or longer seasons,” he said.
“The solution is to use longer-persistence products and mixtures of pre-emergent herbicides.”
Loss of herbicide from the root zone of germinating weeds mostly occurred with more-soluble herbicides, such as metazachlor (Tenet®) and cinmethylin (Luximax®) and generally on lighter soil types, he said.
Generally, sufficient rainfall exacerbated this, but it could be managed using herbicides with lower water solubility (in higher-rainfall areas).
He said too little rainfall after herbicide application could be a problem for less-soluble products, such as pyroxasulfone (Sakura®), propyzamide and aclonifen + diflufenican + pyroxasulfone (Mateno® Complete).
“This typically occurs where there has been good rainfall that allows annual ryegrass to germinate before herbicide application,” he said.
“Then, without sufficient follow-up rainfall after herbicide application, the herbicides are not activated in time to control the weeds.”
Useful mixtures
Professor Preston said herbicide mixtures with different properties could overcome this problem.
“Useful mixtures have been pyroxasulfone (Sakura®) plus tri-allate (Avadex® Xtra) and pyroxasulfone (Sakura®) plus trifluralin.”
He said an early post-emergent application of s-metolachlor + prosulfocarb (Boxer Gold®), prosulfocarb (Arcade®), or aclonifen + diflufenican + pyroxasulfone (Mateno® Complete) could manage potential issues with pre-emergent herbicides.
“These herbicides require rainfall after application to activate them. S-metolachlor + prosulfocarb (Boxer Gold®) is the most water-soluble product, requiring the least amount of rainfall, followed by prosulfocarb (Arcade®), whereas aclonifen + diflufenican + pyroxasulfone (Mateno® Complete) is much less water soluble,” he said.
“S-metolachlor + prosulfocarb (Boxer Gold®) and prosulfocarb (Arcade®) are best applied when annual ryegrass is at the one to two-leaf stage.”
Because of the higher rainfall requirement, aclonifen + diflufenican + pyroxasulfone (Mateno® Complete), he said, was best applied as a strategic application rather than for salvage at the two-leaf stage of the crop, preferably before additional annual ryegrass emerged.
“Aclonifen + diflufenican + pyroxasulfone (Mateno® Complete) will control new emergences of annual ryegrass after rainfall but will not control larger annual ryegrass plants.”
Acknowledgements
Trial hosts; GRDC; Jenna Malone and Patricia Adu-Yeboah (University of Adelaide); Michael Widderick (Queensland Department of Agriculture, Fisheries and Forestry); and Navneet Aggarwal (South Australian Research and Development Institute); South Australian Drought Resilience Adoption and Innovation Hub; Department of Agriculture, Fisheries and Forestry; and Elders.
More information: Chris Preston, christopher.preston@adelaide.edu.au
Resources
Professor Chris Preston's presentation at the 2024 GRDC Update at Wagga Wagga