Herbicide resistance and emerging weed problems

Take home messages

• Rotating pre-emergent modes of action and controlling survivors will delay resistance to pre-emergent herbicides in annual ryegrass.
• Common sowthistle has developed resistance to Group B, Group I and Group M herbicides. Stopping survivors setting seed across the farm will reduce populations.
• Competition from cereal crops is effective at reducing common sowthistle numbers.

Resistance to pre-emergent herbicides

Resistance to trifluralin (e.g. Triflur X^®) is widespread across South Australia and Victoria and is increasing in New South Wales. Annual ryegrass is the main weed with resistance to Group D herbicides, although there is also resistance in fumitory. It takes about 12 to 15 years of use of trifluralin for resistance to evolve to this herbicide. Propyzamide (e.g. Rustler^®) is also a Group D herbicide. While there is some limited cross-resistance between trifluralin and propyzamide, field experience suggests most trifluralin resistant populations are well controlled by field rates of propyzamide.

Resistance to the Group C triazine herbicides, such as atrazine, is present in several weed species in Australia: annual ryegrass, barnyard grass, liverseed grass, silvergrass, wild radish, Indian hedge mustard and stinging nettle. Many of these have occurred from post-emergent use of triazine herbicides; however, pre-emergent triazine use also selects for resistance. Resistance to triazine herbicides typically occurs after 10 to 15 years of use, but in some cases has taken a lot longer to develop.

Resistance to the Group J herbicides, such as Avadex Xtra^® (triallate) has appeared in annual ryegrass in South Australia, New South Wales and Victoria in the past two years. Many of these populations are also resistant to Boxer Gold^® (Group J & K), trifluralin (Group D) and Arcade^® (prosulfocarb; Group J), but Sakura^® (Group K) is still effective (Table 1). Our research suggests resistance to Group J herbicides is likely to occur after 7 to 8 years of use, so will occur more rapidly than Group D or Group C resistance.

Table 1. Survival (%) of eight annual ryegrass biotypes to field rates of pre-emergent herbicides in a pot trial.

There is no field-based resistance to Group K herbicides in weeds in Australia (eg. Sakura^®, Dual Gold^® and Clincher^®), but this may be because this mode of action has not been as widely used as some of the others. However, there are only a few examples of resistance to Group K herbicides throughout the world, indicating this mode of action is generally robust from a resistance perspective. Much of the resistance that does occur appears to be cross-resistance from use of other herbicides; as typically these populations have resistance to multiple modes of action. This cross-resistance certainly represents a risk for growers in Australia.

Given these risks for resistance to pre-emergent herbicides and the increasing reliance on Group K herbicides in the rotation for many growers, it is important that strategies to minimise the effect of resistance to pre-emergent herbicides are put in place. Rotating pre-emergent herbicide options across the crops grown is one useful tactic, rather than using the same mode of action every year. Ensuring pre-emergent herbicides are used in such a way that high levels of weed control are achieved is also useful. Crop competition in cereals and canola are good partners with pre-emergent herbicides, by reducing the amount of seed set by survivors and later emerging weeds. Failure is more likely to occur where pre-emergent herbicides are used in very high weed density situations and these situation should be avoided by reducing weed numbers with other tactics. Reducing the amount of weed seed going into the seed bank as a weed control tactic should be included in your weed control plan as often as practical.

Common sowthistle an emerging weed problem

Common sowthistle (aka. annual sowthistle or milkthistle) is becoming a more problematic weed of legume crops and fallows. This species germinates preferentially from the soil surface whenever conditions are favourable. The widespread adoption of no-till and stubble retention has favoured common sowthistle by leaving the seed on the surface and creating additional microclimates for it to germinate.

Common sowthistle was very susceptible to the sulfonyurea herbicides (Group B) when they were first introduced; however, widespread use of these herbicides means that most populations are now resistant. Our resistance surveys typically find more than 90% of populations in growers’ paddocks are resistant to Glean^® (Group B). The recent increase in the use of imidazolinone herbicides (Group B), like Intervix^® in Clearfield^TM crops, has selected for populations with resistance to these herbicides as well. Again, our resistance surveys are now finding more than 80% of common sowthistle populations are resistant to both Glean^® and Intervix^®.

More recently, common sowthistle has also evolved resistance to the Group I herbicides (eg. 2,4-D and Lontrel^®) . Three populations from irrigated pasture seed production proved to have resistance to both 2,4-D and Lontrel^® (clopyralid) (Figure 1). These populations are resistant to all Group I herbicides.

Figure 1. Response of susceptible (○) and three resistant (●, ■, ▲) populations of common sowthistle to 2,4-D and Lontrel^® (clopyralid) in pot trials.

Resistance to glyphosate (Group M) is also present in a number of common sowthistle populations from fallows in northern NSW and south-eastern Queensland. This means resistance in this weed species is present to all of the major herbicides used in summer fallow management, as well as to key herbicides used in crop. Fortunately, there are no common sowthistle populations with resistance to all three modes of action yet (i.e. Group B, I and M).

While common sowthistle seed is blown by wind and a small amount can move a long way, most of the seed falls within 100m of the parent plant. Spring germinating common sowthistle struggles to establish in competitive cereal crops and will be much more problematic in the less competitive pulse crops that use less soil water. Growing more competitive cereals will reduce the amount of sowthistle that can persist through harvest. Ensuring surviving plants do not set seed in crop fields, as well as pastures and fence lines, will have a large impact in reducing populations for the next cropping season.

Common sowthistle is still susceptible to herbicides and herbicide mixtures from Group C, Group G, Group H and Group L. These are typically more expensive options, so using them in some paddocks to take the pressure off other modes of action will help.

Useful resources

Common sowthistle - GRDC Website

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC — the author would like to thank them for their continued support.

Contact details

Christopher Preston
University of Adelaide
0488 404 120
christopher.preston@adelaide.edu.au