December 19, 2013
A perfect storm is brewing for what could potentially be another big herbicide resistance problem in Australia.
The Group B herbicide sulfosulfuron (Monza®) is about to get a lot cheaper, and many growers are looking to increase their use of Group B Imi tolerant barley and wheat varieties in the near future. Past experience tells us that reducing the price often results in dramatic increases in the use of a given herbicide. This is likely to have a big impact on the evolution of Group B resistance in brome grass and barley grass.
AHRI researcher Mechelle Owen found that 13% of the brome grass samples from the 2010 WA grainbelt herbicide resistance survey were resistant to Monza® but there was no cross resistance to Imi herbicides.
University of Adelaide researcher Dr Chris Preston and Dr Peter Boutsalis have also found high levels of Group B resistance in Brome grass in the South Australian Mallee region.
The Good News
Brome grass self-pollinates. This means that the movement of resistance genes may be slower compared to a cross pollinating species such as annual ryegrass.
The Bad News
Weeds very easily evolve resistance to Group B herbicides such as Monza® (SU) and Imi (Imidazolinone) herbicides. There are several different gene mutations that cause resistance and they naturally occur at a high frequency.
2005 survey – metabolic resistant brome grass
During the 2005 survey of the Western Australia grainbelt, AHRI researcher Mechelle Owen found six samples of brome grass that she confirmed to be resistant to Monza®. The populations had low-level resistance that was likely due to enhanced metabolism by P450 enzymes. Enhanced metabolism was indicated by spraying the brome grass with a P450 inhibitor, the insecticide malathion. Researchers use malathion to turn off the P450 enzymes to determine if a plant is resistant due to P450 enzymes. Brome grass was sprayed with malathion 30 minutes prior to spraying with Monza® and the resistance was reversed.
We know what you are thinking, lets mix malathion with Monza® – problem solved! That is a really bad idea for three reasons:
- Malathion is not registered for this purpose.
- Malathion turns off P450 enzymes in the crop as well so you would end up with dead weeds and a dead crop (plus dead insects as well!)
- Metabolic resistance is not the only mechanism of resistance in brome grass.
In the 2010 resistance survey of the WA grainbelt, Mechelle collected 91 samples of brome grass from the 466 paddocks that she sampled. Twelve populations were resistant to Monza® (sulfosulfuron) and Oust® (sulfometuron).
Cross resistance to Imi herbicides?
The 2010 survey found that there was no cross resistance between Monza® and the Imi herbicides Intervix (Imazamox + imazapyr).
Brome grass resistance in South Australia
A random survey of the South Australian Mallee region in 2012 by Dr Chris Preston and Dr Peter Boutsalis from the University of Adelaide has found high levels of Group B resistance in brome grass. Fifty percent of the brome grass populations from the Northern and Southern Mallee regions were resistant to the group B (SU) herbicide Atlantis® (mesosulfuron). There was no resistance to Imi herbicides detected in this random survey, however, Imi resistant brome grass has been confirmed from other populations in the region sent in by growers where a spray failure was observed.
Chris believes that this high level of SU resistance in brome grass has come about due to the cereal dominant cropping system in this area. Many growers have heavily relied on the Group B herbicides Glean® and Logran® in the Mallee for many years. While these herbicides do not achieve high levels of brome grass control, they are likely to have placed low-level selection pressure on brome grass for many years. In addition to this, the selective herbicides such as Atlantis®, Crusader® and Monza® (all Group B herbicides) have been relied upon heavily in recent years in this cereal dominant cropping system. The use of Imi herbicides in this region has been limited due to the high cost, however, the cost Clearfield® has come down over the past two to three years which has seen more growers beginning to use this technology and the use of Imi herbicides is starting to increase. This trend is likely to continue.
Why was no resistance to Imi herbicides detected?
We speculate that the reason is that Imi herbicide use has been low inWestern Australiain the past. Research by AHRI researcher Dr Qin Yu shows that there are a number of target site mutations that endow resistance to Imi herbicides in annual ryegrass. It is highly likely that these mutations will also be present in brome grass. The specific target site mutation/s that are present in a population will determine the level of cross resistance between SU and Imi herbicides. For example, the 197 mutation endows resistance to SU but not Imi herbicides whereas the opposite is true for the mutations 205, 653, or 654. However, the mutation 574 gives high levels of resistance to both.
This tells us that the mutations that endow Imi resistance are out there, and when we start using a lot of Imi herbicides we will see resistance soon after.
A critical thing to understand is that high herbicide rates mainly select for target site resistance whereas low herbicide rates select for both target site and metabolic resistance. A herbicide such as Monza® typically gives 70 to 80% control of Brome Grass, which is effectively low dose selection. Therefore, we are likely to see both metabolic and target site resistance to Monza®. The Imi herbicides in Imi tolerant crops typically give very high levels of brome grass control, and therefore we are more likely to select for target site resistance to Imis.
Barley grass is also under threat from increased resistance as the usage of Monza® and Imi herbicides increases. Like brome grass, barley grass is also a self-pollinating species. Barley grass resistance to Monza® was confirmed in 2007, and to imis in 2009. Mechelle Owen found Imi resistance in barley grass due to a target-site mutation. The population was highly resistant to Imi herbicides, but had low level resistance to SU’s. More on barley grass in a future edition of AHRI insight.
What does it all mean?
- Brome grass will find it easy to evolve resistance to Monza® and Imis
- The spread of resistance will be slower than a weed such as ryegrass because brome grass self-pollinates
- A mix of metabolic and target site resistance is likely
We need to develop a robust package of tools to use for brome grass to compliment these herbicides. We know that harvest weed seed control is not as effective on brome as other weed species due to early shedding of seed. Past experience tells us that brome grass hates competition, we need to exploit this weakness, and we need to develop other strategies to stay ahead of the game.
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