Take home messages

• Exclusion and eradication are the first two steps in managing these weeds.
• Flaxleaf fleabane and Feathertop Rhodes grass (FTR) have a natural tolerance to glyphosate, so are best controlled when small.
• Glyphosate and paraquat resistance in fleabane has been measured, but paraquat is still useful on FTR.
• Crop competition is a useful tool.
• Pre-harvest herbicide application, for example, registered glyphosates and 2,4-D amine, may aid in management while weeds are small.
• Residual herbicides aid in longer term control but be wary of plantback periods to the following crop.
• Burial by cultivation can work but is only a short-term fix.
• Incorporate non-selective means of management, such as chipping, spot spraying, and wiping.

Background

Some summer weeds are becoming harder to manage out of our growing season, while also starting to become problematic within the growing season (such as fleabane). Effective herbicide options are limited, while common knockdown herbicides are increasingly proving to be inefficient in their control. What have we learnt about fleabane (in 20+ years), and are learning about FTR (in <10 years), that we can maybe implement on emerging weeds like gazanias?

Discussion

Flaxleaf Fleabane

• huge seed production – can produce 100,000+ seeds with 80% viability
• surface germinating – has flourished with the adoption of no-till
• seeds easily dispersed by wind
• seeds have no dormancy, so will germinate whenever conditions are right
• loves no competition – roadsides and fencelines
• naturally glyphosate tolerant
• now glyphosate and paraquat (AP) resistant
• local avoidance of chemicals that have activity on fleabane due to rotational and resistance constraints.

Feathertop Rhodes grass

• huge seed production – can produce ~150,000 seeds/plant
• wind dispersed
• surface germinating – favoured by no-till
• loves no competition – roadsides and fencelines
• matures very quickly when stressed
• naturally glyphosate tolerant
• short dormancy period
• high retention of seed in head – HWSM.

Management

• Identification (FTR)
  • On roadsides as juveniles.
• Exclusion
  • Control it when first seen on roadsides.
  • Do not let it set seed.
• Eradication
  • If you find it has ‘jumped the fence’, do everything to control it then.
• Containment
• Management

Control

• Chemical control options are limited.
• The key is to apply herbicides to juvenile weeds.
• Optimise competition with the weeds, whether it be from the crop, or from residual stubble.
• Fleabane only needs the ground to be wet for a handful of days to stimulate germination.

Some chemical residues provide beneficial control of fleabane after harvest. Is it getting to a point that a secondary herbicide needs to be planned late in the season for some residual control?

Isoxaflutole (Balance®) is a very useful residual product, that also provides control of milk-/sow-thistle.

WeedSmart Big 6

As resistance develops in weeds like fleabane and FTR, 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 that are important for fleabane and FTR include:

• know your resistance status
• double knock fleabane, to preserve paraquat if possible
• test, mix and rotate herbicides (while checking re-cropping intervals)
• increase crop competition
• stop weed seed set, particularly of FTR along any paddock boundaries

In addition, incorporate the following for other weeds in the paddock:

• rotate crops and pastures
• adopt harvest weed seed control

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. The author also thanks Plant Science Consulting’s Peter Boutsalis and Sam Kleemann, WeedSmart’s Greg Condon and the University of Queensland’s Bhagirath Chauhan. WeedSmart is financially supported by its numerous partners, with the GRDC being the principal investor.

Contact details

Chris Davey
Next Level Agronomy Pty Ltd
Wallaroo SA 5556
0428 466 675
@cropdoctor54