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) – resistance has also been confirmed recently to Group J and Group K herbicides.
• 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 seedbank 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 (<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.

Herbicide resistance in the South-East of SA

In 2017 we conducted a survey of crop fields in the South-East of SA to assess the extent of herbicide resistant weeds in this region. The survey showed high levels of resistance in annual ryegrass (Table 1). Of particular concern in the South-East is that 27% of the samples collected had resistance to glyphosate and 7% had resistance to paraquat. This, together with high levels of resistance to the Group A and Group B herbicides, severely restricts the ability to control annual ryegrass post-emergence in this region. There is also resistance to the pre-emergent herbicides in annual ryegrass. Resistance to trifluralin and to the Group J and/or Group K pre-emergent herbicides was also present. Resistance to triallate and metazachlor tended to be more common than resistance to the other Group J and Group K herbicides.

Table 1. Extent of resistance in randomly collected annual ryegrass samples (n = 65) from crop fields in the South-East of SA in 2017. Resistance is defined as >20% survival.

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 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 (UA) 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 seedbanks 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 will encourage non-dormant seeds in the seedbank to germinate. In addition, weeds that survive control tactics 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), Paskeville (medium rainfall) or Roseworthy (medium rainfall) grown in the same environment. Two populations were collected from each district in 2017.

Pre-emergent herbicide performance in the HRZ

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. 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 SA, Victoria and NSW. 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. Biomass of cereal crops is often large, creating an unacceptable fire risk for narrow windrow burning. Frequently, the whole paddock will burn rather than just the windrows, producing a poor result. Also cooler and moist conditions encountered can make it difficult to achieve hot enough burn.

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 approx. 100 plants/m2 had little impact on crop yield.

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

In the HRZ, annual ryegrass matures and sheds a significant amount of seed before wheat maturity and this worsens further south as the growing season gets longer (Table 3). However, shedding of annual ryegrass seed can be reduced by later sowing and harvesting lower can increase the amount of annual ryegrass seed being captured by the header. 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 crops, 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 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 were retained stubble, burning stubble, incorporating stubble and a mouldboard plough operation followed by retained stubble. These were each followed in crop by 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, the rapid increase in ryegrass numbers in subsequent years meant 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 in 2017. Despite this, annual ryegrass seed head production was still significantly higher under the low intensity management strategy compared to the other management strategies.

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

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 invested in five demonstration trial sites across Victoria and SA 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 on The University of Adelaide's website.

Useful resources

The University of Adelaide

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 author would like to thank them for their continued support.

Contact details

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