Vary tactics to beat resistant ryegrass

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Figure 1 Confirmed cases of glyphosate-resistant weeds between 1996 and 2016.

SOURCE: AGSWG

With glyphosate-resistant annual ryegrass present in most cropping areas of the southern region, challenges are being presented to growers in the form of preventing seed-set of the weed. Data from the Australian Glyphosate Sustainability Working Group (AGSWG) indicates populations of glyphosate-resistant annual ryegrass have risen substantially since 1996 (see Figure 1).

AGSWG chair Dr Chris Preston says growers are generally managing resistant weed populations well, despite some issues with resistant ryegrass populations from fencelines and roadsides creeping into paddocks. However, he says growers must remain vigilant against glyphosate resistance by employing a variety of tactics in controlling seed-set. The use of paraquat as an alternative crop-topping option to glyphosate has been explored as part of a GRDC investment with the University of Adelaide.

“One of the issues we have with crop-topping is that in almost all winter crops grown in the southern region we have the opportunity to apply glyphosate,” Dr Preston says.

“All that’s going to do is increase resistance, especially if growers are also using glyphosate as knockdown prior to sowing.

Therefore, when we have opportunities not to use glyphosate we need to take them. For crop-topping, paraquat is only registered for use in field peas, faba beans, chickpeas, lentils, lupins, vetch and pasture, so that is where we should use it.”

Other useful tactics include cutting crops for hay and harvest weed-seed control (HWSC).

Among the HWSC techniques are narrow-windrow burning, chaff carts, the Harrington Seed Destructor (HSD) or Integrated HSD, chaff decks and chaff lining.

As part of a GRDC investment, University of Sydney director of weed research Dr Michael Walsh has previously looked at the success of HWSC techniques across Australia in reducing the ryegrass seedbank (see Table 1). This research found that chaff carts, narrow-windrow burning and the HSD all reduced the subsequent ryegrass germination by 60 per cent, on average.

“To achieve 60 per cent reduction in ryegrass germination in the year following the HWSC treatments means that it is likely that 70 to 80 per cent of the ryegrass seed was removed at harvest,” Dr Walsh says.

“This is because there is a ryegrass seedbank with seed that has carried over from previous seasons. To achieve a 60 per cent reduction in germination we need to achieve greater than 60 per cent removal of weed seeds at harvest.

“We have modelled what happens to profit and the ryegrass seedbank using the Ryegrass Integrated Management model many times and, in just about every case, 60 per cent weed-seed removal is enough when used in combination with herbicides and a range of other weed-control tools.”

Table 1 Annual ryegrass plant densities (plants per square metre) in response to harvest weed-seed control (HWSC) treatments conducted during wheat harvest at southern region sites during 2010 and 2011.
LocationControlChaff cartNarrow windrow burnHarrington Seed DestructorReduction in emergence (%)
Arthurton, SA1221190
Bute, SA8942413855
Cummins, SA29414714616248
Dimboola, Victoria1344568
Dookie, Victoria5164-3246237661
Minnipa, SA 141322923322937
Minnipa, SA 2234-786262
Pinnaroo, SA18174555566

SOURCE: Australian Herbicide Resistance Initiative

GRDC Research Codes UA00149, UWA00171

More information:

Dr Chris Preston,
08 8313 7237,
christopher.preston@adelaide.edu.au

Dr Michael Walsh,
02 6799 2201,
michael.walsh@adelaide.edu.au

Useful resources

GRDC Integrated Weed Management Hub

Australian Herbicide Resistance Initiative