Maintaining the best options with herbicides

Take home messages:

  • Annual ryegrass in the South East is resistant to most herbicides tested.
  • Clethodim resistant ryegrass can be difficult to manage in canola.
  • New pre-emergent herbicides offer new opportunities to control grass weeds, choosing the right herbicide for the situation will maximise control.

Herbicide resistance in annual ryegrass

Surveys across South Australia and Victoria from 2005 show an increase in incidence of resistance to herbicides in annual ryegrass populations (Table 1).  We have recently started testing Intervix® and have found that many annual ryegrass populations are resistant to this herbicide. 

Table 1. Percentage of paddocks with herbicide-resistant annual ryegrass in cropping regions of South Australia and Victoria.

Region

Year

Trifluralin

Hoegrass®

Glean®

Axial®

Select®

Intervix®

Populations with resistance (%)

SA- Mid North

2008

40

76

73

59

40

nt

SA- Nth Mallee

2012

17

9

70

7

0

36

SA- Sth Mallee

2012

69

30

52

16

6

36

SA- South East

2012

78

90

74

80

43

60

SA- Eyre Peninsula

2009

5

30

78

30

11

47

Vic- Western

2010

25

40

73

33

5

18

Vic- Northern

2011

0

55

87

31

8

29

Vic - Southern

2009

0

79

88

68

23

39

In 2012 we tested for glyphosate resistance for the first time.  There was no glyphosate resistance detected in the SA Mallee, but 16% of fields in the South East of SA had glyphosate resistant ryegrass present.

Managing clethodim resistant ryegrass in canola

Clethodim resistance in annual ryegrass is increasing.  In the past clethodim resistance was managed by increasing the rate of clethodim. Unfortunately it is no longer possible to do that.  There are now populations of annual ryegrass that are resistant to 500 mL ha-1 clethodim and some will survive when treated with 2 L ha-1 of clethodim.

As there are no new post-emergent grass herbicides for canola in the pipeline, pre-emergent herbicides will have to take a greater role in managing ryegrass post-emergent. We examined the ability of some currently registered and potential products for controlling annual ryegrass in canola in 2012 (Table 2). None of the pre-emergent herbicides were particularly efficacious against clethodim resistant annual ryegrass and none were better than using clethodim.  Currently, the mix of clethodim plus butroxydim applied after a pre-emergent herbicide offers the best control; despite continuing to select for clethodim resistance. 

Table 2.  Control of clethodim-resistant annual ryegrass in canola at Roseworthy in 2012. POST herbicides were applied 8 weeks after sowing.

Herbicide program

Annual ryegrass 8 weeks after sowing

(plants m-2)

Annual ryegrass spikes at harvest (m-2)

Crop yield

(T ha-1)

1.5 kg ha-1 Atrazine IBS + 500 mL ha-1 Select® POST

387ab

149cd

1.34a

1.5 kg ha-1 Atrazine IBS + 250 ml ha-1 Select® POST

262b

306c

1.13a

1.5 kg ha-1 Atrazine IBS + 500 mL ha-1 Select® + 80 g ha-1 Factor® POST

333b

92d

1.37a

Group K IBS

498a

1105a

0.46d

Group K + 2.0 L ha-1 Avadex® Xtra IBS

298b

775b

0.76c

Group K + 2.0 L ha-1 Avadex® Xtra IBS

235b

260cd

0.88bc

Group K + 250 mL ha-1 Dual Gold® IBS

350ab

802b

0.50cd

Group D IBS

108c

149cd

1.11ab

Abbreviations: IBS, incorporated by sowing; POST, post-emergence; CT, crop-topped

Breaking the ryegrass seed bank

Obtaining high control of seed set is essential for breaking the seed bank of ryegrass.  We conducted a multi-year trial to determine effects of management strategies on ryegrass seed banks.  The treatments used are listed in Table 3. After the first year of oaten hay, we used a low, medium and high level of management (Management Strategies 1, 2 and 3 respectively) and measured the seed bank.   

Table 3. Long-term weed management strategies investigated for the control of annual ryegrass (ARG) at Roseworthy from 2009 to 2011. All herbicides were applied at recommended label rates & timings.

Management strategy

Year

(MS)

2009

2010

2011

2012

1

Oaten hay

Kaspa field pea

Trifluralin IBS

Axe wheat

Boxer Gold® IBS

Scope barley

Trifluralin IBS

2

Oaten hay

Kaspa field pea

Trifluralin IBS + Select® POST

Axe wheat

Sakura® IBS

Scope barley

Trifluralin IBS

3

Oaten hay

Kaspa field pea

Trifluralin IBS + Select® POST

+ Roundup PowerMax® CT

Axe wheat

Boxer Gold® IBS

+ Roundup PowerMax CT

Scope barley

Trifluralin IBS

Abbreviations: IBS, incorporated by sowing; POST, post-emergence; CT, crop-topped 

In this trial, the oaten hay crop reduced the high seed bank of more than 4000 seeds m-2 by more than 80% (Figure 1). However, failure to keep pressure on the population (MS 1) caused the population to rebound immediately. Two years of strong control reduced the seed bank by more than 90% and allowed a further reduction with just a pre-emergent herbicide in the wheat crop (MS 3).

Figure 1. Effect of different long-term weed management strategies on pre-sowing (March) ARG seed bank at Roseworthy from 2009 to 2012 under three different management strategies (MS).

Figure 1. Effect of different long-term weed management strategies on pre-sowing (March) ARG seed bank at Roseworthy from 2009 to 2012 under three different management strategies (MS). Bars represent SE of the mean. Seed bank change from 2009 to 2012 expressed as lambda value (λ); (λ value < 1 = seed bank decline; λ value >1 = seed bank increase).

The more intensive management in the field pea phase, and the subsequent reduction in ryegrass seed bank, resulted in increased yields and gross margins in the subsequent wheat and barley phases (Figure 2). MS 3 resulted in an additional $541.50 ha-1 over three years compared with the lowest intensity strategy (MS 1) and was considerably more profitable than MS 2. 

Figure 2. Effect of long-term weed management strategies on total grain production (kg/ha) at Roseworthy from 2010 to 2012.

Figure 2. Effect of long-term weed management strategies on total grain production (kg/ha) at Roseworthy from 2010 to 2012. Bars represent SE of the mean. Values ($/ha) provided for management strategies 2 and 3 indicate improvement in gross return relative to management strategy 1 ($2237.3/ha). Commodity prices sourced from Farm Gross Margin Guide. 

Getting the best out of pre-emergent herbicides

The movement towards no-till crop seeding and the evolution of resistance to post-emergent herbicides has placed increasing reliance on pre-emergent herbicides.  The changing nature of the crop seeding environment has changed how pre-emergent herbicides work.  In no-till, weed seeds are left close to the soil surface and the herbicides are placed on top of the weed seeds.  This gives most pre-emergent herbicides the ideal opportunity to work.  However, it also creates risks for crop damage that need to be managed.

Two new pre-emergent herbicides, Boxer Gold® (Group J+K) and Sakura® (Group K), have been released in the last 5 years.  These herbicides are alternatives to trifluralin (Group D) and will control trifluralin resistant annual ryegrass. In addition, Nufarm are working towards an extension of the label of Avadex® Xtra (Group J). All of these herbicides control annual ryegrass (Figure 3). Rotation of modes of action of pre-emergent herbicides will be important to reduce the risk of resistance to all pre-emergent herbicides.

Figure 3. Control of annual ryegrass 50 days after sowing with pre-emergent herbicides in wheat at Freeling, SA in 2011.

Figure 3. Control of annual ryegrass 50 days after sowing with pre-emergent herbicides in wheat at Freeling, SA in 2011. 

Boxer Gold® and Sakura® have different characteristics to trifluralin, which need to be considered when using the products.  Boxer Gold® is registered for use in wheat (including durum) and barley.  Boxer Gold® controls annual ryegrass, silvergrass and toad rush. Boxer Gold® is weaker on barley grass and provides limited control of brome grass.  Where these weeds are expected, mixing Boxer Gold® with another pre-emergent herbicide will provide better control.  Sakura® is registered for use in wheat (not durum) and triticale.  Sakura® provides control of annual ryegrass, silvergrass, barley grass, annual phalaris and toad rush. Sakura® has some, but insufficient, activity on brome grass.

Boxer Gold® and Sakura® are more water soluble than trifluralin.  This has advantages in that they will not become tied up on stubble, but disadvantages in that the risk of movement into the crop seed row is higher. Boxer Gold® is more water soluble than Sakura® and should provide better control of ryegrass under conditions of marginal moisture, particularly early in the season. Sakura® has longer persistence than Boxer Gold® and should provide better control in longer growing seasons and higher rainfall districts.  Avadex® Xtra makes an excellent mixing partner for the other three pre-emergent herbicides and often results in better control of ryegrass and wild oats.

Contact details

Christopher Preston
School of Agriculture, Food & Wine, University of Adelaide
christopher.preston@adelaide.edu.au

GRDC Project Code: UA00113, UA00121,