A long-term approach to annual ryegrass management that employs a variety of cultural and chemical tactics remains the best method of keeping the problem weed at bay in south-eastern Australia's high-rainfall zone (HRZ).

The need for cultural tactics to be included as part of ryegrass management in the HRZ is highlighted in paddock surveys conducted in South Australia's south-east in 2017, which uncovered high levels of resistance in annual ryegrass in the region.

Of particular concern for researchers was the fact that 27 per cent of the samples collected had resistance to glyphosate and 7 per cent had resistance to paraquat.

Resistance an issue

University of Adelaide Associate Professor for weed and crop ecology, Dr Gurjeet Gill, says this resistance, together with high levels of resistance to Group A and Group B herbicides, severely restricts growers' ability to control annual ryegrass post-emergence in SA's south-east.

"There is also resistance to pre-emergent herbicides, such as trifluralin and Group J and/or Group K herbicides," he says.

"Resistance to triallate and metazachlor tended to be more common than resistance to the other Group J and Group K herbicides."

Dr Gill says increased dormancy of annual ryegrass seeds will reduce the efficacy of pre-emergent herbicides if such chemicals are the main control option.

He says trials and grower experience have consistently found that pre-emergent herbicide performance can decline quickly during the season in the HRZ.

"Activity of herbicides with short persistence in the environment, such as Boxer Gold® and Butisan®, can fall away quickly, resulting in high weed populations later in the season," he says.

"For this reason, products with longer residual activity are preferred."

Pre-emergent herbicides

Trials conducted in 2011 and 2012 as part of a GRDC investment examined the performance of various pre-emergent herbicide options across the high-rainfall areas of South Australia, Victoria and New South Wales.

According to Dr Gill, these trials demonstrated that while all herbicides can perform adequately, single herbicide applications were more likely to fail than mixtures or sequences.

In the HRZ, annual ryegrass matures and sheds a significant amount of seed before wheat maturity and this worsens in more southern areas where the growing season is longer. - University of Adelaide Associate Professor for weed and crop ecology Dr Gurjeet Gill

"These trials demonstrated that the best-performing options were mixtures of Avadex® Xtra with Sakura® and sequences of TriflurX® or Sakura® followed by Boxer Gold® early post-emergent," Dr Gill says.

"These are likely to be the best pre-emergent herbicide approaches for wheat in the HRZ."

Other trial work conducted by Southern Farming Systems as part of a GRDC investment found there was less impact of harvest weed-seed control on annual ryegrass populations in paddocks with existing high populations of the weed.

"In the HRZ, annual ryegrass matures and sheds a significant amount of seed before wheat maturity and this worsens in more southern areas where the growing season is longer," Dr Gill says.

"However, shedding of annual ryegrass seed can be reduced by later sowing and harvesting at a lower height can increase the amount of annual ryegrass being captured by the header.

"While still reducing weed numbers, the benefits of harvest weed-seed control on annual ryegrass control are not likely to be as great in the HRZ as in other regions.

"It is worth noting, too, that the trials conducted by SFS found there were reductions in harvest efficiency with a seed impact mill due to the amount of material going through the mill, resulting in greater fuel use."

Early sowing

While early sowing has been shown to reduce annual ryegrass seed head production in medium-rainfall zones, Dr Gill says the value of this practice in the HRZ may be lower.

Trials conducted at Lake Bolac, Victoria, in 2016 found there to be no significant effect on annual ryegrass establishment in-crop or annual ryegrass seed head production between different times of sowing.

"Crop competition practices effective in medium and low-rainfall areas may be less effective in the HRZ," Dr Gill says.

A long-term trial at Lake Bolac running since 2012 has looked into the value of pre-sowing tactics - including retaining stubble, burning stubble, incorporating stubble and a once-off mouldboard plough operation followed by retaining stubble - on annual ryegrass populations.

These trials then had three different intensities of herbicide management applied to them - low-cost, mid-cost and high-cost.

"This trial showed that the mouldboard plough operation reduced establishment of annual ryegrass by 95 per cent in the year it was implemented, but the rapid increase in ryegrass numbers in subsequent years meant by 2014 there was no difference in annual ryegrass populations between pre-sowing cultural treatments," Dr Gill says.

"While annual ryegrass seed head numbers increased in all management strategies between 2012 and 2016, they increased less with the most-intensive or high-cost management strategy compared to the other two treatments.

"Crop-topping faba beans prior to harvest in 2016 resulted in greatly reduced ryegrass numbers in 2017, but ryegrass seed head production was still significantly higher under the low-intensity management strategy compared to the medium and high-cost management strategies.

“Over six years, the yield of the high-cost management strategy was 2.8 tonnes per hectare higher than the low-cost management strategy."

Demonstration sites

To help growers gain a greater understanding of ryegrass management in the HRZ, the GRDC has invested in five demonstration sites in the high-rainfall areas of Victoria and South Australia.

Led by University of Adelaide Associate Professor of weed management, Dr Chris Preston, these sites - located at Frances and Millicent in South Australia and Hamilton, Inverleigh and Berrybank in Victoria - will help to identify the most effective and profitable management strategies for annual ryegrass in the HRZ.

More information: Dr Gurjeet Gill, 08 8313 7744, gurjeet.gill@adelaide.edu.au; Dr Chris Preston, 08 8313 7237, christopher.preston@adelaide.edu.au