A range of control methods, coupled with close monitoring, testing and information sharing, is key to responding to the challenges of highly variable seed dormancy and herbicide resistance in barley grass.

University of Adelaide Associate Professor of weed and crop ecology Dr Gurjeet Gill says the most promising management tactics to emerge from a major extension project across Australia’s low-rainfall zones are centered on variety in crop rotations and herbicide use, based on close monitoring and herbicide resistance testing when needed.

The project aims to identify locally suitable, optimum control methods for barley grass in low-rainfall zones. It involves 12 farming systems groups across Western Australia, South Australia, Victoria and New South Wales, with final results due in January 2022.

“The key message is that you can’t expect to solve the problem in a single season,” Dr Gill says. “The barley grass seedbank can persist, so you need a strategy over at least two years.”

Barley grass is also highly mobile through adhering to livestock, machinery and fodder, so preventing new incursions is a challenge.

Testing a wise investment

Dr Gill says herbicide resistance testing is a wise strategic investment if control efforts for barley grass are failing.

“This will help you plan a better strategy and reduce the risk of the resistant weeds spreading around your property, because barley grass seed is easily spread by sheep,” he says.

“In addition to varying rotations and using herbicides, another option is to use Clearfield® varieties that allow you to safely use Group 2 (B) herbicides.”

The trials being run by Ag Innovation and Research Eyre Peninsula (AIR EP) at Minnipa Agricultural Centre found that use of Clearfield® varieties in 2019 resulted in the lowest barley grass plant numbers and weed seed-set compared to other strategies. AIR EP also had success sowing barley, which is more competitive with barley grass than wheat.

In 2019, hay “freezing” was used to create standing hay of Compass barley sown at a high seeding rate, which reduced the barley grass weed seed-set by 75 per cent. Hay freezing is a more reliable tactic for controlling weed seed-set than conventional hay-making. Non-selective herbicides are used to kill both crop and weeds at flowering time to create standing hay and the applications are earlier than if the crop was to be mown for conventional hay-making.

“Using a hay cut and hay freeze may be an option for paddocks with high group 1 (A) resistant barley grass populations,” Dr Gill says.

Cropping frequency a driving factor

Each group involved in the project has a three-year management plan for farm-based replication to implement local barley grass control strategies.

The project builds on previous GRDC investment in research that found a high degree of variability in barley grass populations across the low-rainfall zones of all four states.

Dr Gill says cropping frequency can drive both seed dormancy and herbicide resistance.

“Low-rainfall cropping zones are prone to grass weed infestations because rotations are strongly based on cereals due to a lack of suitable alternative crops,” he says.

“Barley grass has shown that it competes strongly with cereal crops and there are limited herbicide options for in-crop control. Frequent cropping with cereals tends to kill off early germinating barley grass, selecting for later-germinating dormant strains.

“We are seeing high variability in seed dormancy and herbicide resistance within districts as a direct result of farming practices. For example, Eyre Peninsula is having real challenges because of frequent cropping systems with short pasture phases that may have been used on a property for 20 to 30 years.”

Dr Gill says another factor giving barley grass the edge in low-rainfall zones is its tolerance to dry finishes, which enables it to outcompete other weeds and crops. Early sowing could also work in favour of barley grass and its tendency for long dormancy, allowing it to escape knockdown sprays.

He says the lower price of Group A herbicides and the fact they are safe to use in pasture species such as medics means they have been heavily used in some areas, leading to resistance problems.

For example, previous surveys in 2018 and 2019 indicated resistance to Group A and Group B herbicides, but not to glyphosate or paraquat.

Four populations of barley grass from the Victorian Mallee are now being monitored as samples collected in 2020 showed resistance to paraquat, which is used frequently in lucerne.

There were also indications of glyphosate resistance in two barley grass samples supplied in late 2020 by Wimmera farmers participating in the Birchip Cropping Group barley grass project.

Dr Gill says the project demonstrates the value of gathering and sharing information when tackling weed problems. “That’s the strength of farming systems groups; it’s all about sharing information and getting a feel for what has happened,” he says.

Triple threat on EP

Gregor Wilkins’ property at Yaninee, on the Upper Eyre Peninsula, is in the heart of a lower-rainfall zone that appears hardest-hit by the impacts of both resistance and dormancy in barley grass.

He’s fighting back with a triple threat – a rigorous and well-researched approach to herbicide use, a change in rotations, as well as cultivation to bury the seed and promote germination before a knockdown in specific paddocks.

“We first noticed resistance developing about eight years ago,” Gregor says.

“First it was quizalofop, then haloxyfop. We had the seed tested and that showed the resistant barley grass patches on our property were resistant to all the ‘fop’ herbicides.

“We’ve been using grass herbicides here for 30 years. We used to mix broadleaf and grass herbicides, which you shouldn’t do, and that would have also contributed to resistance.

“Now, we do everything by the label. You live and learn.

For the past four years, Gregor has been using clethodim and has not had resistance problems. He has found that Group K chemicals provide good control of barley grass and ryegrass, so he has used these where old barley grass seed has persisted, on 200 hectares out of a total of 1600ha under crop.

He was wary of the local barley grass developing resistance to clethodim, so to help extend its use he switched from a wheat/medic rotation to wheat/barley/medic.

He is also trialling the impact of lightly burying the seed to make it germinate at the break of season, using a cultivator with sweeps and/or a prickle chain.

“We have been direct drilling with nine-inch (22.5-centimetre) spacing, using press wheels and knife-points, but the knife-points bury the barley grass seed so that it germinates in crop,” he says.

“When the seed is buried after using the cultivator or prickle chain early, the barley grass comes up with the first decent rain, then we are able to get a weed kill on the grass with Roundup® pre-seeding because barley grass is germinating at the same time as the other weeds.”

Gregor has also noticed barley grass seed under rocks, where it may only get a sub-lethal dose of herbicide, contributing to resistance.

“So now we roll every stony flat to get it level,” he says. “Stony flats are also prone to frost, which reduces effectiveness of grass herbicides, especially clethodim. Once the minimum temperature gets down to 2°C to 3°C, they don’t work as well.”

He says long seed dormancy in barley grass appears to have been strengthened by the advent of early sowing, made possible by conserving subsoil moisture over summer with chemicals for weed control.

More information: Dr Gurjeet Gill, 08 8313 7744, gurjeet.gill@adelaide.edu.au

Useful resources: Ecology of Major Emerging Weeds; Integrated Weed Management Manual; GRDC podcast, Making barley grass barely a problem