Integrating new herbicides into the rotation

Take home messages

• Well above average rainfall in Victoria in 2022 led to a lot of later germinating weeds (and escapes) and/or weed blow-outs in some paddocks.
• Resistance in annual ryegrass to common herbicides is increasing.
• Other weed species developing resistance include brome grass, barley grass, milk thistle, wild radish and Indian hedge mustard.
• Strategic use of new herbicides is paramount to ensure the longevity of these herbicides and prolong the onset of herbicide resistance.
• Consideration should be given to soil type, tillage system, varietal difference and forecast rainfall, when choosing the right pre-emergent herbicide for your crop.

Background

Pre-emergent herbicides have become more important for the control of grass weeds, particularly annual ryegrass, in the past decade, as resistance to post-emergent herbicides has increased. Resistance to trifluralin is now common across many cropping regions of South Australia and Victoria and is increasing in NSW.

Worryingly, resistance to the Group 15 pre-emergent herbicides has also be detected in random weed surveys. In some parts of South Australia and Victoria, resistance to triallate is becoming common. It is likely resistance will further increase, making it more difficult to control annual ryegrass with the current suite of herbicides available.

New pre-emergent herbicides offer an opportunity to expand the suite of products that can be rotated. However, it is important that these are used well to optimise performance while also maintaining their longevity.

Results and discussion

Resistance testing

Weed resistance testing by Plant Science Consulting, through both GRDC random surveys, and targeted sampling by growers and agronomists, has shown an increasing amount of herbicide resistance to a range of weeds.

New pre-emergent herbicides

We have been blessed to have been given the opportunity to use new herbicides that have been recently released into the market. These include Luximax®, Overwatch®, Mateno® Complete, Callisto®, Reflex® and Voraxor®, to name a few.

The introduction of these products has given growers access to brand new chemical mode of action (MOA) groups that have not been utilised before in broadacre cropping in Australia. A few of these new products have different characteristics when applied to the soil, like their solubility and mobility/binding to soil and organic matter. Thought should be given to where the new products best fit in your rotation, with regards to crop type, problem weed species and soil influences.

WeedSmart Big 6

As new chemistry becomes available, it is crucial for all involved to protect the longevity of any new products and minimise the risk of resistance. The WeedSmart Big 6 brings together weed research data with grower experiences to create a set of practical guidelines focused on minimising the weed seedbank without compromising profit.

The WeedSmart Big 6:

• rotate crops and pastures
• double knock – to preserve glyphosate and paraquat
• test, mix and rotate herbicides
• increase crop competition
• stop weed seed set
• adopt harvest weed seed control.

Best practice agronomy is a key component of the Big 6 and pulls together all the aspects of profitable no-till cropping such as precision seeding, timely sowing, targeted nutrition, soil amelioration and crop competition, so that crops have the edge over weeds. Tactics such as harvest weed seed control, cutting hay and diverse rotations are also essential to complement herbicide use including the mix and rotation of herbicides, double or triple knock, and late season crop-topping.

Grower success in reducing weed seedbanks but staying profitable has been achieved through stacking Big 6 tactics over an extended period of time. For example, a diverse rotation with pulses, competitive barley and early sown hybrid canola combined with pre-emergent herbicides, opportunistic double knocks, crop-topping and chaff decks has all the Big 6 tactics stacked together.

Conclusion

New herbicides will continue to form part of our weed management strategies in the future.

The key is to know how to get the best out of them, optimising weed control, while also preventing the onset of herbicide resistance from poor application and overuse.

Old chemistries may still have a fit on your farm, for certain weed species where resistance has not evolved. Resistance test any problem areas of weeds so that you know what you are dealing with, and what herbicides are still active on that weed type.

Adopt as many of the WeedSmart Big 6 principles as possible into your farming system, to complement your herbicide management strategies.

Acknowledgements

Plant Science Consulting, Peter Boutsalis and Sam Kleemann
ICAN, Mark Congreve
WeedSmart *, Greg Condon
University of Adelaide, Chris Preston

* WeedSmart has investment from the GRDC and commercial companies and delivers science-backed weed control solutions. GRDC is a Platinum Partner in WeedSmart..

Useful resources

WeedSmart Big 6

Pre-emergent herbicides fact sheet 

Soil behaviour of pre-emergent herbicides in Australian farming systems: a reference manual for agronomic advisers

Contact details

Chris Davey
1-3 Kennett Street North, Kadina, SA 5554
0428 466 675
@cropdoctor54