Long dormancy adds to wild radish headache
GroundCover™ Issue: 112 | Author: Sarah Cole
‘Bankrupting’ the wild radish seedbank is the only long-term solution to managing this significant weed in paddocks; northern growers can benefit from Western Australia’s experience by getting on top of the weed and its herbicide resistance
Wild radish is a weed that already significantly affects northern growers, and with increasing herbicide resistance developing, it is demanding more attention from growers to manage.
Yield losses from the weed can be severe: between 10 per cent and 90 per cent in wheat crops.
Although Geraldton, Western Australia, is known as the home of wild radish in Australia – with its acidic, sandy soils – areas of the northern grains region with lighter textured acid soils are not immune from wild radish. Research from WA is giving northern growers a ‘leg-up’ in efforts to control this weed.
The real problem with wild radish is that most of the seed produced does not emerge for two years. It will germinate and grow 12 months of the year if sufficient soil moisture is available.
The weed is extremely competitive, particularly against legume and canola crops and it is resilient. Wild radish is developing resistance to many of the most common herbicides across the country.
But it is the long dormancy of the seeds that creates the biggest problem. Some seeds can stay dormant for up to 10 years in the soil. “So if you let any seed go back into soil for one year, you’ll be dealing with the problem for another five, six or seven years,” says Mark Congreve, a senior consultant with ICAN.
“Even when weed numbers are very low, those remaining plants can return thousands of seeds back into the soil, and these will rapidly drive up numbers in the following couple of seasons if not controlled.”
Outlasting the seedbank with long-term strategies is the best way for growers to beat wild radish, Mr Congreve says.
“When weed numbers are very low, growers can introduce integrated non-chemical strategies, which might not be as cost-effective for high weed numbers.”
With good practices, growers can erode the seedbank by up to 50 per cent each year, reaching 95 per cent depletion in five years.
It makes sense that the heartland of wild radish in Australia, Geraldton, was the first area to develop serious herbicide resistance. The unpleasant discovery of glyphosate resistance in wild radish in WA was confirmed in 2010.
Now New South Wales has seen confirmed cases of phenoxy-resistant wild radish, and experts believe glyphosate resistance is inevitable in the east. Resistance to multiple herbicide groups in the same paddock (also called ‘stacking’) is also increasing because wild radish can cross-pollinate.
Herbicide resistance expert Peter Newman, from the Australian Herbicide Resistance Initiative, says there is only one answer to slow the development of herbicide resistance in eastern Australia: a low seedbank.
“Growers need to aim to put the seedbank in decline by using diverse tactics. They need to jump on wild radish immediately – even if it’s only a small patch – and not let survivors set seed,” Mr Newman says. “Growers need to do whatever it takes to stop a weed-seed blowout.”
For WA growers, it is too late to only use herbicides in the fight against wild radish.
But with the right choice of herbicide, timing and application, northern growers can keep this key tool working for longer.
Keep seedbanks low
Mr Congreve emphasises that the biggest problem in ‘losing’ herbicides to resistance is that it greatly increases the cost of weed control.
“For example, with pockets of phenoxy-resistant wild radish recently confirmed in NSW, the cost of control will increase because MCPA and 2,4-D are the mainstays of most weed-control programs,” Mr Congreve says.
As the seeds of wild radish hang on for up to 10 years in soil, the best management strategy is to keep the wild radish seedbank very low.
“If growers can get the seedbank right down, there may be years when they don’t have to spray at all – crop competition and harvest weed-seed control could be enough. And if that means not using any herbicides, that will reduce the herbicide selection pressure too.”
In the west, Mr Newman has seen growers successfully control wild radish by aggressively reducing the seedbank.
If growers want to use chemicals, spraying small plants twice is absolutely essential, he says. The first spray should be applied as soon as the crop is big enough, usually at the two to three leaf stage, depending on the herbicide used, then again three to four weeks later.
However, experts warn that using herbicides only is not a long-term, sustainable strategy. Integrated management should include various combinations of:
- pre-harvest control: manuring crop-topping and mowing;
- crop competition: competitive cultivars, east–west row orientation of crops, narrower row spacing, higher seeding rate; and
- harvest weed-seed control: windrow burning, chaff carts, diversion of chaff to permanent tramlines in controlled-traffic systems, destruction in a Harrington Seed Destructor.
Mr Congreve is cautiously optimistic about northern growers controlling wild radish through learning from growers’ experiences in the west: “Some northern growers are still relying on herbicides to do a ‘good enough’ job. But allowing even a few survivors to return seed to the soil will mean herbicide resistance is inevitable and growers will be battling wild radish for many years to come,” he says.
“However, western growers have been capturing and destroying seeds successfully for quite a few years now, so northern growers can learn from this,” he says.
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A fact sheet on Wild radish is available at: www.grdc.com.au/GRDC-FS-WildRadishManagement
A fact sheet on Herbicide resistance is available at: www.grdc.com.au/GRDC-FS-HerbicideResistance
GRDC Project Code ICN00016
Region North, South, West