How best to tackle herbicide resistance with new and old chemistries in Tasmanian farming systems?

Take home messages

• Resistance to the Group A herbicides is increasing in annual ryegrass in Tasmania.
• New pre-emergent herbicides are becoming available; however, it is vital that these are used appropriately to get the best results.
• Rotating pre-emergent herbicide modes of action and using other weed management practices will be essential to managing resistance to these new herbicides.

Herbicide resistant weeds in Tasmania

Tasmania has several weed species with herbicide resistance: barley grass with resistance to paraquat and diquat, mainly from lucerne paddocks; wild radish with resistance to the Group B herbicides including the imidazolinone herbicides; and annual ryegrass resistant to Group A and Group B herbicides.

A recent survey of crop paddocks in Tasmania conducted by Charles Sturt University found resistant populations to the Group A herbicides diclofop-methyl and clethodim, but no populations resistant to glyphosate (Table 1). There has been a substantial increase in resistance to the Group A fop herbicides since 2014 in Tasmania. There has also been a reduction in the number of populations with complete susceptibility to all three herbicides tested (Table 1).

Table 1. Extent of resistance in annual ryegrass populations to diclofop-methyl, clethodim and glyphosate from Tasmania in 2014 and 2019.

New pre-emergent grass herbicides for annual ryegrass and their fit for Tasmania

There are several new pre-emergent herbicides that have been released or will be released in the next few years. It is important to understand their behaviour to get the best use out of these herbicides. Not all products will be as suitable for high rainfall environments like Tasmania. For Tasmania, the key characteristics to look for in a pre-emergent herbicide for ryegrass control will be lower water solubility and longer persistence.

Devrinol-C®

Devrinol-C®, active ingredient napropamide, is a Group K herbicide from UPL registered in 2019. Devrinol-C® is registered for annual grass weed control in canola.

Its active ingredient, napropamide, is not as water soluble as metazachlor (Butisan®) and has less movement through the soil. Canola has much greater tolerance to napropamide compared to metazachlor making it much safer in the high rainfall zone. Devrinol-C® offers an alternative pre-emergent herbicide to propyzamide or trifluralin for canola.

Luximax®

Luximax®, active ingredient cinmethylin, is a new mode of action herbicide (Group Z) from BASF registered in 2020. Luximax® is a pre-emergent herbicide for annual ryegrass control in wheat, but not durum. It will also provide some suppression of brome grass and wild oats. In our trials, control of ryegrass is as good as with Sakura®.

Its active ingredient, cinmethylin, has higher water solubility than many other wheat pre-emergent herbicides. This means cinmethylin will move readily into the soil with rainfall events. Less rainfall will be required to activate the herbicide similar to Boxer Gold® (prosulfocarb + S-metolachlor). Persistence of Luximax® is generally good.

Cinmethylin has quite high binding capacity to soil organic matter and this is important in achieving crop safety. Wheat is not inherently tolerant of cinmethylin, so positional selectivity (keeping the herbicide and the crop seed separate) is crucial. Knife-points with press-wheels is the only safe seeding system and the crop seed needs to be sown 3cm or deeper. Heavy rainfall in the first few days after application can also result in the herbicide causing crop damage. Mixtures with trifluralin, triallate and prosulfocarb are good and can provide some additional ryegrass control; however, mixtures with Sakura®, Boxer Gold® or Dual Gold® are likely to cause crop damage and need to be avoided.

Overwatch®

Overwatch®, active ingredient bixlozone, from FMC is a Group Q herbicide that will be available for sowing in 2021. Overwatch® controls annual ryegrass and some broadleaf weeds and will be registered in wheat, barley and canola. Some suppression of barley grass, brome grass and wild oats may occur.

Wheat is most tolerant to bixlozone, followed by barley and then canola. The safest use pattern will be incorporation by sowing (IBS) with knife-points and press wheels to maximise positional selectivity, particularly with canola. Some bleaching of the emerging crop occurs often, but in our trials, this has never resulted in yield loss. In situations where the crop grows poorly, for example, water logging, high root disease, etc., the crop may have more difficulty growing away from the initial bleaching effect.

Overwatch® has a little more water solubility than Sakura®. The level of ryegrass control in our trials has been just behind Sakura®. Mixtures with other herbicides can increase control levels and in our trials in the high rainfall zones, the mixture of Overwatch® plus Sakura® has been very good.

Ultro®

Ultro®, active ingredient carbetamide, from Adama is a Group E herbicide that will be available from 2021. Ultro® will be registered for the control of annual ryegrass, barley grass and brome grass in all pulse crops.

Pulses have reasonable tolerance to Ultro®, so crop damage should be rare. Ultro® provides the best control of annual ryegrass when used pre-emergent. Ultro® has relatively high water solubility, so is more effective on weeds like brome grass that tend to bury themselves in the soil. Persistence of Ultro® is shorter than Sakura®.

Mateno® Complete (BAY167)

Mateno® Complete is a new product from Bayer containing the active ingredient aclonifen. It will be a new mode of action pre-emergent and early post-emergent herbicide for the control of grass and some broadleaf weeds in wheat and barley. Registration is expected in 2022.

The behaviour of this herbicide in the soil will be more similar to Sakura® than Boxer Gold®. It will require more rainfall to activate and will have similar persistence to Sakura®. It will most likely work best for annual ryegrass as a pre-emergent IBS herbicide. The timing of the early post-emergent application will be similar to Boxer Gold®, at the 1 to 2-leaf stage of annual ryegrass. The post-emergent timing will require more rainfall after application than what Boxer Gold® does, so will suit higher rainfall regions like Tasmania.

Managing resistance to pre-emergent herbicides

To date, weed resistance surveys have not identified resistance to any of the main ryegrass pre-emergent herbicides in Tasmania. However, annual ryegrass with resistance to the Group D, Group J and Group K herbicides is present in Victoria and South Australia. One of the keys to slowing the evolution of herbicide resistance is to rotate herbicide modes of action. In Tasmania, a common choice will be to mix pre-emergent herbicides to obtain better annual ryegrass control. This will make rotating modes of action more challenging, but it is important to reduce reliance on any one mode of action. Additional weed control activities, such as crop competition, harvest weed seed control and crop topping, will have to be included wherever they are practical.

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, and the author would like to thank them for their continued support.

Contact details

Dr Chris Preston
School of Agriculture, Food & Wine
University of Adelaide
0488 404 120
christopher.preston@adelaide.edu.au