Herbicide resistance challenges and management strategies southern

Author: Gurjeet Gill, Peter Boutsalis, Samuel Kleemann and Christopher Preston (School of Agriculture, Food & Wine, University of Adelaide) | Date: 09 Feb 2016

Weed resistance surveys

A survey for herbicide resistant weeds was conducted in late 2014 across the Eyre Peninsula. Weed populations collected were tested for resistance during 2015. Annual ryegrass was found in 85 per cent of crop fields surveyed. Resistance was identified in annual ryegrass collected from fields to all herbicides tested except Sakura® and propyzamide (Table 1). There is considerably more resistance in ryegrass in the southern compared with the northern Eyre Peninsula. This reflects the greater presence of ryegrass in the south and the heavier reliance on cropping.

Since the last Eyre Peninsula survey in 2009 there has been an increase in resistance to trifluralin, with resistance to other herbicides changing little. Resistance to Avadex® Xtra was tested for the first time and several resistant populations were identified. One of these had cross resistance to Boxer Gold®.

Table 1: Annual ryegrass populations resistant to herbicides for the Eyre Peninsula survey in 2014. Populations are considered resistant if 20 per cent of individuals survived the herbicide.

Herbicide Southern EP Northern EP
Populations resistant (% tested)
Total
Trifluralin 51 10 34
Propyzanide 0 0 0
Sakura® 0 0 0
Avadex®Xtra 3 0 2
Boxer Gold® 1 0 1
Diclofop-methyl 73 10 47
Axial® 32 0 18
Clethodim 7 0 4
Chlorsulfuron 85 75 80
Intervix® 53 39 47
Glyphosate 1 0 1

Brome grass was detected in 28 per cent of paddocks visited and was concentrated in the northern Eyre Peninsula. Of these populations 52 per cent were resistant to Atlantis® and four per cent resistant to Targa® (Table 2). No resistance was identified in the survey to either Intervix® or glyphosate. Both Bromus diandrus and B. rigidus were present among resistant populations.

Table 2: Brome grass, barley grass and wild oats populations resistant to herbicides for the Eyre Peninsula survey in 2014. Populations are considered resistant if 20 per cent of individuals survived the herbicide.

Herbicide Brome grass Barley grass
Populations resistant (% tested)
Wild oats
Targa® 4 4 -
Topik® - - 4
Atlantis® 52 16 3
Intervix® 0 0 -
Glyphosate 0 - -

Barley grass was detected in 37 per cent of paddocks visited, confirming the increasing importance of barley grass as a weed of cropping. Both Hordeum leporinum and H. glaucum were present. For barley grass there was a low frequency of resistance to Group A herbicide, but 16 per cent of populations were resistant to Group B herbicides. Wild oats was detected in 18 per cent of paddocks visited with both Avena sterilis and A. fatua present. A small number of populations had resistance to Topik® and to Atlantis® (Table 2).

Two broadleaf weeds were collected during the Eyre Peninsula survey: Indian hedge mustard and sowthistle. These species were present in 18 and 11 per cent of surveyed paddocks respectively, and therefore are much less common than grass weeds. High frequencies of resistance to the Group B herbicides were present in both species (Table 3). Indian hedge mustard populations were also resistant to diflufenican, atrazine and 2,4-D.

Table 3: Indian hedge mustard and sowthistle populations resistant to herbicides for the Eyre Peninsula survey in 2014. Populations are considered resistant if 20 per cent of individuals survived the herbicide.

Herbicide Indian hedge mustard
Populations resistant (% tested)
Sowthistle
Glean 64 75
Eclipse® 74 -
Intervix® 14 -
Diflufenican 36 -
2,4-D 7 -
Atrazine 7 0
Glyphosate 0 0

Annual ryegrass resistant to Avadex® Xtra

Poor control of annual ryegrass with Avadex® Xtra (triallate) in a pre-emergence herbicide field trial conducted on the Yorke Peninsula was observed in 2014. This population was tested for resistance to pre-emergent herbicides. The results (Table 4) confirmed resistance to Avadex® Xtra, but also low level cross-resistance to Boxer Gold®. At the full rate Boxer Gold® was still effective, but control was poor at the half rate. This population has resistance to Boxerɸ (prosulfocarb) as well as Avadex® Xtra, suggesting resistance occurs across the Group J herbicides. Sakura® was still effective.

ɸ n.b. Boxer is not registered in Australia.

Table 4: Response of a resistant population of annual ryegrass from the Yorke Peninsula (YP-R) to Avadex® Xtra and other pre-emergent herbicides compared to a susceptible population (S) and a multiple-resistant population (SLR31).

Herbicide product and rate (ml/ha or g/ha) S SLR31 Control (%) YP-R
Avadex®Xtra
800 100 96 6
1600 100 100 47
3200 100 100 91
Boxer Gold®
1250 100 96 68
2500 100 100 100
Boxer*
2500 100 100 65
5000 100 100 82
Sakura®
59 100 96 97
118 100 100 100

*Boxer (prosulfocarb) is not registered for use in Australia and is shown here for comparison purposes only.

Several other annual ryegrass populations with resistance to Avadex® Xtra have been identified during our weed resistance surveys, suggesting resistance to it in annual ryegrass is becoming more common.

We conducted a trial in 2011 where an annual ryegrass population not previously exposed to herbicides was treated with four times the field rates of each of four pre-emergent herbicides. The only survivors were to 12L/ha of Avadex® Xtra at a frequency of about one in 1.6 million seed treated. This suggested that resistance to Avadex® Xtra was likely in annual ryegrass after six to eight years of intense selection.

Most of the populations with resistance to Avadex® Xtra can be controlled by the full rate of Boxer Gold® in pots. However, under non-ideal field conditions Boxer Gold® may fail on these populations.

Resistance to Group J herbicides represents a looming risk to effective pre-emergent herbicide control of ryegrass in southern Australia.

Managing clethodim resistance in canola

The increasing resistance to clethodim is creating problems with the control of annual ryegrass in break crops. Our trials with canola have identified problems with reliance on pre-emergent herbicides alone to control annual ryegrass in canola.

One management tactic that has the potential to augment pre-emergent herbicides is crop competition. A trial was conducted at Roseworthy (SA) in 2015 comparing several canola cultivars for their ability to suppress annual ryegrass seed production in combination with pre-emergent herbicides.

The trial examined three TT canola cultivars: ATR-Stingray (open-pollinated); Hyola 559TT (a hybrid); and Hyola 750TT (a high biomass hybrid). There were three herbicide management strategies: no herbicides; Atrazine (1.6kg/ha) pre + Clethodim (500ml/ha) post; and Rustler (1L/ha) pre + Clethodim (500ml/ha) + Factor (80g/ha) + Atrazine (1.1kg/ha) post.

The high biomass canola (Hyola750TT) significantly reduced the number of ryegrass spikes at harvest compared with the other two cultivars in the absence of herbicides, demonstrating the impact of extra competition (Figure 1).

The annual ryegrass populations at the site were resistant to clethodim and so the post-emergent herbicide treatments were not that effective. For herbicide treatments two and three, there was about twice the number of ryegrass spikes at harvest in the ATR-Stingray plots compared with the two hybrid cultivars. Simply changing from an open-pollinated to a hybrid canola variety has the potential to reduce ryegrass seed set by half.

Figure 1: Effect of cultivar and herbicide treatment on annual ryegrass spike numbers at harvest at Roseworthy in 2015. Herbicide treatment 1: no herbicides. Herbicide treatment 2: Atrazine (1.6kg/ha) pre + Clethodim (500ml/ha) post. Herbicide treatment 3: Rustler (1L/ha) pre + Clethodim (500ml/ha) + Factor (80g/ha) + Atrazine (1.1kg/ha) post. Effects of herbicide treatment (p <0.0001) and cultivar (P<0.0001) were significant.

Figure 1: Effect of cultivar and herbicide treatment on annual ryegrass spike numbers at harvest at Roseworthy in 2015.

Herbicide treatment 1: no herbicides. Herbicide treatment 2: Atrazine (1.6kg/ha) pre + Clethodim (500ml/ha) post. Herbicide treatment 3: Rustler (1L/ha) pre + Clethodim (500ml/ha) + Factor (80g/ha) + Atrazine (1.1kg/ha) post. Effects of herbicide treatment (p <0.0001) and cultivar (P<0.0001) were significant.

As South Australia experienced a hot and dry spring canola yields in the trial were low. The early finish to the season did not suit the longer season varieties and Hyola 750TT was affected by frost. There were significant effects of herbicide treatment (p <0.0001) and cultivar (p=0.042) on canola yield, but the highest yield was only 1.17t/ha for Hyola 559TT with herbicide treatment 3.

Managing clethodim resistant ryegrass with Roundup Ready® Triazine Tolerant (RT) canola

While not available to growers in South Australia, canola resistant to both glyphosate and the triazine herbicides (RT canola) offers an opportunity to manage clethodim resistant annual ryegrass.

We have been running long-term management trials in Victoria to develop strategies for using this technology. We have a long-term trial site at Lake Bolac, which offers particular challenges to managing annual ryegrass. The ryegrass is resistant to all post-emergent herbicides including clethodim, and the high rainfall and long growing seasons put pressure on pre-emergent herbicides.

RT canola was sown in 2014 and wheat was sown in 2015. Low, medium and high intensity management strategies were employed (Table 5).

Table 5: Management strategies used in long-term annual ryegrass trials at Lake Bolac in 2014 (canola phase) and 2015 (wheat phase).

Management strategy Canola phase Wheat phase
Low intensity Trifluralin (3L/ha)
(2.2kg/ha) pre
Clethodim (500ml)
Trifluralin (3L/ha) + Avadex®Xtra
(2L/ha) + Dual Gold® (250ml/ha) pre
Medium intensity Trifluralin (3L/ha) pre RoundupReady® (900g/ha) cotyledon
RoundupReady® (900g/ha) + Atrazine (1.1kg/ha) 6 leaf
Sakura® (118h/ha) pre
High intensity Rustler (1L/ha) pre
RoundupReady® (900g/ha) cotyledon
RoundupReady® (900g/ha) + Atrazine (1.1kg/ha) 6 leaf
Weedmaster® DST® (2.5L/ha) crop top
Sakura® (118g/ha) + Avadex®Xtra (2L/ha) pre
Boxer Gold® (2.5L/ha) post

ɸ Rates listed in this table are for trial purposes. If using these products commercially ensure you follow the rates listed on the registered labels.

It is clear that in circumstances like this that high intensity management of annual ryegrass is required every year just to maintain the annual ryegrass population. While annual ryegrass populations are still high the high intensity management strategy was able to reduce annual ryegrass spike numbers by 70 per cent compared with the low intensity management, and 50 per cent compared with the medium intensity management (Figure 2).

The ability to attack ryegrass at least three times in canola was important for reducing the population size when only pre-emergent herbicides are available in cereals.

Figure 2: Effect of management strategy on annual ryegrass spike numbers at harvest at Lake Bolac in 2015. RT canola was sown in 2014 and wheat in 2015. Management strategies are listed in Table 5. Different letters indicate significant differences between means.

Figure 2: Effect of management strategy on annual ryegrass spike numbers at harvest at Lake Bolac in 2015. RT canola was sown in 2014 and wheat in 2015. Management strategies are listed in Table 5. Different letters indicate significant differences between means.

Acknowledgements

The authors would like to thank the GRDC for supporting this research, collaborators at Southern Farming Systems and MacKillop Farm Management Group for hosting and managing long-term trials, as well as Pacific Seeds for providing canola varieties and funding additional weed suppression and Integrated Weed Management trials.

Contact details

Gurjeet Gill
School of Agriculture, Food & Wine, University of Adelaide
08 8313 7744
gurjeet.gill@adelaide.edu.au