The use of cultural tactics to drive down weed seedbanks, manage herbicide resistance and protect available chemistry is proving to be a powerful tool for growers and advisers.
That is according to Australian Herbicide Resistance Initiative (AHRI) southern extension officer Greg Condon.
Mr Condon presented a practical overview of methods growers and advisers could use to manage herbicide resistance - with the WeedSmart 'Big Six package the feature - at a December 2018 Centre for Crop and Disease Management (CCDM) Crop Protection Forum in Adelaide.
He says people are starting to think beyond herbicides and adopt a mixture of tactics from the WeedSmart Big Six suite, which consists of:
- Diverse rotations
- Double knock
- Mix and rotate herbicides
- Crop competition
- Stop seed set
- Harvest weed seed control (HWSC).
Stacking cultural tactics is really powerful, Mr Condon says.
Weve got some growers who are increasing crop competition, some doing HWSC, then throwing in some diverse rotations with double breaks and the results are just mind-blowing.
Mr Condon says an example of the worth of diverse rotations was a rotation he observed in South Australia called the triple break.
This grower had a triple break of cereal hay, faba beans and canola to drive down resistant populations of ryegrass he couldnt control with trifluralin, he says.
It shows the value of rotations, not only for weeds but for other things such as fungicide resistance.
Rotation and diversity comes in different forms.
It comes in the form of the herbicides used, the rotations of crops and even the different integrated weed management strategies used.
Weve got some growers who are increasing crop competition, some doing HWSC, then throwing in some diverse rotations with double breaks and the results are just mind-blowing
The double knock method engages one tactic to kill the majority of weeds present and a different follow-up tactic to kill any survivors.
An example of this is paraquat being used as a double knock through a sprayer with automatic weed seeking sensors following the application of glyphosate, Mr Condon says.
The double knock strategy is a really critical tool in our toolbox to help protect glyphosate, one of the most valuable herbicides in no-till farming systems.
Mr Condon says the mix and rotate tactic involves mixing and rotating chemicals at full lethal rates to get full value rather than just rotating herbicide groups.
Crop competition incorporating narrow row spacings while maintaining a standard plant population is a tactic Mr Condon says will reduce weed seed set and increase the value of HWSC.
This is because the increased crop competition keeps grass weeds upright, enabling the harvester to more easily capture weed seeds at harvest.
When it comes to HWSC, Mr Condon says many growers are starting to move away from narrow windrow burning to chaff lining and chaff decks.
The bottom line is it doesnt matter what HWSC tactic growers are using, as long as they are using a tactic, Mr Condon says.
Narrow windrow burning has been well adopted because of its low cost and it kills weed seeds. The challenges with it are the burning, the smoke and the loss of ground cover.
While burning straw is effective for mitigating disease resistance, the soil can drift from loss of cover and ground cover is a fairly high priority which narrow windrow burning compromises.
Mr Condon says chaff-lining placing the chaff into a narrow line and either grazing or leaving it to rot is an effective tactic, especially for mixed farming, while chaff decks are a popular option for controlled traffic farmers.
The ultimate in HWSC is seed impact mill technology, or harvester-integrated weed seed destruction technology, Mr Condon says.
Rather than leaving the seeds to rot, burning or baling, this technology enables growers to capture weed seeds at harvest which are then destroyed by a mill mounted on the harvester, with the residue spread out the back of the header.
This technology enables high levels of weed seed removal with almost no removal of residue.
"But there are still a few challenges with harvester setup and it is most suited to continuous cropping systems rather than mixed farms, as it destroys all the seeds that livestock would otherwise eat.
The role of resistance monitoring
An ever-sharpening rise in cases of insecticide resistance is a major reason to improve monitoring and forecasting of pest risk.
This was the overarching message from cesar research scientist Dr James Maino, who says the increasing cost of chemical control and the increasing cost of discovery for chemical products were other reasons to engage in predictive activities that increase precision of control and monitoring efficiency.
Dr Maino has been involved in recent work looking at 90 different common grains pests and gathering information about predictors associated with resistance evolution.
The first one of those was evolution potential, he says.
The more plant families an insect feeds on, the more plant chemicals it gets exposed to and theres been emerging research saying that actually pre-adapts them to resistance evolution.
Dr Maino says they also examined targeted insecticide use and the number of times that pest was reported using the PestFacts reporting service.
Another aspect we examined was non-targeted pesticide application, he says.
Just because that pest is not being targeted, it doesnt mean its not getting hit when other pests are being controlled.
This study assigned a high-risk rating of future resistance evolution to many pests that have already evolved resistance including green peach aphid, redlegged earth mite and diamondback moth.
Interestingly, several other pests were also identified that are currently not known to have resistance but have a lot of these traits that predict resistant pests, Dr Maino says.
And we have hypothesised they have a mechanistic link with how resistance evolves.
Just because that pest is not being targeted, it doesnt mean its not getting hit when other pests are being controlled... Several pests were identified which are currently not known to have resistance but have a lot of traits which predict resistant pests and we have hypothesised they have a mechanistic link with how resistance evolves
Dr Maino says such pests include oat aphid, lucerne flea, cabbage aphid and cowpea aphid.
The upshot of knowing this is when we do see these pests in the field, we can pay more attention when monitoring them, he says.
Particularly their responses to chemical application and really start thinking about whether chemical applications are warranted in those situations.
The 2018 Crop Protection Forum was organised by CCDM, AHRI and cesar.
With fungicide resistance on the rise, researchers, practitioners, growers and agronomists attended the function to discuss the important questions around crop disease control and management.