By Cindy Benjamin

High-level infestation of Phalaris (Phalaris paradoxa), aka paradoxa grass, can reduce wheat yield by 40 per cent and add 3100 weed seeds/m² to the weed seedbank.

Phalaris is considered the second most difficult-to-manage winter weed in the northern grains region, after wild oats. 

Bhagirath Chauhan, professor at the University of Queensland’s Centre for Crop Science, says the increasing prevalence of Phalaris with resistance to Group 1 and 2 herbicides makes controlling this weed more difficult and demands a more integrated approach.

“We have conducted a series of weed ecology studies to better understand the conditions for germination and growth of Phalaris in southern Queensland and northern NSW,” he says. 

“Field trials in wheat and chickpea crops have also demonstrated the efficacy of different pre- and post-emergence herbicide options and increased crop competition to suppress Phalaris establishment, growth and seed production.”

These field trials showed the standard practice of a single post-emergent herbicide application gave variable results. 

However, adding more diversity with pre-emergence herbicides and increasing crop competitiveness provided excellent weed control and increased wheat and chickpea yields compared to the control.

“Once again, taking an integrated approach using the WeedSmart Big 6 strategy can keep weed numbers low – using both herbicide and agronomic tactics,” Bhagirath says. 

“We plan to conduct further studies into the biology and management of Phalaris and are looking for growers who have a problem with this weed and are happy to host trials.”

This year, Geraldton is hosting WeedSmart Week on 29 and 30 July, showcasing how growers can implement the Big 6 tactics to tackle hard-to-control weeds.

What Phalaris control options work in wheat?

At Gurley, NSW, LRPB Raider wheat was sown on April 29 last year on 30 cm row spacing at two plant densities – 100 and 150 plants/m² – into a natural seed bank of Phalaris suspected to be resistant to Group 1 herbicides (e.g. Axial).

Mesosulfuron-methyl (Group 2, e.g. Atlantis) was chosen as the post-emergence herbicide; however, when used alone, Atlantis did not provide commercially acceptable control of Phalaris.

Four weeks after the post-emergence herbicide application, the control plots (no herbicide) had 374 Phalaris plants/m², and the Atlantis-treated plots had 286 plants/m².

In contrast, the Phalaris plant density and seedhead production were significantly reduced in plots treated with pre-emergent herbicides (Sakura [pyroxasulfone] and Mateno Complete [aclonifen/diflufenican/pyroxasulfone]).

Sequential herbicide applications (i.e. pre-emergence followed by post-emergence) provided the most consistent and near-complete Phalaris control. 

These plots generally produced higher yields, 16 to 27 per cent greater than in the Atlantis-alone plots.

Control plots produced more than 80 seedheads/m², compared to four and 15 seedheads/m² in plots treated with Mateno Complete and Sakura, respectively.

Increasing the crop density (seeding rate) also reduced seedhead production by more than 30 per cent across the herbicide treatments. The same trends were observed in the replicated field trial at Gatton.

Can Phalaris be controlled in chickpeas?

A similar trial was established on 28 May 2024 at Garah, NSW, to assess Phalaris control options in chickpeas (cv. CBA Captain). The chickpea crop was sown into a natural seedbank of Phalaris on 30 cm row spacing to establish two crop densities – 30 and 45 plants/m².

Four weeks after applying the post-emergence herbicide treatment, haloxyfop (Group 1, e.g. Verdict), the Phalaris density was around 120 plants/m² in the control (nil herbicide) plots and 30 plants/m² in the Verdict-only plots, demonstrating that haloxyfop is a valuable stand-alone option.

However, the pre-emergence herbicides tested (Sakura and Rifle [pendimethalin]) reduced the weed density by a further 80 to 85 per cent and almost 100 per cent control was achieved when sequential pre-emergence and post-emergence herbicides were applied.

Relative to the untreated control, all pre-emergence herbicide treatments achieved over 95 per cent Phalaris control.

Herbicide treatments and chickpea crop density also had a significant impact on Phalaris seedhead numbers. Increased crop density significantly suppressed Phalaris seedhead numbers but did not provide a commercially acceptable level of control on its own.

Combining crop competition with sequential pre- and post-emergence herbicides reduced Phalaris seedhead numbers to 0–1 per m².

Increasing the crop density to 45 plants/m² increased chickpea yield in the untreated control and several of the herbicide treatments. Although Verdict alone achieved 75 per cent Phalaris control, it did not provide a yield advantage over the untreated control.

A combination of increased crop competition and pre-emergence herbicides is highly effective in reducing Phalaris density and seed production and retaining chickpea crop yield potential. 

The same trends were observed in the replicated field trial at Gatton.

Are there other tactics for the integrated management of Phalaris?

In addition to prioritising weed-free crop establishment using effective pre-emergence herbicides and adjusting seeding rates and row spacing to enhance crop competitiveness, growers may also have other tactics at their disposal.

Weed ecology studies at Gatton have found:

• Phalaris prefers cooler temperatures for germination
• Rapid exposure to high heat (over 150°C) destroys the seed
• Only 10 per cent of seed on the soil surface germinates
• Phalaris is effectively controlled by a range of pre- and post-emergence herbicides (for use in crop or fallow situations)
• Phalaris tolerates drought and saline conditions

When preparing an integrated weed management strategy for Phalaris, growers could consider taking these findings into account. For example, stubble or windrow burning could reduce the weed seedbank after a weed blow-out.

Early planting of wheat (e.g. end of April) could allow a competitive wheat crop to establish before the emergence of Phalaris.

Leaving seed on the surface should minimise germination; however, deep tillage (more than 8 cm inversion) may be useful if the weed seedbank is high.

Although several pre-emergence herbicides effectively control Phalaris in bare soil, stubble loads exceeding 2 t/ha compromise the efficacy of all options in wheat. 

At 2 t/ha residue cover, only pyroxasulfone (e.g. Sakura) provided effective control of Phalaris.

How to ask a WeedSmart question

Ask your questions about using the WeedSmart Big 6 to manage the weed seed bank on social media using @WeedSmartAU (Twitter, Bluesky and Facebook).