National survey quantifies herbicide resistance in key weeds

The latest screening results from the national survey of weeds have quantified the extent of herbicide resistance in several key grass and broadleaf weeds in grain cropping systems.

Dr John Broster, weeds researcher at Charles Sturt University, says the 2020 random weed survey was the first to be fully coordinated across Australia with standardised protocols for collection and herbicide resistance screening.

The Grains Research and Development Corporation (GRDC) invests in these surveys every four years.

“Data from the national surveys shows the extent of herbicide resistance in a range of weed species to a regional level,” says John. “Although the rise in resistance to several important herbicide modes of action requires attention, the good news is that weed densities are generally low.”

The information from the random survey is particularly valuable to weed researchers and agronomists, helping direct agronomic and product use pattern research to support sustainable use of herbicide modes of action.

“On the other hand, commercial testing of weed samples collected by farmers and agronomists provides both herbicide resistance and susceptibility results they can use to make immediate and future weed management decisions,” he says.

Herbicide resistance testing is a critical tactic within the ‘mix-and-rotate’ pillar of the WeedSmart Big 6 integrated weed management program to minimise the risk of herbicide resistance. Knowing which herbicide modes of action are effective on the weeds present in a field helps growers to make more informed decisions for future crop rotation choices and herbicide programs.

National weed collection and density survey 2020 and 2024

In 2020, research teams led by John, Michael Widderick (Qld DPI), Peter Boutsalis (Adelaide Uni) and Mechelle Owen (AHRI) collected over 3500 weed samples – ryegrass, wild oats, barley grass, brome grass, sowthistle, wild radish and Indian hedge mustard – from 2588 paddocks across all cropping regions. The national, state and regional herbicide resistance screening of weeds collected during the 2020 random survey of winter cropping paddocks is now complete.

In 2024, almost 4000 weed samples, this time also including annual phalaris, were collected from 2061 paddocks. Screening of these samples will continue through this year and 2027.

Weed densities at the pre-harvest collection time in both 2020 and 2024 were very low or low, in fact well below 1 plant/m², for all species except ryegrass. The 2024 survey represents the first national data set for weed density. Around 21 per cent of fields where ryegrass was collected had weed densities over 10 plants/m², and in one-third of these paddocks, ryegrass was dominating the crop. This is very important for herbicide resistance risk management, where it really is a numbers game to keep weed numbers low by stopping weeds that escape herbicide control from setting seed.

Ryegrass resistance status across Australia

Glyphosate (Grp 9) resistance in annual ryegrass has increased nationally from 4 per cent resistant in 2013-17 to 16 per cent in 2020. An additional 22 per cent of ryegrass populations were developing resistance to glyphosate in 2020.

Nationally, screening showed major increases in the extent of ryegrass resistance to post-emergent herbicides (Grp 1 clethodim and pinoxaden; Grp 2 SU and Imi) from 2013-17 to 2020. Previous surveys showed that over 90 per cent of ryegrass samples are resistant to Group 1 ‘fop’ herbicides, so samples from the 2020 survey were not screened for this herbicide sub-group. Only two ryegrass samples were resistant to paraquat (Grp 22), with another six samples developing resistance nationally.

Resistance to pre-emergent herbicides is less common in ryegrass than for post-emergent herbicides, but the increasing trend in resistance to Group 15 (Prosulfocarb + s-metolachlor) in ryegrass is concerning. Trifluralin (Grp 3) resistance in ryegrass is most widespread but seems relatively stable. No ryegrass samples were resistant or developing resistance to bixlozone (Grp 13), cinmethylin (Grp 30) and propyzamide (Grp 3).

Resistance in other grass weeds

Nationally, resistance in wild oats has remained stable. Resistance to ‘fop’ herbicides is the most widespread, with 16 per cent of samples resistant and 15 per cent developing resistance. Resistance is developing in wild oats to flamprop, tri-allate (Grp 15) and pinoxaden (Grp 1).

Of the 381 brome grass samples collected, screening showed that if a population had resistance to sulfosulfuron (Grp 2), it was also resistant to another Group 2 herbicide, pyroxsulam. Almost 30 per cent of populations were resistant to sulfosulfuron, and a further 10 per cent were developing resistance. Pyroxsulam resistance was found in 37 per cent of brome grass samples, with a further 16 per cent developing resistance. Resistance to the other herbicides was much lower (less than 5%), and glyphosate resistance was detected in two brome grass populations.

Barley grass populations across Australia were generally susceptible to all herbicides (less than 5% resistance). Resistance to sulfosulfuron was 4 per cent (and 3% developing resistance) nationally, with resistance levels in Western Australian samples notably higher than the national average (6% resistant and 10% developing resistance). Resistance to Group 1 and paraquat was detected in barley grass.

Pre-emergent herbicides, fops/dims and imidazoline herbicides, as well as the knockdown herbicides, provide good control of most brome and barley grass populations.

An additional survey was conducted in northern NSW and Queensland just prior to the harvest of the 2020-21 summer crops to determine the resistance status of summer weed species. Screening of barnyard grass, feathertop Rhodes grass and sweet summer grass samples showed glyphosate resistance in 83, 98 and 79 per cent respectively in populations of these grass weeds.

Herbicide resistance testing informs decision-making

Growers and agronomists find value in routine herbicide resistance testing along with keeping paddock records of weed patches. The ‘quick test’ allows in-season testing while seed tests provide data for the following season’s herbicide program planning sessions.

Key times to collect weed samples for testing are after post-emergent spraying (quick test) and while harvesting crops (seed test). Have your resistance sampling kit ready in the header.

Testing is also very useful when growers take on new lease paddocks or purchase a new farm, particularly if historical test results are not available.

Knowing the susceptibility of the weeds in each paddock to the relevant herbicide groups will save growers spending money on ineffective herbicides and prevent a potential weed blow-out season.