Long grass break turns the tables on herbicide-resistant ryegrass

Bec says that herbicide resistance testing has been informing their weed management program for several years, not just their herbicide choices.

“In 2017, a string of weather events in the Mallee resulted in a ryegrass blow-out on our farms and revealed resistance to three of our key herbicides,” she says. “We knew we needed to act quickly. In 2018, we started building a more diverse farming system that restricts ryegrass seed set throughout the rotation.”

The Marshalls’ 2200 ha dryland farming system includes oaten hay, lentils, barley and wheat. They also have 400 ha under irrigation, where they grow canola, faba beans, cereals and sometimes oaten hay. Bec says their irrigation paddocks present additional weed challenges and are often where herbicide resistance evolves within their farm.

The Marshalls have built ‘long grass breaks’ into their crop rotation to manage the seedbank of grass weeds. Bec says that a double-break is not enough, particularly when the ryegrass population has extended dormancy, as well as herbicide resistance. Pre-emergent herbicide use selects for ryegrass plants that naturally germinate later, as these plants typically emerge after the pre-emergent has started to degrade in the soil. If these late-germinating weeds set seed the can progressively add to the seedbank, adding pressure to future weed control efforts.

To counter this, the Marshalls string together crops that are suited to pre-harvest weed control strategies such as windrowing, croptopping and haymaking. If grass weeds are building up in a paddock, the Marshalls implement a three or four-year ‘long grass break’ phase of lentils, oaten hay (either one year or two back-to-back) or barley, then back to lentils to drive down the weed seedbank. All these crops are profitable in their own right, so there is no penalty for undertaking this weed control strategy. Bec says once they lock into the long grass break strategy, they have a pact not to deviate. For example, if a paddock is sown for hay, they ignore any temptation to take it through to grain harvest.

The aim of the game is to run down the weed seed bank, targeting the seed that has longer dormancy. Otherwise, one year the grass weeds don’t seem so bad and then the next year there’s a really high level of emergence.

“Oaten hay has proven to be our best weed control tool because it achieves 100 per cent control of all ryegrass cohorts within one season,” says Bec. “This makes a huge impact on the weed seedbank without using herbicide to control the weed. We don’t apply pre-emergent herbicide, and we only use glyphosate as a pre-cut treatment.”

To maximise competitiveness, the Mulgara oats is cross-sown on 300 mm row spacing, and the Marshalls are very confident to dry sow the crop. Bec says Mulgara has good early vigour and grows tall, shading out weeds while producing high-yielding, good quality export hay.

“Export hay has provided good returns, and the weed management benefits are outstanding,” she says. “Recently, we grew hay in a high-pressure ryegrass paddock and were able to sow lentils the following year with just a pre-emergent and two in-crop clethodim applications, with very few weeds present at harvest. A remarkable result in just 12 months.”

The Marshalls also look for very competitive barley varieties, usually favouring the prostrate growth habit of varieties like Neo to cover the inter-row quickly. They are also bulking up the new Bigfoot ClearField variety, which combines a tall, shading growth habit and imi-tolerance.
Including another highly competitive crop in the rotation maximises the impact of their herbicide program. Bec says the combination of crop competition and a diverse pre-emergent herbicide program is a powerful strategy that is helping them keep the pressure off selective herbicides like clethodim.

“We know it works and are prepared to invest in new actives that diversify the modes of action in our pre-emergent program,” she says. “When we have lentils alongside an oats paddock, we see just how much heavy lifting the pre-emergent herbicide does in the lentils compared to where there is no pre-emergent in the oats crop.”

The Marshalls also crop-top all their lentil crops to ensure no resistant ryegrass sets seed. Similarly, they double-knock glyphosate with paraquat plus a Group 14 to prevent glyphosate-resistant weeds from setting seed. In the hay crop, glyphosate is used to terminate the crop and paraquat is applied after the hay is removed from the paddock.

Herbicide resistance management has been more challenging in their irrigation cropping program, with clethodim used in both TT canola and faba beans. Bec says Roundup Ready canola was not an option because they already had glyphosate resistance, but in 2024, they grew their first Liberty Link canola crop. This gives them the option to use glufosinate instead of clethodim in the canola, and nowhere else on the farm. Bec says the glufosinate gave very good grass control.

“The weed seedbank is always there, and it only takes one wet spring and harvest, as we had in 2022, to create a seedbank ‘hangover’,” says Bec. “We are still working on driving down the ryegrass numbers from that seedbank replenishment event. The difference now is that whether it’s high numbers or herbicide resistance, we have tools that we know will work if we act early.”

The Marshalls are set up to narrow windrow burn when the need arises. Having years of experience using this harvest weed seed control tool, they know when a paddock would benefit from one more, non-herbicide tactic at the end of a high-pressure season. Bec says they typically use this tactic in lentils as required. Although it is time-consuming to burn the windrows the Marshalls know it works.

Fencelines are high-pressure areas that the Marshalls know are a source of herbicide resistance. Bec says they struggle to manage their fenceline weeds in the way that they want to.

“We know that fencelines and roadways have been a source of herbicide-resistant weeds on our farm, particularly glyphosate-resistant ryegrass,” says Bec. “Our aim is to get onto fenceline spraying as soon as possible after cropping, but sometimes it’s done later than is ideal.”

The Marshalls frequently rotate away from the glyphosate/paraquat double-knock, usually to paraquat plus a residual, such as Alliance [Grp 22, 34] plus Terrain [Grp 14] tank mix. They check the result of the spray job to see if there are survivors growing through a herbicide treatment. Bec says they also have concerns about having completely bare fencelines, as this brings with it other problems, such as erosion.

They use information from regular herbicide resistance testing to plan ahead, and also to problem-solve when susceptible weeds survive a herbicide treatment for some other reason. Reviewing the performance of each herbicide application, gives them an early warning of reduced herbicide efficacy or to be aware of sections of a paddock that have been missed for any reason.

The WeedSmart Big 6 is seen in action on the Marshalls’ farms, not as isolated tactics but as an integrated approach to managing weeds so they can pursue profitable cropping options.

For more detail, check out this WeedSmart webinar featuring Bec as a guest presenter – The rise and fall of clethodim.