Managing annual ryegrass in the high rainfall zone

Take home messages

• Annual ryegrass has evolved resistance to most post-emergent herbicides in the high rainfall zone (HRZ).
• Individual pre-emergent herbicides tend to have variable efficacy making mixtures and sequences better.
• Crops mature later in the HRZ meaning that more than 50% of the annual ryegrass seed can shed prior to harvest. This makes harvest weed seed management practices less effective in the HRZ than other regions.
• Annual ryegrass can rapidly replenish the seed bank in the HRZ. This makes pre-sowing cultural tactics less effective unless they are coupled with stopping weed seed set.
• Double break crops in rotations are effective at reducing annual ryegrass population, due to the employment of crop topping.
• Moderate populations ( less than 100 plants/m2) of annual ryegrass do not greatly reduce crop yield, so strategies that drive annual ryegrass to low levels are not always the most profitable.

^ɸExtra technical comment by Protech Consulting Pty Ltd

Herbicide resistance in Tasmania

Like most high rainfall cropping regions of Australia, resistance to the post-emergent herbicides is increasing in annual ryegrass in Tasmania. Random sampling shows resistance to Group A herbicides is common and resistance to Group B herbicides is increasing (Table 1). On the other hand, pre-emergent herbicides are mostly still effective. While the extent of herbicide resistance in annual ryegrass in Tasmania is lower than other high rainfall cropping regions on the mainland, increasingly pre-emergent herbicides will have to be relied on for annual ryegrass control with cereal production.

Table 1. Extent of resistance to herbicides in annual ryegrass in Tasmania from randomly collected samples in 2014 and 2019 (Data courtesy of Dr John Broster, Charles Sturt University).

Biology of annual ryegrass in the HRZ

There are anecdotal comments from growers and advisers that the ecology of annual ryegrass is different in the HRZ compared to other growing regions. Trial data from other regions suggests that annual ryegrass populations in continuously cropped regions have changed their emergence pattern to greater dormancy, with some of the population not emerging until after sowing. Where pre-emergent herbicides are the main control option, increased dormancy will reduce their efficacy. Some preliminary research from the University of Adelaide suggests that the changes in dormancy in annual ryegrass are less evident in higher rainfall regions than in medium rainfall regions (Figure 1).

Annual ryegrass populations tend to be larger in the HRZ and if seed dormancy has not changed, then later emergence of weeds is likely related to high weed seed banks and longer growing seasons. Rainfall tends to be higher in spring in the HRZ than in other growing regions and temperatures stay lower for longer. Both of these environmental conditions will encourage residual seeds in the seed bank to germinate. In addition, residual weeds in crops in the HRZ are able to take advantage of the extra moisture and cooler conditions to set more seed.

Figure 1. Emergence of annual ryegrass populations sourced from Hilltown (high rainfall), Paskerville (medium rainfall) or Roseworthy (medium rainfall) grown in the same environment.

Pre-emergent herbicide performance in the HRZ

Trials and grower experience has 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. For this reason, products with longer residual activity are preferred.

Trial work conducted as part of GRDC project UA00113 examined the performance of various pre-emergent herbicide options for annual ryegrass control in 2011 and 2012 in six trials across higher rainfall districts of South Australia, Victoria and New South Wales. These trials showed that while all herbicides can perform adequately, single herbicide applications were more likely to fail than mixtures or sequences (Figure 2). The best performing options were mixtures of Avadex® Xtra with Sakura® and sequences of TriflurX® or Sakura® followed by Boxer Gold® early post. These are likely to be the best pre-emergent herbicide approaches for annual ryegrass control in wheat in the HRZ.

Figure 2. Performance of pre-emergent herbicides across six trials at Manoora, Yarrawonga and Wagga Wagga in 2011 and Saddleworth, Lake Bolac and Wagga Wagga in 2012. Data are presented as box and whisker plots. The line across the box is the mean of all trials. The top whisker is the best performing trial and the bottom whisker the worst performing trial. BG = Boxer Gold.

Harvest weed seed control in the HRZ

Harvest weed seed control (HWSC) is a set of practices that remove or destroy weed seeds that are collected by the harvesting operation. Some of these practices can be difficult to use in the HRZ because the biomass of cereal crops is often large, creating unacceptable fire risk for narrow windrow burning (form of HWSC). Frequently, the whole paddock will burn rather than just the windrows, producing a poor result.

Trial work conducted as part of GRDC project SFS00032 examined the applicability and use of HWSC in the HRZ. This work found that there were reductions in harvest efficiency with the Integrated Harrington Seed Destructor (iHSD) due to the amount of material going through the mill, resulting in greater fuel use. There was little impact of HWSC on annual ryegrass populations in fields with existing high annual ryegrass populations (Table 2). However, in these trials, annual ryegrass populations of about 100 plants/m2 had little impact on crop yield.

Table 2. Annual ryegrass populations at 60 days after seeding/sowing (DAS) of the following crop after use of the iHSD at harvest in the previous crop.

In the HRZ annual ryegrass matures and substantial amounts of seed are shed before wheat maturity and this gets worse further south (Table 3). However, shedding of annual ryegrass seed can be reduced by later sowing and the amount of annual ryegrass seeds that are caught by HWSC can be increased by cutting lower. While still reducing weed numbers, the benefits of HWSC are not likely to be as great in the HRZ as they are in other regions.

Table 3. Amount of annual ryegrass seed shed in HWSC trials in the HRZ prior to harvest.

Crop competition for annual ryegrass management

Crop competition can help reduce seed set of annual ryegrass. There are several options for increasing crop competition against annual ryegrass. These include changing crop type, changing crop variety, reducing row spacing, increasing seeding rates, changing row orientation or changing planting times. Several of these tactics can vary greatly in efficacy in different environments.

Early sowing of wheat can reduce annual ryegrass seed production in medium rainfall zones; however, its value in the HRZ may be lower. A trial conducted at Lake Bolac in 2016 found no significant effect on annual ryegrass establishment in-crop or annual ryegrass seed head production between sowing times (Table 4). This demonstrates that competition practices effective in the medium and low rainfall zones may be less effective in the HRZ.

Table 4. Effect of time of sowing of wheat on annual ryegrass plant numbers and seed heads at Lake Bolac in 2016.

Long term Integrated Weed Management

A long-term trial at Lake Bolac has run since 2012. This trial initially examined the value of pre-sowing cultural tactics on annual ryegrass populations. These tactics were: retained stubble, burning stubble, incorporating stubble and a mouldboard plough operation followed by retained stubble. These were each followed by an in-crop treatment of either three different intensities of herbicide management (Table 5). The trial showed that the mouldboard plough operation reduced establishment of annual ryegrass by more than 95% in the year that it was implemented. However, in subsequent years the weed population continued to increase and by 2014 there was no difference in annual ryegrass populations between the pre-sowing cultural treatments.

Annual ryegrass seed head numbers increased in all management strategies between 2012 and 2016. They increased less with the most intensive management (MS 3) than with the other management strategies (Figure 3). Following crop topping of faba beans for all strategies in 2016, weed numbers were greatly reduced during 2017. Despite this, annual ryegrass seed head production was still substantially higher under the low intensity management strategy compared to the other management strategies.

Table 5. Herbicide and other treatments used as for the management strategies at Lake Bolac between 2012 and 2017.

IBS=incorporated before sowing
ɸaTreatment listed are for trial purposes ONLY as rates and/or products are not as stated on the label for use within this crop, and therefore, are unregistered. For commercial use of products please adhere to label recommendations.
ɸbUnspecified concentration of active.

Figure 3. Annual ryegrass seed heads at harvest from 2012 to 2017 at Lake Bolac for the three different management strategies (MS1, MS2 and MS3) employed. See Table 5 for details of strategies.

Higher annual ryegrass populations in MS 1 resulted in lower crop yields at Lake Bolac (Figure 4). Yield over six years for MS 2 was 1.5t/ha more than MS 1 and for MS 3 was 2.8t/ha more than MS 1. These increases in yield were 6 to 12% of the yield of MS 1.

Figure 4. Effect of management strategy intensity on accumulated yield of crops at Lake Bolac between 2012 and 2017. MS1 was low intensity; MS2 medium intensity; and MS3 high intensity management.

The GRDC has funded five demonstration trial sites across Victoria and South Australia in the HRZ to identify effective and profitable strategies for the management of annual ryegrass in the HRZ. Information about the trials and other information about management of herbicide resistant annual ryegrass in the HRZ can be found at The University of Adelaide's weed science website.

Useful resources

The University of Adelaide's weed science website.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC — the authors would like to thank them for their continued support.

Contact details

Gurjeet Gill
University of Adelaide
(08) 8313 7744
gurjeet.gill@adelaide.edu.au