Innovative strategies for narrowleaf weed management

Innovative strategies for narrowleaf weed management

Take home messages

  • The late start to the 2024 growing season across many parts of SA and Vic is likely to have implications for weed control by pre-emergent herbicides.
  • The most recent random weed survey and data from commercial testing indicate that resistance in ryegrass and barley grass from the SA Mallee is lower than in higher rainfall cropping areas.
  • Resistance testing, such as the in-season Quick-Test, can identify effective post-emergent herbicides.
  • Crop-topping is an effective strategy to reduce ryegrass seed viability with herbicides such as glyphosate and paraquat.
  • Other strategies to destroy or capture seed, such as use of a Seed Terminator, burning, chaff lines, hay or silage, can be effective in reducing weed seed infestations.

Resistance in ryegrass and barley grass in the SA Mallee

The incidence of herbicide resistant ryegrass in the SA Mallee is presented using two data sets. The first shows the incidence of resistance to six pre-emergent and six post-emergent herbicides from the national GRDC random weed survey conducted in 2020 (Table 1). Resistance to herbicides in the SA Mallee was lower than in higher rainfall regions. The highest resistance was detected for Hussar® OD (71%), with 29% of ryegrass samples not resistant. No resistance was detected for five pre-emergent herbicides in ryegrass, only for trifluralin (44%). However, 8% of samples were resistant to glyphosate, which is extremely concerning. The results nevertheless suggest that opportunities exist to control ryegrass with post- and pre-emergent herbicides in the SA Mallee. Clethodim is one of the most important Group 1 herbicides, with only 2% of samples exhibiting resistance to clethodim.

The incidence of resistance in barley grass was 5% or less for the Group 1 herbicides and only 1% for paraquat. Resistance to other modes of action was not detected, with populations remaining susceptible to important herbicides like Intervix®. Barley grass can be problematic due to delayed emergence making control with knockdown and pre-emergent herbicides difficult. Its ability to mature before the crop also makes control with late season techniques difficult (Gill et al. 2013).

Table 1: Resistance detected in ryegrass and barley grass from GRDC funded random weed survey conducted in 2020 in the SA Mallee. Results are the percentage (%) of paddocks where samples of ryegrass or barley grass tested the following season exhibited ³20% survival.

RYEGRASS

     

Trifluralin

Boxer Gold®

Sakura®

Rustler®

Luximax®

Overwatch®

1.5L/ha

2.5L/ha

118g/ha

1L/ha

500mL/ha

1.25L/ha

44

0

0

0

0

0

Axial®

Clethodim

Hussar OD

Intervix

Glyphosate

Paraquat

300mL/ha

0.5L/ha

100mL/ha

750mL/ha

1.5L/ha

1.2L/ha

29

2

71

50

8

0

BARLEY GRASS

Targa®

Clethodim

Intervix

Monza®

Glyphosate

Paraquat

250mL/ha

0.5L/ha

750mL/ha

25g/ha

1.5L/ha

1.2L/ha

5

3

0

0

0

1

Table 2 shows the results from ryegrass samples sent to Plant Science Consulting for testing. In some cases, poor weed control had prompted the testing. Testing the effect of numerous herbicides on a particular weed sample can identify herbicides that still remain effective and those that are no longer effective. Like the random weed survey findings, out of the pre-emergent herbicides tested, resistance was only identified for trifluralin (Table 2). Opportunities to control ryegrass with post-emergent herbicides in the SA Mallee is possible. In a season with delayed emergence of crops, such as in 2024, knowledge of the resistance status of ryegrass to post-emergent herbicides can provide valuable information for effective herbicide choice. In situations where glyphosate-tolerant canola has been sown, use of a glyphosate + clethodim tank-mix is likely to be an effective strategy.

Table 2: Resistance detected in ryegrass tested from seed or plant (Quick- Test) samples originating from SA Mallee growers and tested by Plant Science Consulting. Only samples exhibiting ≥20% survival are presented. These samples were received between the 2017 and 2023 growing seasons.

Herbicides and rates /ha

% resistant samples

Number of samples tested

(resistant = ≥20% survival)

Axial

64

69

Clethodim 500mL*

16

69

Hussar OD 100mL

57

10

Intervix 750mL*

61

44

Glyphosate 2L*

35

34

Paraquat 1.2L

1

16

Trifluralin 2L*

32

44

Trifluralin 1.5L + AvadexXtra® 2L

0

18

Boxer Gold 2.5L

0

54

Sakura 118g

0

29

Atrazine 2kg

0

17

Overwatch 1.25L

0

17

Luximax 500mL

0

17

Ultro® 1.8kg

0

17

Rustler 1L

0

25

*data include eight samples tested for Mallee Sustainable Farming from the 2023 growing season

Between 2017 and 2023, only three barley grass samples were received from the SA Mallee region. Two samples were resistant to Group 1 herbicides, with no resistance to Group 2, glyphosate or paraquat detected.

Other in-crop weed control strategies

The use of pre- and post-emergent herbicides are the two main strategies used to control weeds. Other techniques are also available to control certain weed species that have not been controlled by pre-emergent and post-emergent herbicides. They include reducing seed-set (crop-topping), capturing seed (chaff cart, hay, silage, chaff lines) or destroying the seed (seed mills, for example, Seed Terminator, or burning). These techniques are more successful on species that do not readily shed prior to harvest, or mature slower than the crop, such as ryegrass. In contrast, barley grass usually matures well before the crop, reducing the effectiveness of late seed set control measures . However, the effectiveness of these strategies, even on more suitable species (that is, ryegrass, wild radish and brome) can differ from one season to the next.

Pre-harvest non-selective herbicide use for ryegrass seed sterilisation

Crop-topping is the application of non-selective herbicides, such as glyphosate or paraquat, prior to harvest for crop desiccation (that is, even the maturation of the crop) and to sterilise developing weed seeds. The technique can be particularly useful on species such as ryegrass when applied between the flowering to early grain fill (soft dough) stage. The presence of ryegrass late in the season can result from later germination (that is, dormant ryegrass) or surviving herbicide-resistant plants. The selectivity of crop-topping is dependent on there being a sufficient gap in the physiological maturity between the crop and weed. However, there are several factors that determine the effectiveness, including developmental stage, environmental conditions, product choice and herbicide resistance. It is also important to be aware of the maximum residue levels (MRLs) that vary for different crops and different markets. Growers are urged to check with their grain handlers before undertaking crop-topping. More information can be found in the ‘Pre-harvest herbicide use fact sheet’ (see Reference list).

Herbicide vs crop choice

Wheat

Unlike glyphosate, paraquat is not registered for crop-topping in wheat. In Australia, there are hundreds of glyphosate products. While the maximum registered rate is 975g ai/ha for most products registered for this use, higher rates are registered for Roundup UltraMax® (3.4L/ha = 1938g ai/ha), Weedmaster® DST® (4.1L/ha = 1927g ai/ha) and Crucial® (3.2L/ha = 1920g ai/ha). In addition, the withholding period of these products is 5 days compared to 7 days for other glyphosate products.

Barley (except malting barley)

Paraquat is not registered for crop-topping in barley. No glyphosate products are approved for use on malt barley, only for feed barley on crops at the late dough stage. As for wheat, Crucial, Weedmaster DST and Roundup UltraMax are registered in feed barley. As the maturation stage between barley and ryegrass tends to be greater than wheat, the opportunity for crop-topping is greater, with less risk of reducing the crop yield.

Canola

Paraquat is not registered in canola for any use. Roundup UltraMax, Weedmaster DST and Crucial are registered in canola. Apply to mature standing crop from early senescence prior to windrowing or direct harvest. Application can also be applied at the time of windrowing, utilising windrow equipment fitted with spray booms. Additionally, Crucial and Weedmaster DST have a 5-day withholding period, whereas there is no withholding period for Roundup UltraMax when used as per the label.

Pulses

Paraquat is registered to reduce seed set of annual ryegrass at 400 or 800mL/ha, depending on which product. Gramoxone rates are 400 or 800mL/ha. The optimum stage to spray ryegrass is when the majority of the seed heads at the bottom of the plants have emerged and the majority are between flowering and no later than soft dough stage, usually between October to November. Using the higher registered rate provides a greater reduction in seed set. In seasons where the crop is less advanced than the ryegrass, a yield reduction may occur, therefore crop-topping may not be practical every season. Diquat is also registered in pulses for crop desiccation but not for ryegrass seed set control. Note that not all glyphosate formulations are registered for crop desiccation and crop-topping, therefore refer to the specific product labels for more detailed information.

Crop-topping timing field trials with glyphosate

Field trials were conducted at Blyth and Spalding in canola in 2010 after paddocks had been windrowed. Glyphosate was applied at three rates at the time of windrowing. Seeds were collected from tillers and tested for germination to determine the effectiveness of the glyphosate treatments (Table 3). Although ryegrass seed was slightly more advanced at Blyth (late milky dough to early soft dough), spraying conditions were much better (dT = 7) resulting in better sterilisation compared to Spalding site (dT = 6-11), where temperatures exceeded 30°C (Table 4). This indicates that, as well as the developmental stage of the ryegrass, the weather conditions at crop-topping can play an important role. Targeting ryegrass at the optimum developmental stage during cooler conditions is likely to result in the greatest reduction in ryegrass seed viability.

Table 3: Results for visual viability x germination of ryegrass (% of Nil). Means with different letters are significantly different at P=0.05.
Ryegrass stage at Blyth: oldest tillers – late milky to early soft dough stage
Ryegrass stage at Spalding: oldest tillers – seed at milky dough stage, youngest tillers anthers not emerged yet, but could be seen when glumes were peeled off.

Treatment

Ryegrass viable and able to germinate (% Nil)

Glyphosate

Blyth

Spalding

Nil

100 a

100 a

0.648kg/ha

24 bc

46 abc

1.296kg/ha

7 c

25 bc

1.944kg/ha

13 bc

11 c

Table 4: Weather conditions at each trial site.

Blyth

Spalding

Conditions

Beginning

End

Beginning

End

Max Wind Speed (km/h))

4.3

2.3

2.5

1

Temp (°C)

18-21

18-20

22.8

30

Wet Bulb (°C)

12

12.3

14.5

17

Delta T

7.3

7.2

6

11

RH (%)

46

44

50

32

Windrow date

14 Nov 2010 (pm)

19 Nov 2010 (am)

Weeds not controlled with pre- or post-emergent herbicides compete with the crop and produce seeds that enter the seedbank, making control in the following season(s) more difficult. Stopping seed-set reduces subsequent infestations. Crop-topping (sterilising) the seed is one such strategy. Sacrificing an infested crop by cutting for silage or hay is also effective in removing weed seeds before they mature. Alternatively, a planned hay or silage crop is an effective management tool for removing weed seeds before they develop. For species that don’t readily shed before the crop matures, the seeds can be processed during the harvest operation. This can include collecting the chaff (chaff carts) or passing the chaff/fines fraction including weed seeds, through a mill operating at around 3000rpm to kill the seed. Seed Terminator, Seed Destructor (iHSD), Seed Redekop and Weedhog offer this technology (see ‘Weed control: chaff mills kill seeds before they set’, in reference list).

Other techniques involve:

  • Chaff lining: the chaff and weed seed fraction is separated from the straw and directed into a narrow line behind the harvester. This fraction remains on the soil surface to rot, whereas the straw is spread back across the paddock.
  • Chaff decks: similar to chaff lining with the chaff and weed seed fraction directed into dedicated wheel tracks, common in control traffic farming.
  • Narrow windrow burning: the straw and chaff fraction are concentrated into a narrow windrow that are burnt the following autumn. Higher temperatures (up to 500oC for 10 seconds) than that with standing stubble can be achieved, creating a greater weed seed kill.

More information on these techniques is available at ‘Harvest weed seed control – beyond windrow burning’, in the reference list.

Conclusion

In 2024, across most of the SA Mallee, the first rains to germinate sown crops were in mid-June. Crops had been sown from April, so it is of concern whether the pre-emergent herbicides used were as effective as expected. Lower detected resistance in ryegrass from the SA Mallee suggests there is a greater opportunity to control ryegrass in-crop. Including late season weed control strategies, such as crop-topping, windrow burning, and the use of seed destructor mills, need to be considered if large weed burdens prevail this season.

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. The authors also thank the advisors who made this research possible.

References

Gill G, Shergill L, Fleet B, Boutsalis P, Preston C (2013) Brome and barley grass management in cropping systems of southern Australia. Proceedings GRDC Grains Research Update, GRDC project code: UA00105, UA00121.

Pre-harvest herbicide use fact sheet

Weed control: chaff mills kill seeds before they set

Harvest weed seed control – beyond windrow burning

Contact details

Peter Boutsalis
University of Adelaide
Waite Campus, Glen Osmond SA 5064
peter.boutsalis@adelaide.edu.au
Plant Science Consulting
0400 664 460
@PBoutsalis

GRDC Project Code: UCS2008-001RTX,