Time to open the herbicide resistance toolkit
By GRDC northern region panellist Penny Heuston
Harvest weed seed control (HWSC) is one of the best tools in the kit to help growers combat the rapidly-increasing challenge of herbicide resistance.
Herbicide resistance poses a major, if not the major, threat to the long term viability of the northern grains industry with resistance issues across a multitude of weed species and modes of action.
Its management is a key investment area for the Grains Research and Development Corporation (GRDC) which is funding trial work into the extent of herbicide resistance to weed types and multiple modes of action; the effectiveness of non-herbicide tactics in supressing weed growth and driving down the weed seed bank; and integrated weed management programs.
HWSC plays an important non-chemical role in stopping weed seeds from entering the soil and can dramatically reduce the emergence of hard-to-kill weeds in the following season. Chemicals alone will not control herbicide resistant weeds, other management tactics need to be employed to drive numbers down.
It involves collecting, destroying or burning weed seeds that are present at harvest and is particularly effective in problem species such as black oats, wild radish and annual ryegrass.
Windrow burning is a cost effective, non-chemical HWSC tactic that has been widely and successfully adopted in Western Australia and is being increasingly used by northern growers. Done well, windrow burning can kill 99% of wild radish and annual ryegrass seeds.
The practice has been the focus of significant research and extension work undertaken by the GRDC-funded grower solution group Grain Orana Alliance (GOA), which has given growers access to practical information and support via on farm-field field days and demonstrations to ensure their windrow burning activities are safe and effective.
At the outset, a weed “audit” should be conducted in every paddock prior to harvest, whether windrow burning is to be undertaken or not. This will help growers ascertain if they have weeds that have escaped in-crop sprays or pre-emergent activity but also offers growers the opportunity to assess the disease status of their paddock for issues such as crown rot.
This is also the time to collect weed seeds for herbicide resistance monitoring if your levels of resistance are not fully understood.
Once the weed status of paddocks is determined, the focus needs to be on planning for an effective harvest and then ensuring conditions are suitable for a successful burn.
Harvest height should be 10cm-20cm – a good rule of thumb is to stick to the height of a standard Australian beer can – and windrows should be 500mm-600mm wide. Machinery modifications can be simple and extremely cost effective and some handy `how to’ videos can be found on the GRDC YouTube channel.
Burning should be undertaken in autumn when there’s a light cross breeze blowing. For annual ryegrass, the fire should burn at 400 degrees C for 10 seconds while to kill wild radish seeds, the fire needs to burn at 400 degrees C for 30 seconds. A slow burn is ideal as this will endure that the windrow is burnt right down to the soil surface where the weed seeds will be.
It’s important to remember that you can still windrow burn even if you have a large cereal crop but be aware that there is more chance of the fire getting away.
Pulse crops and canola are ideally suited to windrow burning and even light chickpea crops can offer an effective burn. The oil in the canola stubble makes it a great target crop and will burn the hottest.
There are always questions over what rainfall means for windrow burning, however GOA work has shown that even after good summer rainfall, windrow burning can still be effective.
GOA’s work into windrow burning is continuing and research is currently being conducted into windrow burning’s effect on fallow moisture efficiency.
This on-going work is critical as windrow burning offers one of our best defences against herbicide resistance and the preservation of our northern farming systems.
Penny Heuston, GRDC Northern Region panellist, Warren
Sarah Jeffrey, Senior Consultant Cox Inall Communications