Effective fleabane double-knocks tested in Western Australia

Growers attending WeedSmart Week 2025 in Geraldton ranked flaxleaf fleabane as the top summer weed problem in Western Australia.

Dr Harmohinder Dhammu, senior research scientist with the Department of Primary Industries and Regional Development, Western Australia, says their recent GRDC-funded research* has identified alternative double-knock strategies that can provide excellent control, even of mature fleabane plants. 

“Fleabane has a strong taproot system that allows the plant to survive under high moisture stress over summer and to extract water from deep in the soil profile,” he says. “This reduces the moisture and nitrogen available for the following winter crops and reduces yield by 10 to 30 per cent, depending on the seasonal rainfall patterns.”

Flaxleaf fleabane also possesses several other attributes that make it difficult to control. In 2018, herbicide resistance surveys conducted by Mechelle Owens at AHRI found that 11 of the 94 fleabane populations collected in Western Australia were glyphosate-resistant.

In 2023-24 and 2024-25, field trials were conducted near Scadden and Walkaway, respectively, to test double-knock strategies on existing fleabane populations. Each double-knock treatment was applied at two spray volumes (50 and 100 L/ha) and two application times (night and day). The trials also included a mowing treatment to simulate grazing and an untreated control.

“Flaxleaf fleabane is well-suited to zero-till farming systems where surface germination can occur from spring to autumn in Western Australia,” says Harmohinder. “Double-knock strategies are one of the few WeedSmart tactics available to control this weed in summer fallow. With rising glyphosate resistance, our trials suggest that other registered knock-down mixes can provide over 95 per cent control of mature flaxleaf fleabane plants and reduce seed viability.”

Mixing and rotating herbicide modes of action is a pillar of the WeedSmart Big 6 integrated weed management program to minimise the risk of herbicide resistance.

Alternative double-knock strategies – including 2,4-D LV ester

The fleabane plants at the Scadden site were mature, with around 10 per cent of the flowers open when the treatments were applied in February 2024. This site had a fleabane density of 1 plant per m².

Three double-knock strategies were applied, with the second spray applied one week after the first:

1. Glufosinate + 2,4-D LV ester followed by (fb) paraquat,
2. Glyphosate + 2,4-D LV ester fb paraquat,
3. Glyphosate + saflufenacil + adjuvant fb paraquat.

Four weeks after the second spray application, the treatment plots were assessed for weed survival and recovery. All herbicide treatments significantly reduced survival, dry biomass and seed germination compared with the untreated control.

The herbicide treatments achieved 100, 95 and 76 per cent control of fleabane, respectively. The third treatment achieved better control when the double-knock sprays were applied at night (86%) than during the day (67%), as saflufenacil is known to perform better in cooler, low-light conditions. The other strategies were not affected by the time of spraying.

Alternative double-knock strategies – without 2,4-D LV ester

The fleabane plants at a site near Walkaway, WA, were still in the vegetative phase with less than 10 per cent of flowers open when the double-knock treatments were applied. The fleabane had germinated in the spring of 2024, before the lupin crop was harvested. These plants were cut during the harvest operation and were actively regrowing from side branches. The fleabane density at this site was 10 plants per m².

Sensitive crops were located adjacent to this site, so no 2,4-D LV ester was applied in the double-knock strategies. The strategies tested were:

1. Glyphosate + glufosinate fb paraquat + amitrole,
2. Glufosinate + saflufenacil + adjuvant fb paraquat + amitrole,
3. Glyphosate + saflufenacil + adjuvant fb paraquat + amitrole.

These treatments achieved 96, 95 and 71 per cent control of fleabane, respectively. The timing of application (day and night) and spray volume did not result in a significant difference in herbicide efficacy at this site.

Herbicide reduces flaxleaf fleabane seed viability

Herbicides generally reduce the viability of both mature and immature seeds on treated fleabane plants. This is a valuable weed seed bank management outcome because flaxleaf fleabane is a prolific seed producer, and in the absence of herbicide applications, 80 per cent of its mature seed is viable.

The three herbicide strategies tested at Scadden all reduced seed germinability on surviving plants to almost zero.

At Walkaway, seed that matured on the plants that survived the glyphosate + glufosinate fb paraquat/amitrole treatment had the lowest germinability (3%), compared with 13–17 per cent germination of seed collected from plants surviving the other herbicide treatments.

Resistance testing informs herbicide choice

In the 2023-24 summer, seeds were collected from flaxleaf fleabane populations at four locations in Western Australia’s grain belt – Northam, Badgingarra, Scaddan and Albany. Seedlings from these populations were screened at the rosette stage for susceptibility to glyphosate and 2,4-D ester.

The populations were tested against the standard rate of glyphosate registered for the control of common annual weeds in fallow (1x). The four populations varied from highly tolerant to glyphosate (10% injury) to highly sensitive (93% injury) at the standard rate.

All populations were sensitive to 2,4-D ester, but the level of injury caused by the standard rate varied from 63 to 98 per cent. This variability highlights the value of herbicide susceptibility testing before investing in chemical control strategies.

WeedSmart provides a practical planning framework and farming systems approach to integrated weed management to keep weed numbers low, maximise crop yield and improve long-term farm profitability.