Can annual ryegrass be beaten in the north?

Annual ryegrass is a relatively new weed in the northern grain growing regions. With its capacity to produce enormous amounts of seed and readily evolve herbicide resistance, growers and agronomists are keen to avoid widespread infestation, Garry Onus* writes.

A critical first step is to be vigilant about scouting paddocks and farm borders for small patches of annual ryegrass and be aware of biosecurity hazards like contract machinery and hay.

If you find annual ryegrass on your farm, it is reasonable to assume that it will have resistance to some herbicides. The only reliable way to know your herbicide options is to have those plants or their seed tested. Getting the testing done early will give you the best chance of eradicating small areas of this weed using herbicides that will work on that population.

Annual ryegrass is the most costly weed in southern and western grain growing regions, reducing profitability through lower crop yield and increased weed control costs.

“There are ways that growers can realistically eradicate ryegrass from cropping paddocks using tactics in the fallow and in-crop. Preventing seed set is the key for prolific seeding species like ryegrass. Get in early, treat small weeds and seriously consider physical removal with a chip hoe or plough.”

The WeedSmart Big 6 strategy to grow more crop and fewer weeds offers a range of herbicide and non-herbicide tactics that growers can deploy against annual ryegrass incursions.

What are some practical tactics to use in fallow?

Annual ryegrass typically germinates from late autumn through to early spring. A winter fallow offers an excellent opportunity to prevent seed set and to drive down an existing weed seed bank. Ryegrass seed is relatively short-lived in the soil (4–6 years) and will not germinate from a depth of 10cm or more. This trait makes strategic tillage a solid option to avoid a blow-out event, particularly if large plants have established.

While the ryegrass plants are small, there are several herbicide options. A double-knock of glyphosate followed with paraquat (Group 22 [L]) and saflufenacil (Group 14 [G], e.g. Voraxor, Sharpen or similar). The Group 14 herbicides have residual activity in the soil, so be aware of plant-back restrictions.

Applying higher rates registered for optical sprayers is a great option to hit small weeds hard in the fallow. Ensure there are no survivor plants left to set seed. If some plants get established, a ‘conservation’ tillage operation or employing a team of chippers could be money well-spent.

Are there reliable in-crop tactics?

The number one tool for in-crop ryegrass control is crop competition. Annual ryegrass is poorly competitive but will take full advantage of any gaps in the crop. Consider higher seeding rates and do everything possible to set the crop up for success.

Pre-emergent herbicides can provide early protection for the crop, reducing ryegrass germination prior to crop canopy closure. A robust pre-emergent package such as pyroxasulfone (e.g. Sakura) and prosulfocarb (e.g. Arcade) in wheat is one example of an excellent pre-em mix. In this case, the mix outperforms the single mode of action partners by 11 and 29 per cent, respectively.

Be prepared to follow up with effective post-emergent herbicides in-crop.

As a cautionary note, most pre-emergent options for ryegrass are registered with an IBS (incorporated by sowing) use pattern. This removes the pre-emergent herbicide from the seeding trench and can result in weed germination concentrated along the row.

Consider using a herbicide registered for early post-emergence (EPE) application to protect the row. Examples of herbicides registered for EPE are Mateno Complete (Group 32, 15 and 12) and Boxer Gold (Group 15) in wheat and barley.

What are effective non-herbicide tools?

In addition to excellent crop competition and potentially chipping or cultivation, harvest weed seed control (HWSC) is very effective against annual ryegrass seed. Strong crop competition also improves weed seed capture at harvest because the seed heads are held high in the canopy and are less likely to lodge.

In the northern region, many growers have used narrow windrow burning or a blanket burn in autumn to kill weed seeds after harvest. While narrow windrow burning is an effective HWSC tactic, it is labour-intensive, and there is a risk of fire escapes.

An increasing number of growers in the northern region are already using, or are contemplating, chaff lining, chaff decks and seed impact mills.

Of these, chaff lining is the easiest to implement, and most growers sow through the chaff line the following season, allowing crop competition to suppress weed germination.

Seed impact mills are the most expensive up-front cost, but all crop residue is retained and spread at harvest, avoiding some of the downsides of other HWSC methods. There is a cost calculator on the WeedSmart website designed to assist growers in choosing a HWSC tool.

Finally, take farm hygiene seriously. Scout for any plants where contract machinery is unloaded or starts working, or where hay is delivered and fed out, and chip them out. Annual ryegrass is definitely a weed that you don’t want on your farm.

*Garry Onus, senior agronomist, Nutrien Ag Solutions, Moree 

About WeedSmart

‘WeedSmart’ is the industry voice delivering science-backed weed control solutions to enhance on-farm practices and promote the long term, sustainable use of herbicides in Australian agriculture.

WeedSmart has support from the Grains Research and Development Corporation, major herbicide, machinery and seed companies, and university and government research partners, all of whom have a stake in sustainable farming systems.

The GRDC is a Platinum investor in WeedSmart to ensure Australian grain growers have access to world class research in strategies to mitigate weeds and control herbicide resistance.

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