Australia
April 4, 2012
Source: Australian Herbicide Resistance Initiative (AHRI)
Background
In 2010, the fourth herbicide resistance survey in Western Australia was conducted to determine the current level of resistance of key weed species to different herbicide modes of action and to assess the change in resistance frequency over time.
In 1998, the first resistance survey in WA was conducted in a relatively small area of the WA wheatbelt to determine the extent of Group A & B herbicide resistance in annual ryegrass and wild radish. In 2003, another survey was conducted across the whole WA wheatbelt area and examined a range of herbicide chemistries. Annual ryegrass and wild radish seed samples were collected from five hundred cropping paddocks and showed a dramatic increase in herbicide resistance levels. The first wild oat survey, conducted in 2005, found widespread resistance to the Group A herbicide diclofop-methyl and low resistance levels to other Group A herbicides tested.
What we did
The latest survey (2010) was conducted to assess the current state of herbicide resistance to commonly used herbicides in the WA wheatbelt for five important weed species. Over 15,000 km was travelled, extending from Binnu in the north to Esperance in the south, visiting 466 cropping paddocks and collecting mature seed heads from ryegrass, wild radish, wild oat, barley grass and brome grass.
While previous surveys had been done randomly and anonymously, the 2010 survey required grower involvement, however, paddock selection was still random to allow comparison with the data on resistance occurrence. Growers involved will receive results for their paddocks used in the survey.
Mature weed seeds were gathered at harvest from randomly selected paddocks on each grower’s property. During the 2011 growing season, annual ryegrass seedlings were screened for resistance to commonly used herbicides including clethodim, trifluralin, diclofop-methyl, sulfometuron and glyphosate at recommended field rates. Wild radish, brome grass, barley grass and wild oat will be assessed in 2012/13.
What we found
Annual ryegrass resistance levels were found to be extremely high for the group B herbicide sulfometuron, with 98% of populations containing resistant plants (Table 1) and most populations having a high proportion of plants surviving. This is a 10% increase in the number of resistant populations since the last survey conducted in 2003. A similar result was found for the Group A herbicide diclofop, with 96% of populations containing resistant plants, an increase of 30% since the last survey in 2003. The greatest increase in resistance came from southern cropping regions for both herbicides.
Two label rates were used for the herbicide clethodim: the first rate used was 250ml of Select as it was the label rate in 2003 and allows direct comparison with the 2003 survey. The label rate has since changed to 500ml/ha and so this rate was also used in the 2010 survey. Under the 250ml rate of Select, 65% of populations contained resistant plants (Table 1), a large increase of 57% since 2003; a further 42% of populations also survived the 500ml rate. This herbicide had the greatest increase in resistance levels since 2003 (Table 1), with resistance becoming widespread across the state rather than being isolated to the northern agricultural region. Higher levels of resistance at the higher label rate were observed in the northern agricultural region.
The pre-emergent herbicide trifluralin remained effective on 73% of populations. Only 1% of the resistant 27% of populations had high plant survival, and these populations were controlled by the pre-emergent herbicides Boxer Gold and Sakura. Only 2% of populations contained plants that were resistant to atrazine, while all populations were susceptible to paraquat. These results are similar to the 2003 survey results.
Table 1 Change in herbicide resistance levels for annual ryegrass from the Western Australian wheat belt over an 11 year period. Populations are classed as: Susceptible (S) (0% survival) or Resistant (R) (1-100% survival). (NT- herbicide was not tested)
|
Herbicide
|
1999
|
2003
|
2010
|
|
R
|
S
|
R
|
S
|
R
|
S
|
|
Diclofop
|
46
|
54
|
68
|
32
|
96
|
4
|
|
Clethodim
|
0.5
|
99.5
|
8
|
92
|
65
|
35
|
|
Sulfonyl urea
|
64
|
36
|
88
|
12
|
98
|
2
|
|
Trifluralin
|
NT
|
NT
|
25
|
75
|
27
|
73
|
|
Atrazine
|
NT
|
NT
|
1
|
99
|
2
|
98
|
|
Glyphosate
|
NT
|
NT
|
1
|
99
|
7
|
93
|
The number of glyphosate-resistant populations increased from 1% in 2003 to 7% in 2010, with all resistant populations coming from higher rainfall coastal regions in the southern cropping region between Albany and Esperance.
The number of populations with resistance to both the Group A and B herbicides had increased to 95%, a reflection of the increase in resistance to sulfometuron and diclofop particularly in southern cropping regions.
Have resistance levels changed over time?
Overall, it can be seen that there is a large increase in the level of resistance to the Group A and B herbicides, while, more encouragingly, atrazine and trifluralin resistance remains low and has not changed greatly over the past 7 years (Table 1). While glyphosate resistance is evident, it was confined to high rainfall southern cropping regions, and growers from these regions should be particularly cautious in their use of glyphosate.
Unlike previous surveys, the 2010 survey required grower participation and we would like to extend our thanks to all the growers involved in the survey. We would also like to thank all those who helped in promoting the work and distributing survey participation forms to growers. This work was funded by GRDC.
Photo: Herbicide resistance screening of ryegrass populations conducted at UWA during 2010 (May-September)
Topics
Species
