Risk of addiction to IMIs (Group B imidazolinone herbicides)

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.

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 

GRDC Project Code: UA00144,