Beyond herbicides – experience shared
GroundCover™ Issue: 105 | Author: Tom Dixon
With herbicide-resistant weeds an increasing problem across the northern grains region, experts are looking to Western Australia for alternative methods of controlling weeds.
Maurie Street from the Grain Orana Alliance says herbicide resistance is an increasing problem in northern grains production because many farming systems have moved to continuous cropping with nearly 100 per cent reliance on herbicides for weed control.
Mr Street and Glenn Shepherd from IMAG Consulting in central-west New South Wales were among several presenters at the recent GRDC Grains Research Update in Coonabarabran, NSW, who explored herbicide alternatives for controlling weeds.
“The commonly promoted approach to managing herbicide resistance through herbicide rotations does not actually prevent resistance,” Mr Shepherd says. “It just slows down the rate of resistance developing.”
There are many examples of herbicide resistance across Australia, most notably in Western Australia, where varying chemical use has not prevented resistance developing and many weeds have developed resistance to multiple herbicide groups.
Some populations of ryegrass can no longer be effectively controlled with in-crop herbicides, Mr Shepherd says.
“Group A and B herbicides failed a long time ago, and clethodim, the most recent viable Group A option, is no longer effective in many areas,” Mr Street says. “This leaves WA growers with only a small suite of pre-emergent chemicals at their disposal, but with moderate levels of trifluralin resistance in ryegrass, herbicides such as Boxer Gold® and Sakura® may be all that is left.”
Modelling has shown that resistance to Group A and Group B herbicides can occur after as few as four applications.
- There is no magic bullet for weed control – all weed management strategies, including windrow burning, need to involve an integrated approach with a variety of tactics
- Windrow burning and harvest weed seed management techniques are effective management tools for all weeds
- Windrow burning is cheap and easy to set up, allowing growers to test the ‘proof of concept’ on their farms before making large investments in chaff carts or seed destructors
“We need to start looking at new techniques for weed control – we shouldn’t be expecting or relying on new herbicides to fix the problem,” Mr Street says.
Mr Shepherd agrees and says that growers entering the industry now will be the first in more than a generation with no access to chemicals to control some problem weeds. Although chemicals can be expensive, they are a relatively easy solution to most in-crop weeds.
“We need to develop novel solutions to weed problems to complement and extend the life of current practices and the effectiveness of herbicides,” Mr Shepherd says.
Growers in the northern region still have effective herbicide options available. Mr Street urges growers to protect these herbicide options, so they can continue to be used.
“In northern NSW, we haven’t yet had the problems seen in WA,” Mr Street says.
“But if we don’t want to be in the same predicament as WA in five years’ time, we have to act now.
“No new chemical modes of action have been developed in the past 20 to 30 years.
“We must look outside the square – the answer cannot come ‘from the chemical drum’ – we must look at alternative methods for control,” Mr Street says.
Weed seeds a target at harvest
Alternative methods of control available to growers include selecting different crops, adjusting planting rates and using narrower row spacings, but these methods only reduce the number of weed seeds that germinate, rather than eradicate them.
WA growers and researchers have developed harvest weed seed management that aims to intercept weed seeds at harvest to prevent them competing with the next crop.
Growers in the west have used two primary methods of weed seed management at harvest:
- killing weed seeds with fire, using chaff carts and windrow burning; and
- destroying seeds mechanically, for example with the Harrington Seed Destructor.
Both options have been shown to be effective in sterilising seed.
Windrow burning involves dropping the chaff into windrows at harvest where they are later incinerated. It is said to be the cheapest and easiest non-herbicide strategy available because no expensive equipment is needed, so growers can try it for a season or two at very little cost.
“All it cost me was $100 for a few bits of steel to modify the header to drop chaff into the right place,” Mr Street says. “But the windrowing is the easy part; burning the windrows without igniting the whole paddock will take some skill.
“The key to successful windrow burning is to burn early enough in the year to get the temperature hot enough to kill the seeds,” Mr Street says.
In WA, many growers use the Department of Fire and Emergency Services fire danger index to select a suitable day and time for burning. The index takes into account wind speed, temperature and humidity to calculate fire potential.
“The index is a terrific tool for selecting the right conditions for an effective burn,” Mr Street says.
“A Department of Agriculture and Food, WA (DAFWA), video on YouTube features a grower, Doug Smith, discussing windrow burning in WA,” he says. “I would encourage any grower looking for information to look at it and consider the information presented.” An article by Mr Smith on the technique is available on the DAFWA website (www.agric.wa.gov.au).
Mr Smith suggests generating a fire that does not move too quickly. A fire that is fanned by strong winds can ‘run’ along the top of the windrow and either does not generate enough heat to kill seeds, leaves some material unburnt, or increases the risk of the fire escaping.
Two seasons of burning
Following the 2011 harvest, Mr Street trialled windrow burning wheat stubbles on his property near Dubbo, NSW, with encouraging results. He also windrowed and burnt wheat and canola stubbles from the 2012 harvest.
“I was concerned that windrow burning wouldn’t work as well for us in the east with our high summer rainfall compared with the drier summers in WA,” Mr Street says. “But having burnt now for two seasons, I am more confident that summer rainfall did not hamper the outcomes.
“More than 90 millimetres of rain fell early in March 2012 and another 10mm just 10 days prior to burning,” he says. “However, the windrows dried out well, possibly because the windrow had thatched itself, repelling much of the water rather than letting it through.”
Mr Street says that even though in some cases the soil surface under the windrows was moist, as long as conditions were good for burning, the fire burnt completely.
In areas that see almost weekly rain through March and April, it can be problematic to dry windrows enough for them to burn. “But in my experience, if you keep an eye on the forecasts, it is more than likely there will be an opportunity to burn,” he says.
Burning temperature and duration
Trials have shown that to achieve the temperatures and fire duration required to kill weed seeds, the trash needs to be dropped into a narrow windrow rather than just dropped off the harvester sieves.
Ryegrass required 10 seconds above 400°C to destroy seed, while wild radish needed 10 seconds above 500°C. To achieve these temperatures and durations, narrow windrows are more effective than standing stubble or conventional windrows, Mr Street says (Figure 1).
The duration of heat intensity in a narrow windrow suggests this technique will be effective against most weeds, and less than 10 per cent of the field is exposed to erosion.
“By experimenting with and perfecting techniques, such as windrow burning within our systems, we can help extend the life of herbicides and put the brakes on herbicide resistance on most farms,” Mr Street says. “This may mean you need to windrow burn only some paddocks each year.
“The alternative, if we fail to adopt change, can be found in WA, where many growers have to windrow burn every paddock every year to help control weed populations and continue to farm successfully,” he says. “There are very few solutions left available to them in a chemical drum and the implications for the cost of production and labour are profound.”
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The full paper, Windrow burning for weed control by Maurie Street and Glenn Shepherd, presented at the GRDC Update is available on the GRDC website.
Doug Smith’s experience of windrow burning in WA, including tips for how to burn successfully, is available on the DAFWA website:
Video of Doug Smith on windrow burning in WA
Video of Maurie Street’s experience of windrow burning in northern NSW
GRDC Project Code GOA00001
Region North, West