One downside of the widespread production of canola is the increase in clethodim resistant annual ryegrass populations on many farms across southern Australia.
What’s the best option for crop topping canola?
Dr Chris Preston, University of Adelaide (UA) Associate Professor—Weed Management says growers are noticing that in some areas clethodim may be only suppressing rather than controlling ryegrass. “All tactics in the weed management program need to minimise seed set,” he says. “End of season control measures have been limited to narrow windrow burning in canola and this is not always a practical option for growers.”
Through a GRDC funded trial Dr Preston investigated the pre-harvest use of a number of herbicides, looking at efficacy, crop safety and residue levels. “Our trials concluded that only glyphosate was effective and safe to use for pre-harvest weed control in canola,” he says.
This research supported the registration of Nufarm’s weedmaster® DST® as the only glyphosate product registered for pre-harvest application in canola.
Dr Preston, who is also Chair of the Australian Glyphosate Sustainability Working Group says that, once again, glyphosate has proved to be a valuable chemical but a word of caution is required. “Pre-harvest weed control with glyphosate must not be over-used in the rotation. It is essential that many other non-glyphosate measures are also being used in a weed management strategy.”
“Keep track of how often you are applying glyphosate across the rotation and include as much diversity as possible,” he says. “For example, if you use glyphosate for pre-harvest control in canola it would be wise to use a different harvest weed seed control tactic in your cereal crop and consider paraquat as a better choice to crop top pulses.”
“Both over the top and under the windrow applications are equally effective as weed seed set control measures,” he says. “Efficacy is reduced in hot, dry weather conditions so an over the top crop topping application offers some extra flexibility provided growers have access to a self-propelled boom with sufficient clearance. In some situations this will make direct harvesting a more practical option too.”
A harvest weed seed operation, such as narrow windrow burning, will also assist to remove any survivors and help prolong the efficacy of glyphosate across the rotation.
Nufarm field development manager (broadacre) at Horsham, Mark Slatter says the expanded registration of weedmaster DST for harvest aid and weed control applies to triazine tolerant (TT), CLEARFIELD®, Roundup Ready® and conventional canola varieties. “Label rates for applications of weedmaster DST to standing canola or under the cutterbar at windrowing @ are 1.4 – 4.1L/ha, however Nufarm trials showed best results were achieved at rates of 2.8 – 4.1L/ha with an adjuvant (LI 700®) to increase penetration into the crop canopy and for drift management,” says Mr Slatter. “The recommended water rate for ground application is at least 80 L/ha applied to standing canola.”
The product is also registered for aerial application at a maximum rate 3.1L/ha.
Mr Slatter says the timing of the application is critical and must not occur before there has been a minimum of 20% grain colour change across the paddock as going in too early will cause yield reductions. “No withholding period applies when the product is applied under the windrow but direct harvest must not occur until five days after application to a standing crop,” he says.
While annual ryegrass is a key target weed for this use pattern, other key target weeds controlled include wild radish, sow thistle and many other annual grass and broadleaf weeds.
Mr Slatter says that Nufarm has developed this new pre-harvest registration as an additional tool for reducing weed seed set of annual weeds because a ‘zero tolerance’ approach to weed seed set is one way of reducing the risk of herbicide resistance developing. “It is very important to use a pre-harvest application of weedmaster DST as part of a broader integrated weed management (IWM) strategy to minimise the risk of glyphosate resistance,” he says.
“Crop safety trials proved that this use pattern has no negative effects on yield or oil content in canola, however it must not be used on crops intended for seed because germination and vigour is affected,” he says. “Nufarm’s extensive MRL trials also showed residues are well below maximum levels for all canola systems so there will be no impact on product export suitability.”