Herbicide resistance in annual ryegrass and wild oats

| Date: 14 Feb 2008

Herbicide resistance in annual ryegrass and wild oats – current situation and what to do about it, and everything old is new again – trifluralin + triallate

Andrew Storrie, Weeds Agronomist, NSW Department of Primary Industries

• Annual ryegrass - widespread Group A resistance, scattered Group B resistance and some glyphosate resistance in the Coonamble area
• Wild oats - widespread resistance to Group A, with some Mataven® (Group K) resistance and Group B resistance is developing. A number of populations are resistant to more than one MOA.
• The focus on weed control must include seed bank management (stopping seed set), as well as making a profit, and keeping weed numbers low
• Weed control MUST be continued through the season to prevent weeds seed setting seed - more than one weed control operation will be necessary each crop season.
• Using tank mixes of trifluralin + Avadex® Xtra improves control of both wild oats and annual ryegrass compared to when they are used alone
• Trifluralin + Avadex® Xtra, and other soil active herbicides can be successfully used in no-till if their characteristics are fully understood.

Herbicide Resistance

Herbicide resistance (HR) is not a new phenomenon. The first cases of resistance in Australia was to Hoegrass® (Group A) in 1980 and shortly after to Glean® (Group B) in ARG. This was only 4 years after the release of this new herbicide mode-of-action (MOA)! The first population of wild oats resistant to Group A herbicides was in 1986. The first case of Group A resistance in wild oats in northern NSW was 1996.
 
The world’s first case of glyphosate resistance was found in ARG at Echuca in 1995, followed 3 years later on the Liverpool Plains.
 
General rules of thumb for the time for HR to develop are shown below in Table 1.
 
Table 1: The “average” number of years for herbicide resistance to develop for different herbicide MOA’s (based on Preston et al 1999)
Herbicide group
Years of application
Herbicide resistance risk
A
6–8
High
B
4
High
C
10–15
Medium
D
10–15
Medium
F
10
Medium
I
Not known
Low
L
>15
Low
M
15
Low
 
The number of years will vary from weed population to weed population depending on:
  1. Initial frequency of the resistant gene in the population.
  2. MOA group being applied
  3. What percentage of the weeds sprayed are killed (herbicide efficacy) – this will depend on how well it is applied and environmental stress.
  4. Size of the weed population being treated.

Resistance to more than one MOA in the same weed population

Weed populations can be resistant to more than one herbicide MOA. This can occur by two separate paths:
  1. multiple resistance – where a range of different MOAs have been used on the weed population and weeds containing resistant genes to 2 or more MOA’s are selected
  2. cross resistance – where use of one MOA gives resistance to that group of herbicides plus at least one other MOA group. This second MOA group has not been used on that population before. WARNING: because a herbicide MOA has never been used before, doesn’t mean that it will be effective. Therefore herbicide testing is essential.

Annual ryegrass (ARG)

Annual ryegrass is one of the major crop weeds for more southern areas of Australia. ARG is a genetically variable species that must receive pollen from another ARG plat (out crossing). ARG can produce up to 45,000 seeds m-2, capable of having populations over 1000 plants m-2. These factors make it a prime candidate for developing herbicide resistance.
 
In Australia there are now populations of ARG resistant to MOA groups A, B, C, D, and M. Table 2 shows the percentage of samples tested at Charles Sturt University that are resistant to one of 5 MOA’s. Trifluralin (Group D) resistance is rapidly increasing in South Australia and Victoria. In 2007, there were 7 populations resistant to trifluralin however none were from NSW.
 
The Coonamble area has ARG populations resistant to Groups A, B and M. There will be populations resistant to more than one MOA. There are at least 2 populations of glyphosate resistant ARG on the local area.
 
Table 2: Percentage of ARG samples resistant or developing resistance to each of five herbicide groups. (J. Broster, CSU)
 
2004
2005
2006
2007
A (fops)
77
90
97
91
A (dims)
10
27
16
41
B
48
88
87
81
C
0
0
0
0
D
13
9
5
7

Wild oats

Wild oats has been generally slower to develop herbicide resistance, however at the time of writing, about 20% of paddocks have Group A resistance. This has lead to a swing to sulfonylurea (Group B) herbicides such as Hussar® and Atlantis®. Currently levels of wild oat control by Hussar® are rapidly falling in many cases, while Atlantis® remains effective.
 
Mataven® (Group K) resistant wild oat populations began to appear in 2003 and have increased quickly in the past 3 years (Table 3).
 
Table 3: Group A resistance percentage for wild oat samples since 2003 (number tested in brackets) (J. Broster, CSU)
 
2003
2004
2005
2006
2007
 
% (no.)
% (no.)
% (no.)
% (no.)
% (no.)
‘fops’
88 (24)
96 (29)
93 (51)
77 (51)
100 (22)
‘dims’
5 (20)
4 (26)
7 (50)
5 (42)
15 (33)
K
0 (18)
9 (23)
14 (28)
9 (22)
12 (15)
 
Currently there are a few wild oat populations with multiple resistance to Groups A, B and K. At least some of the Group K resistance appears to be cross-resistance from the commonly used of ‘fop’ Group A cereal herbicides. Research is being conducted to clarify this development.

Testing – know what you are dealing with!

The main reason for testing for herbicide resistance is to know what still works!
Just because one herbicide from a MOA doesn’t work, doesn’t mean all herbicides from that MOA are ineffective. For example wild oats resistant to Wildcat® and Topik® might still be susceptible to Verdict®, which are all Group A ‘fops’.
 
As stated previously, herbicide resistance is not a new thing, however many growers fail to face up to the challenge and keep throwing the same herbicide at the weeds when they don’t die. It is not uncommon that when the first application fails growers will use the same herbicide again in the same season, and usually with poor control.
 
If resistance is suspected, some form of testing is advised. Testing can be strips of different MOA’s in the paddock, sending young plants for testing (Quick-test™) or collecting seed at the end of the season for testing.
 
Each type of testing has good and bad points, which should be discussed with your adviser (preferably one that has done a 2-day Integrated Weed Management course).

Economic impact of not controlling resistant ARG and wild oats

Wild oats and ARG have the potential to reduce crop yield by over 50% if uncontrolled. Resistant weed infestations start as individual plants that set seed, which then become small patches that turn into large patches, then paddocks, so yield loss will be determined by when action is taken.
 
Switching to a different herbicide MOA when one herbicide fails is the preferred management choice of growers, however this practice usually ends in multiple resistant weed populations. This practice defers the inevitable development of resistance, however it does limit the management choices available to growers. The end result could be the inability to successfully grow crops such as chickpeas, or that stubble burning or inversion tillage might become necessary options.
 
The costs of herbicide resistance therefore may not be immediately obvious.

Managing herbicide resistance

The key to managing herbicide resistance can be summarised by the following three points:
  • Keep weed numbers low – this greatly reduces the risk of resistance developing.
  • Monitor weed control – if you don’t know weeds have survived, how can management be adjusted to control them. Once the header is in the paddock it is generally too late.
  • Stop the seed set of all weeds – one pre-emergent or early post emergent herbicide in a crop will not do it!! The survivors could be resistant so they must be controlled.
 
Get a copy of “Integrated weed management in Australian cropping systems”.

Soil residual herbicides in no-till

The use of soil active herbicides in no-till poses a number issues depending on the characteristics of the herbicide, level of incorporation, proportion of the soil covered by plant material or clods and amount of moisture near the soil surface.
 
Crop safety also needs to be considered with incorporation by sowing. Treated soil needs to be thrown away from the planting row to cover the herbicide and move the herbicide away from the emerging seedlings. Too much throw will place treated soil in the adjacent crop row leading to crop damage. 
 
Herbicide characteristics
·         Volatility – herbicides such as trifluralin, pendimethalin and Avadex®Xtra will evaporate from the soil surface if not incorporated, or covered by soil. These herbicides are not suitable for use with disk seeders. Herbicide effectiveness will be determined by the amount of soil that can be thrown by the tine to cover the herbicide. Soil throw is controlled by travel speed, soil type and soil moisture content. There will no herbicide in the planted row and weeds can germinate in the crop row.
·         Solubility in water – Herbicides that dissolve in water will have the ability to be moved into position in the soil with rainfall. Group B herbicides, Dual® Gold, Balance® and atrazine are examples. On the other hand heavy rain after application can move the herbicide into the crop root zone and cause crop damage. These herbicides are less affected by stubble.
·         Binding to stubble and plant material – herbicides like trifluralin and pendimethalin will bind to stubble and not reach the soil surface. Rainfall will not wash these herbicides off the stubble and into the soil. This is one reason why IBS rates of trifluralin are much higher than full incorporation rates.
·         Surface soil moisture – Soil active herbicides need moisture in the topsoil to be active and kill weeds. Dry conditions often lead to reduced levels of control from these herbicides, with a proportion of the weeds managing to emerge through the herbicide band.

Avadex® Xtra and trifluralin in no-till

These herbicides are coming back into widespread usage as resistance to Groups A, B and K increase. Widespread resistance of trifluralin in South Australia has lead to the practice of tank-mixing Avadex® Xtra to give an extra 10% ARG control – an expensive option.
 
Use of these herbicides has increased in no-till and as long as their behaviour in the soil and stubble is accounted for they can give high levels of weed control.
 
A number of experiments in northern NSW have shown that trifluralin and Avadex® Xtra, used alone or as a tank-mix, and incorporated by sowing (IBS) can give good control of grass weeds without reducing crop yield. Table 4 shows a range herbicide rates and gave good control of wild oats up to 11 weeks after treatment. There were no statistical differences between any treatments for control or yield, except the untreated. The nil treatment (100 wild oat plants m-2) gave a 62% reduction in wheat yield over the herbicide treatments.
 
Table 4: Control of wild oats in wheat by soil active herbicides - either incorporated before sowing or incorporated by sowing (IBS) 11 weeks after treatment. Coonamble 2006.
Herbicide Treatment
Wild oats m-2
Nil
76
Trifluralin 0.8 L/ha Incorporated
18
Avadex 2 L/ha Incorporated  
13
Trifluralin 2 L/ha   IBS
12
Trifluralin 1.2 L/ha Incorporated
8
Trifluralin 2 L/ha + Avadex 2 L/ha IBS
7
Avadex 1.6 L/ha Incorporated  
6
Trifluralin 1.5 L/ha + Avadex 1.6 L/ha IBS
5
Trifluralin 1.5 L/ha   IBS
3
Trifluralin 0.8 L/ha + Avadex 1.6 L/ha + Glean 20 g/ha Incorporated  
3
Trifluralin 1.5 L/ha + Avadex 1.6 L/ha + Glean 20 g/ha IBS
2
Trifluralin 0.8 L/ha + Avadex 1.6 L/ha Incorporated  
2
lsd (pr.= 0.05) 
20
 
In 2007 the Northern Grower Alliance conducted a series of experiments in northern NSW comparing the effectiveness of these herbicides alone and tank-mixed IBS on wild oats. These experiments show a 10-15% improvement in control by using a tank-mix.
 
Figure 1: Comparison of trifluralin and Avadex® Xtra IBS alone and tank-mixed on wild oat control, 19 and 51 days after planting. Average of 3 trials. Northern Grower Alliance 2007.
 
In both these trials it should be noted that while pre-emergent herbicides give cost effective weed control, however the surviving wild oats will produce sufficient seed to perpetuate the weed problem.

Contact details

Andrew Storrie
Weeds Agronomist
NSW Department of Primary Industries
Tamworth Agricultural Institute
4 Marsden Park Rd., Calala. 2340
Ph: 02 6763 1100
Fx: 02 6763 1222
Email: andrew.storrie@dpi.nsw.gov.au