Developments in Herbicide Resistance and New Products

| Date: 25 Jun 2008

preston

Chris Preston1,2, Peter Boutsalis2 and Angela Wakelin2
1CRC for Australian Weed Management and
2School of Agriculture, Food & Wine, University of Adelaide, PMB1 Glen Osmond SA 5064.

Take home messages

• High levels of herbicide resistance are present in annual ryegrass in South Australia and Victoria.
• Trifluralin resistance is high in South Australia, but lower elsewhere.
• There is no cross resistance to the new pre-emergent herbicides Boxer Gold and Bay 191.
• Mataven resistance in wild oats appears related to use of fenoxaprop and clodinafop.
• Fencelines and other uncropped areas around the farm are at risk of glyphosate resistance.
• Using robust rates of paraquat will reduce the risk of selecting paraquat-resistant annual ryegrass.

Surveys of herbicide resistant annual ryegrass

Recent surveys of cropping regions in South Australia, Victoria and WA have indicated that most farms have annual ryegrass with resistance to Groups A or B (Table 1). The worst examples of resistance are occurring in continuously cropped areas where Group A and B herbicides have been intensively used for many years. Resistance to Dims is increasing, including resistance to Select. Resistance to trifluralin is high in South Australia, but lower in other areas.

Table 1. Resistance in Victoria, South Australia and Western Australia fields.

 
Victoria
(2005)
Victoria
(2006)
South Australia (2003)
Western Australia
(2003)
 
Mallee/ Wimmera
North central and north eastern
Mid-North and YP
Entire cropping zone
Sampled paddocks
125
118
187
503
 
 
 
 
 
HERBICIDE
% of paddocks with resistance
Trifluralin
4.5
1.5
49
0.2
Hoegrass
35
40
77
37
Glean
57
43
75
68
Hoegrass + Glean
21
31
60
31
Achieve/ Sertin*
28
NT
45
13*
Axial
29
36
36
NT
Select
12.5
11
NT
0.5


Cross resistance within Group A herbicides

Axial (active ingredient pinoxaden) a ‘den’ herbicide was introduced in 2006. While Axial has different chemistry to the fops and dims, its resistance profile in annual ryegrass is similar to Sertin and Achieve (Table 2). This means Axial is unlikely to control Dim resistant annual ryegrass.

Table 2. Frequency of annual ryegrass populations resistant to various Group A herbicides. From a collection of 108 populations tested by Plant Science Consulting.

Herbicide
Fops
Achieve
Select
Axial
Populations resistant (%)
78
47
11
31


Mataven resistance in wild oats

There is a lot of Group A resistance in wild oats as well as annual ryegrass. As yet, much of the resistance in wild oats is to the Fop chemistry only so Dims and Dens will still work. However, the last few years have seen an increase in reports of wild oats with resistance to Achieve and Mataven. The current thinking is that selection of wild oats with Wildcat or Topik has tended to select for Mataven resistance. A survey of Fop resistant wild oat populations collected in 2005-2006 found 42% with resistance to Mataven, despite many populations not having previous exposure to Mataven (Table 3). In contrast, Mataven was not detected at all in Fop resistant wild oat populations collected in 1990-1992.


Table 3: Survival (%) of selected Fop-resistant wild oat populations to Wildcat, Topik, Axial, Atlantis and Mataven.

 
Wildcat
300 ml/ha
Topik
75 ml/ha
Axial
200 ml/ha
Atlantis
330 ml/ha
Mataven
2.5 L/ha
Resistant samples
(%)
83
63
21*
4*
42

*Survivors to Axial and Atlantis were classed as possessing weak resistance- survivors exhibited heavy herbicide damage but recovered by the production of new tillers).



Group D resistance in annual ryegrass

There has been a significant increase in the amount of trifluralin being used to control annual ryegrass in Australia. Much of this is being used in no-till cropping systems and where there is existing resistance to the Group A and/or Group B herbicides. This increased use of trifluralin is inevitably applying additional selection pressure for trifluralin resistance in annual ryegrass.

A recent survey has identified an emerging problem with trifluralin resistance in annual ryegrass in South Australia (Table 4). Almost 50% of all populations had some level of resistance to trifluralin. At present, South Australia seems to be the worst affected state. There is some trifluralin resistance apparent in Victoria, but little yet in New South Wales or Western Australia. The challenge for the future will be to manage populations that are resistant to all the selective herbicides in wheat.


Table 4. Frequency of resistance to trifluralin by region in a random sample of annual ryegrass populations from South Australia collected in 2003 and Victoria collected in 2005 and 2006.

Region
Moderate resistance
(% of samples)
High resistance
(% of samples)
SA- Lower North
44
8
SA- Mid North
19
6
SA- Yorke Peninsula
84
22
Vic- Wimmera
2
0
Vic Mallee
10
1
Vic- North East
1.5
0
High resistance is defined as all samples with survival greater than 21% at 400 g ai ha-1 trifluralin. Moderate resistance is defined as all samples with survival greater than 21% survival at 200 g ai ha-1 but less than 21% survival at 400 g a.i. ha-1.

New pre-emergence herbicides

Since 2005 the efficacy of trifluralin has been compared to alternative pre-emergence herbicides including a new herbicide KIH-485 discovered by Kumiai Chemical Industry Co., Ltd (now Bayer BAY-191) and Boxer Gold (Prosulfocarb + S-Metolachlor) to be marketed in 2008 by Syngenta. A number of field trials have been conducted in South Australia showing these herbicides are effective at controlling annual ryegrass on sites where trifluralin resistance is present. A pot study has shown that both new herbicides control trifluralin resistant ryegrass (Figure 1).


Figure 1: Effect of Trifluralin (��), BAY-191 (��) and Boxer Gold (��) on 20 trifluralin-resistant ryegrass populations, 6 weeks after treatment. A standard susceptible (S) and two previously confirmed trifluralin-resistant biotypes (R1 & R2) were used. No survival was detected with BAY-191 or Boxer Gold.


Glyphosate Resistance

There are now 73 populations of annual ryegrass from around Australia with known resistance to glyphosate (Table 5). Many of these are from winter fallow systems in northern NSW; however, an increasing number are from fencelines and other uncropped parts of the farm. Glyphosate resistance occurs when annual ryegrass populations are treated intensively with glyphosate, where no other herbicides are applied and where there is little or no tillage.

The good news is that despite early forecasts, the number of resistant populations from no-till cropping systems is still low. The glyphosate resistant populations seem not to perform well under crop competition. However, in areas with little competition, like fencelines, resistance does occur. Once resistance is present on the fenceline, it can be dragged into the cropped area with harvest and seeding equipment creating a problem throughout the paddock.

In addition to glyphosate resistance in annual ryegrass, two populations of awnless barnyard grass have been confirmed resistant to glyphosate in a summer cropping/ fallow situation in Northern NSW. Resistance in awnless barnyard grass has major implications for the management of summer fallows and weed control in summer crops.

Table 5. Occurrence of glyphosate resistant annual ryegrass in Australia

Situation
 
Number of sites
States
Broadacre cropping
Chemical fallow
24
NSW
 
No-till winter grains
10
NSW, Vic, SA, WA
Horticulture
Tree crops
3
NSW
 
Vine crops
14
SA, WA
Other
Driveway
1
NSW
 
Fenceline
11
NSW, SA, Vic
 
Firebreak
2
SA, NSW
 
Irrigation channel
6
NSW
 
Airstrip
1
SA
 
Railway
1
WA
From Preston, C. (2007) Australian Glyphosate Resistance Register. National Glyphosate Sustainability Working Group.Online. Available from www.weeds.crc.org.au/glyphosate

Resistance to 2,4-D in Indian hedge mustard

2,4-D resistance in Indian hedge mustard has been confirmed in South Australia. Plants from this population survived 5.6 kg a.e. ha-1 of 2,4-D dimethylamine, whereas two susceptible populations were completely controlled with 250 g a.e. ha-1. This population is resistant to Group B herbicides and to other Group I herbicides (Table 6). This is a worrying development as Group I herbicides are often used to control Group B resistant broadleaf weeds. There is a second population with suspected resistance nearby.

Table 6. Estimated LD50s (g a.i. ha-1) and resistant ratios for populations from Roseworthy (S), Port Broughton (R) and Tumby Bay (R to Group B herbicides only) treated with various herbicides.

Herbicide
S
R
R/S
Tumby Bay
R/S
 
LD50 (g ha-1)
 
LD50 (g ha-1)
 
2,4-D
81
2009
25
91
1.1
MCPA
72
1510
21
75
1.0
Chlorsulfuron
0.18
90
545
>1000
>5000
Metsulfuron-methyl
0.18
5.69
32
7.35
42
Imazethapyr
9.5
37
4
>1000
>100
Metosulam
0.45
192
425
232
513
Florasulam
0.31
7.04
23
31.6
103


Resistance to paraquat in annual ryegrass

There are many paraquat-resistant populations of barleygrass in Australia, but so far no paraquat-resistant annual ryegrass. Paraquat resistant annual ryegrass has occurred in vineyards in South Africa. One of the reasons for the lack of paraquat resistant annual ryegrass in Australia is that paraquat rates have been sufficiently high in Australia to control individuals containing only one copy of the resistance allele (Figure 2). Our research indicates that robust rates of paraquat for the conditions should be used to ensure that paraquat resistance stays rare in Australia. Using rates of less than 200 g ha-1 consistently will encourage the evolution of paraquat resistant annual ryegrass.

Figure 2: Response of paraquat resistant and susceptible populations of annual ryegrass and their cross to paraquat.
Figure 2: Response of paraquat resistant and susceptible populations of annual ryegrass and their cross to paraquat.


Contact: Chris Preston
Ph: 08 8303 7237
Email: christopher.preston@adelaide.edu.au


GRDC Projects UA00075 and UA00088