Background
Imidazolinone herbicides like SUs are Group B herbicides. A weed species resistant to one IMI is generally resistant to other IMIs. In some circumstances, a weed population that is resistant to SUs can be cross-resistant to IMIs even if the population has never been exposed to IMIs.
The amount of cross resistance to IMIs can vary. Random weed surveys across SA, Victoria, and NSW have shown that most annual ryegrass populations resistant to SUs are also resistant to IMIs, whereas the amount of cross-resistance in other species can be lower. The availability of a wide range of Clearfield® resistant crops (wheat, barley, and canola) and winter legume tolerance to selected IMIs has resulted in greater reliance on these herbicides.
Good control of weed species such as brome and barley grass, which can be difficult to control with other herbicides in cereals, is possible using the Clearfield® crops.
Table 1: Percentage of paddocks with Group B herbicide resistant annual ryegrass from random weed surveys. The work was conducted by the University of Adelaide. Paddocks were scored as resistant if plants from seeds collected exhibited >20% survival in the pot test. The samples that exhibited 1% to less than 20% survival were scored as non-resistant.
Region |
Year |
Glean 20kg/ha |
Intervix® 750ml/ha |
---|
South Australia |
---|
Mallee |
2012 |
59 |
29 |
---|
South East |
2012 |
70 |
62 |
---|
Mid-North |
2013 |
71 |
83 |
---|
Eyre Peninsula |
2014 |
76 |
44 |
---|
NSW
|
---|
Plains/Riverina |
2010 |
53 |
38 |
---|
Slopes |
2013 |
75 |
54 |
---|
Victoria
|
---|
Western |
2010 |
73 |
18 |
---|
Northern |
2011 |
87 |
29 |
---|
Southern |
2014 |
96 |
33 |
---|
Incidence of resistance
Ryegrass
In some SA regions, such as the south east and mid north, the incidence of resistance to SU herbicides is similar to IMIs (Table 1). In these regions, use of IMIs to control ryegrass may result in poor efficacy. However, in other regions IMI herbicides may control some SU resistant ryegrass populations. Herbicide resistance testing is important to verify if an IMI is effective.
Table 2: Percentage of paddocks with Group B herbicide resistant brome and barley grass from random weed surveys. The work was conducted by the University of Adelaide. Paddocks were scored as resistant if plants from seeds collected exhibited >20% survival in the pot test. The samples that exhibited 1% to less than 20% survival were scored as non-resistant.
Year |
Species |
Region |
Atlantis® 330ml/ha |
Intervix® 750ml/ha |
---|
2012 |
Brome |
SA Mallee |
51 |
0 |
---|
2013 |
Brome |
SA-Mid North |
40 |
0 |
---|
2014 |
Brome |
SA-Eyre Peninsula |
52 |
0 |
---|
2014 |
Barley grass |
SA-Eyre Peninsula |
16 |
0 |
---|
Brome grass
Resistance to the cereal selective SU (Atlantis®, Monza ®) and sulphonamide (Crusader®) herbicides has increased dramatically over the past decade. Logran® and Glean have been used since the 1980s to target ryegrass. The exposure to these latter herbicides may have contributed to the rapid development of resistance to second generation SUs such as Atlantis® and Monza® (Table 2). In contrast to ryegrass, no cross-resistance between brome biotypes resistant to SUs or Crusader® and Intervix® have been detected in randomly collected brome samples across south east Australia. This means that IMIs can be effective for the control of SU-resistant populations of brome grass.
However, a few samples of brome grass sent in for commercial herbicide resistance testing from SA and Victoria have been confirmed as highly resistant to IMIs. One sample from Loxton in the SA Mallee was not killed with three times the field rate of an IMI (Figure 1). With increased use of IMIs to control brome, resistance to these herbicides is likely to increase dramatically over the next few years. Implementing Integrated Weed Management (IWM) strategies that are not heavily reliant on IMIs is critical to ensure they remain effective.
Figure 1: IMI resistant brome (left) and susceptible brome (right) after treatment with three times the field rate of an IMI herbicide.
Barley grass
Barley grass has only recently become a target weed for collection in random weed surveys. A large proportion of paddocks in the northern Eyre Peninsula contained barley grass in 2014. In those samples, 16 per cent were confirmed resistant to SUs with none resistant to IMIs. Resistance to IMIs has not been reported in barley grass to date; therefore use of this chemistry may be effective for a few years. However, as for brome, overuse of IMIs on barley grass will result in resistance.
Table 3: Percentage of paddocks with Group B herbicide resistant broadleaf weeds from random weed surveys. The work was conducted by the University of Adelaide. Paddocks were scored as resistant if plants from seeds collected exhibited greater than 20% survival in the pot test. The samples that exhibited 1% to less than 20% survival were scored as non-resistant.
Year |
Species |
Region |
Glean 20g/ha |
Intervix® 750ml/ha |
---|
2012 |
Wild turnip |
SA Mallee |
55 |
33 |
---|
2013 |
Indian hedge mustard |
SA-Mid North |
25 |
13 |
---|
2014 |
Indian hedge mustard |
SA-Eyre Peninsula |
64 |
14 |
---|
2014 |
Wild radish |
Victorian-Southern |
43 |
0 |
---|
Broadleaf weeds
Resistance to both SU and IMI herbicides in broadleaf weeds (wild turnip, Indian hedge mustard, wild radish) collected in random weed surveys has been confirmed (Table 3). Whilst resistance to SUs is more prevalent, resistance to IMIs is of concern, particularly in wild turnip and Indian hedge mustard.
As for SUs, there is a high frequency of IMI resistant weeds in populations before they are ever treated with herbicides. This means that there are only a few years of use of IMI herbicides before resistance is likely to occur. For IMIs to remain a valuable tool for hard to control weeds like brome grass, it is essential that as many other weed control practices as possible are included.
Acknowledgements
The work presented in this paper was funded by the GRDC project UA00144.
Contact details
Peter Boutsalis
University of Adelaide
0400 664 460