Controlling problem weeds while dealing with herbicide resistance is a top priority in a new GRDC-invested northern region project

Priorities identified by grain growers in northern New South Wales and southern Queensland are being addressed in a new two-year project tackling the vexed issues of weed control and herbicide resistance.

The usual summer suspects – feathertop Rhodes grass, barnyard grass, fleabane and sow thistle – are at the top of the list. Priorities for the winter season are being worked on. Investigations into potential alternatives to glyphosate (Group 9) and paraquat (Group 22), especially for managing weeds in summer fallow situations, are also on the agenda.

The project has been split into two collaborating parts, north-east and north-west NSW managed by ICAN senior consultant Mark Congreve, and south-east and south-west Queensland managed by MCA Agronomy agronomist Brendan Burton.

It kicked off with consultation meetings at Dalby, Goondiwindi, Gunnedah, Burren Junction, Pallamallawa and Narrabri in winter 2024. Growers and agronomists who attended were asked to nominate and prioritise summer and winter weeds and issues of most concern.

MCA Agronomy agronomist Brendan Burton. Photo: courtesy Brendan Burton

New plots add to trial data

As part of the project, Mr Congreve will review trial data generated by the Northern Grower Alliance (NGA), which has wound up after 19 years of running industry-driven applied agronomic research.

He will produce summaries and fact sheets based on that research, along with technical manuals for fleabane and sow thistle to add to a growing library that includes the manual for feathertop Rhodes grass, updated in 2020.

Mr Burton is coordinating a series of small-plot scientific research trials each season that are based on priorities identified at biannual meetings.

The first of the trials investigating feathertop Rhodes grass control in sorghum got underway in early August at Kioma, near Toobeah, west of Goondiwindi. Other trials will assess glufosinate efficacy against summer weeds in fallow, options for controlling glyphosate and 2,4-D-resistant sow thistle and Queensland bluegrass in fallow, the impact of adjuvants and water volumes on glufosinate and Group 1 mixtures for summer grass control.

The next round of meetings held across southern Queensland in late 2024 focused on winter weed issues. “The idea was to capture those issues while they’re fresh in everyone’s mind before they got too far into summer and everyone forgot about the winter we’d just been through,” Mr Burton says.

Advanced sow thistle in wheat stubble north-west of Yelarbon. The plants are too large to expect control from any herbicide. Photo: Mark Congreve

Glufosinate interest

The renewed interest in glufosinate follows significant price reductions and increasing glyphosate resistance.

Mr Burton says trials will investigate methods of improving glufosinate efficacy, such as water volume, adjuvants and timing of application.

“It offers good efficacy against feathertop Rhodes grass, so rather than having to use a Group 1 followed by paraquat, if we can go in with one pass of glufosinate and knock it out, that’s obviously desirable.

“The concern is, if we start mixing other products with glufosinate to get broadleaf weed control, does it knock the grass control around?”

Growers will be invited to look at trial sites and results will be presented at field days and consultative meetings across southern Queensland. There will also be opportunities for growers to attend training programs in northern NSW to help better understand feathertop Rhodes grass, sow thistle and fleabane. Resources will be made available  on the ICAN website.

Phalaris in a paddock of chickpeas. Photo: Mark Congreve

Unique northern issues

Mr Congreve says weed management continues to be one of the biggest priorities for northern region growers because of the reliance on stored soil moisture.

Heavy glyphosate use – with frequent application at high rates – has contributed to resistance in a range of summer and winter grass and broadleaf weeds and has driven the uptake of optical spray technology, more varied herbicide mixtures, and the use of residual herbicides in summer fallow.

“We are a long way down the track now of glyphosate resistance. That’s changed a lot of our weed management over the past 10 to 15 years, and it’s still changing,” he says.

“As a result of all of that – the somewhat unique northern problems – there’s a need for continual education, both in the issues that growers face, how we’re going to deal with those issues, and trying to wrap it into a management program for our weeds. All the weeds we’re focusing on have got significant levels of glyphosate resistance in the paddock.”

National scope

Although this is a northern project, the lessons learned through this work may also help growers in other regions avoid the resistance issues. Mr Congreve, who provides technical training for agronomists across Australia, says not all weed blowouts can be blamed on glyphosate resistance.

“I don’t think we’ve got a really good understanding out in the paddock of the impact of things like water quality, droplet size, temperatures, weed size, and how these all interrelate to get our control,” he says.

“One of the key objectives for me is to explain to growers why water quality is important, why the right adjuvant is important, why the right tank mix is important, and what happens when you get it all wrong.

“We might get an extra 10 per cent control by doing things a bit smarter. And that, in a lot of situations, can be the difference between a substandard result and ... something you can live with.”

The project is a GRDC investment through the National Grower Network.

More information: Mark Congreve, mark@icanrural.com.au
Brendan Burton, brendanburton@mcagoondi.com.au

Resource: Integrated Weed Management Manual