Weed warriors – latest research on problem weeds and herbicide effectiveness

Take home messages

• Annual ryegrass with resistance to the Group D, J and K pre-emergent herbicides will make management in cereals difficult.
• Indian hedge mustard populations have resistance to Group B, C, F and I herbicides, greatly reducing control options in break crops.
• Crop competition coupled with pre-emergent herbicides can be an effective management option.

^ɸExtra technical comment by Protech Consulting Pty Ltd

Resistance to pre-emergent herbicides in annual ryegrass

There has been increased reliance on pre-emergent herbicides for annual ryegrass control due to resistance to the post-emergent herbicides. Pre-emergent herbicides are now one of the most important components of annual ryegrass management. Resistance to trifluralin has been present in South Australia (SA) for many years and by 2005 resistance to trifluralin was widespread. This resulted in early adoption of Boxer Gold® when it was released in 2008 and of Sakura® in 2012. The heavy dependence on Group J herbicides in recent years has led to resistance to this mode of action. Resistance to Group J herbicides in annual ryegrass has occurred in SA, Victoria (Vic) and New South Wales (NSW).

These populations tend to have similar levels of resistance to both triallate (Avadex® Xtra) and prosulfocarb (Arcade®) suggesting broad resistance across the Group J herbicides (Table 1). There is quite a bit of variation in the level of resistance between populations meaning mixtures of pre-emergent herbicides may work for some populations. In most cases, these populations also have resistance to trifluralin, suggesting that once trifluralin has failed, selection pressure shifts to other pre-emergent herbicides.

Table 1. Concentration of Group J herbicides required for 50% control (LD50) of resistant and susceptible annual ryegrass populations with resistance index (RI). The field rate for Avadex® Xtra is 1500g/ha triallate and for Arcade® is 2400g/ha prosulfocarb.

*RI = LD50 of R population divided by LD50 of S populations.

A worrying development is that some of these populations also have resistance to the Group K herbicides (Table 2). Some of the populations will be adequately controlled by Group K herbicides, but others will not. In general, Sakura® is likely to be more effective on these populations than Butisan®.

Table 2. Concentration of Group K herbicides required for 50% control (LD50) of resistant and susceptible annual ryegrass populations with resistance index (RI). The field rate for Dual Gold® is 480g/ha S-metolachlor, for Butisan® is 900g/ha metazachlor and for Sakura® is 100g/ha pyroxasulfone.

*RI = LD50 of R population divided by LD50 of S populations.

There are annual ryegrass populations resistant to the Group D, J and K herbicides, making their control exceptionally difficult. It has been found that mixtures of pre-emergent herbicides can offer better control, for example Boxer Gold® controls some Group J resistant populations and Sakura® plus Avadex® Xtra controls some Group K resistant populations. However, some populations have strong resistance to all three modes of action. Propyzamide (Edge® or Rustler®) is still mostly effective and can be usefully used in break crops.

Rotating and, where appropriate, mixing pre-emergent modes of action will be crucial to delaying further resistance to these herbicides. The aim should be to not use any single mode of action more than twice in four years. In addition, control of ryegrass seed set through crop competition, crop topping and harvest weed seed control will help reduce the amount of resistant weed seed returning to the seed bank. Lastly, it is important to know which of the pre-emergent herbicides are still working. Every population seems to have a different pattern of resistance, so resistance testing can be an important tool to know which herbicides are still effective.

Multiple resistance in Indian hedge mustard

Indian hedge mustard has been a problematic broadleaf weed in SA and Vic for some years. It evolved resistance to the Group B herbicides early and in recent years populations with resistance to 2,4-D, atrazine and diflufenican have been identified. Resistance to all of these herbicides is turning up in the random weed surveys being conducted across SA and Vic (Table 3). The frequency of samples with resistance to the Group C and Group F herbicides appears to be increasing rapidly.

Table 3. Extent of herbicide resistance in Indian hedge mustard populations from South Australia and Victoria. Samples collected randomly at harvest from single fields. Resistant samples had >20% survival at the normal field rate in testing.

Multiple resistance across all of Groups B, C, F and I was also present (Table 4). Of the 50 populations collected in random surveys since 2013, only 38% were susceptible to all herbicides, 36% had resistance to one mode of action, 18% had resistance to two modes of action, 6% had resistance to three different modes of action and one population had resistance to all four modes of action.

Table 4. Extent of multiple herbicide resistance in 50 Indian hedge mustard populations from South Australia and Victoria collected randomly at harvest between 2013 and 2015.

Clearly, this multiple resistance is going to make managing Indian hedge mustard more difficult. There remain some herbicide options that are still effective, in particular herbicide mixtures with bromoxynil are providing effective control in cereals; however, options for pulse crops are limited. Crop topping can be used to reduce seed set in canola and pulse crops. Good control will have to be achieved in the cereal phase to reduce the weed seed bank heading into break crops.

Crop competition and pre-emergent herbicides for grass weed control

Over the past four years we have run a series of trials at Hart, Roseworthy and Lake Bolac in collaboration with the Hart Fieldsite Group and Southern Farming Systems to look at the role of competition from early sowing to aid pre-emergent weed control. In the trials, wheat was sown about one month apart with several different pre-emergent herbicide options used.

In all the trials, time of sowing (TOS) had no effect on the number of annual ryegrass establishing in crop. This is despite an additional knockdown herbicide application between the two times of sowing. For example, the 2016 trial at Roseworthy was sown into paddock with annual ryegrass resistant to trifluralin and all the post-emergent herbicides. The first time of sowing (TOS) was 6 May 2016 and the second TOS 1 June 2016 (Table 5).

Table 5. Annual ryegrass plant counts in August 2016, head counts in October 2016 and grain yield in Mace wheat sown on 6 May 2016 (TOS1) or 1 June 2016 (TOS2) at Roseworthy, SA.

* TOS = time of sowing; fb = followed by. ɸLabel rate is 3L/ha unless combined with trifluralin then it is 1.6 to 2.4L/ha

Annual ryegrass plant counts in August were not significantly different between the two TOS (Table 5). However, in October annual ryegrass head counts were significantly different, with on average twice the number of annual ryegrass seed heads in the second TOS compared with the first TOS. Sakura® + Avadex® Xtra coupled with the early TOS was particularly effective at reducing annual ryegrass seed set.

Hybrid canola crops can also provide competition to reduce annual ryegrass seed set. A trial was sown at Roseworthy, SA on 14 May with ATR StingrayA and Hyola 559TT as the open pollinated and hybrid cultivars, respectively, using a variety of herbicide strategies based around pre-emergent herbicides and post-emergent atrazine (Table 6). There was no significant difference in the number of annual ryegrass plants established in the hybrid canola compared to the open pollinated canola. However, the number of seed heads produced was on average 40% lower in the hybrid Hyola 559TT compared to the open pollinated ATR Stingray.

Table 6. Annual ryegrass plant counts in August 2016, head counts in October 2016 and grain yield in open pollinated canola (ATR Stingray) compared with hybrid canola (Hyola 559TT) at Roseworthy, SA in 2016.

* OP = open pollinated. ɸLabel rate is 3L/ha unless combined with trifluralin then it is 1.6 to 2.4L/ha

None of the pre-emergent herbicide strategies used were particularly successful at controlling annual ryegrass in canola in this trial. This meant that just relying on pre-emergent herbicides in canola is ineffective at managing annual ryegrass. Our long-term management trials have shown that it is essential to include clethodim post emergent (even though the annual ryegrass is resistant to it), or where available use Roundup Ready® canola, and follow that up with seed set control to reduce annual ryegrass numbers for future crops.

Useful resources

Pre-harvest herbicide use GRDC factsheet

Managing the weed seedbank GRDC factsheet

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

Christopher Preston
University of Adelaide
(08) 8313 7237
christopher.preston@adelaide.edu.au