With no crop competition in the fallow, weeds are free to grow bigger and produce about ten times more seed per plant than they can if they grow in-crop.
As a weed technical specialist with NSW DPI, Tony Cook has seen the effect of a serious weed blow-out in fallow. “With grass weeds particularly, the control measures used in the fallow make a huge difference to the amount of seed in the weed seed bank going into the next crop,” he says. “Grasses can quickly form dense patches if just a few plants are allowed to grow and produce seed.”
Not only do these patches become a problem in crop and at harvest, they can also represent a biosecurity risk for the rest of the farm.
“In a demonstration a few years ago we monitored the effect of treating 11 glyphosate resistant ryegrass plants with glyphosate in a fallow situation compared to treating another 11 resistant plants with different chemistry,” he says. “The glyphosate-treated plants survived and produced large quantities of seed. Within two years the 11 plants had become a dense patch of ryegrass, like a bowling green. The 11 plants treated with different chemistry were killed and no new seed was added to the weed seed bank.”
Glyphosate is relatively cheap and still effective, why change?
Short answer: To preserve the efficacy of this useful chemistry.
Longer answer: We know that repeated use of the same chemistry results in resistance to that mode of action. If we want to keep using glyphosate we must include other herbicide and non-herbicide treatments in the fallow.
What are the advantages of a fallow weed management program?
Short answer: Weeds are easier to see and to treat in the fallow.
Longer answer: In the fallow it is easier to monitor weed presence and full sweep treatments will reduce the chance of weed escapes. The key to success is to target small weeds when using post-emergent herbicides and to check two weeks later to see how effective the treatment has been. Follow-up treatments are essential to remove any weeds that escape as no single tactic is likely to be 100 per cent effective.
What other control options are available for fallow weed control?
Short answer: Pre-emergent chemistry, optical sensor technology and strategic cultivation.
Longer answer: Pre-emergent herbicide applied soon after harvest is an effective fallow management tactic that can also reduce time pressures if you are doing double-knock treatments in some paddocks. Many factors, such as stubble cover, organic matter and rainfall, influence the level of control achieved with pre-emergent products and 100 per cent control is unlikely. Check for weed growth a few weeks after rain to assess the level of escapes. A follow up with spot spraying using optical spray technology, or strategic cultivation when weeds are small will further reduce seed set in the fallow.
What’s new in pre-emergent chemistry?
Short answer: More registrations are now in place.
Longer answer: The GRDC and chemical companies have been investing in research and the registration of pre-emergent herbicides for fallow use. Check for label updates and also be aware of plant back periods and of the effect of environmental factors on herbicide breakdown in the soil.
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