WITH glyphosate prices on the rise and weeds of all kinds expected to be an issue after rainfall events, Australian graingrowers should consider a summer weed management strategy as a matter of priority.
Weeds present one of the biggest costs to growers, so investing in proactive management can help maintain profitability and preserve soil moisture for the 2022 winter cropping season.
ICAN senior consultant Mark Congreve, whose work is supported by Grain Research and Development Corporation (GRDC) investment, said spraying weeds when they were young was the most effective way to control them and the best way to ensure growers get the maximum value from their herbicides
"With high populations of weeds and rising glyphosate prices, growers should be discussing management options with their advisers and implementing targeted strategies to get the best results," Mr Congreve said.
Weed identification
The first step to tackling early weed emergence was identifying weed species and determining their resistance levels from prior testing and paddock observations based on the level of control from previous spray applications.
This significantly influences management options, herbicide selection and other application factors such as dose rates, droplet size and sprayer set up.
The GRDC encourages the identification of weed species as a priority to ensure correct management decisions are made in a timely and cost-effective manner.
Correct identification of weeds allows the correct choice of herbicides management tactics.
Optimal glyphosate application
It's common for growers to consider what other mixing partners they should add to their glyphosate applications when paddocks have established weeds.
However Mr Congreve said the best way to maximise glyphosate efficiency was to apply it straight under optimal conditions.
"Growers should try to avoid mixing glyphosate where possible, as other herbicides can often hinder its performance," he said.
"In particular, mixes with Group 4 (I) herbicides such as 2,4-D and fluroxypyr can be antagonistic for control of glyphosate resistant grasses and broadleaf weeds, such as sow thistle, under summer application conditions.
"Group 14 (G) herbicides may also reduce glyphosate performance when mixed and applied under high light intensity conditions."
Adding mixing partners to glyphosate can be effective when weeds are small and glyphosate susceptible, but negative interactions are more likely to occur when dealing with resistant weeds.
Mr Congreve said if growers were unable to apply herbicide under optimal conditions, they would need to increase glyphosate rates to compensate.
"This isn't something growers would want to hear considering high prices but getting a great result the first time could save additional sprays throughout the season," he said.
Double knock applications
Mr Congreve recommends a double knock immediately after harvest if paddocks have weeds from the winter crop, or summer weeds are emerging.
When dealing with resistant weeds, it's important growers use a second knock that's able to control any weeds that survived the first pass.
For growers using a double herbicide knock, Mr Congreve said the second pass was a better time to mix other herbicides when targeting specific weeds, as opposed to mixing herbicides in the first knock with glyphosate.
"Paraquat is generally more compatible with a wider range of broadleaf herbicides," he said.
"Growers could also add a residual herbicide to the paraquat pass, which may minimise additional knockdown sprays - this could be extremely valuable if glyphosate supply is limited."
Ideal timing between the first knock to the second should be somewhere between three to 14 days as this allows enough time for the glyphosate to fully translocate but not too much time that weed regrowth commences.
Applying glyphosate via air if paddocks are too wet to traverse, can achieve good results soon after weed emergence.
Mr Congreve said with significant weed activity, most growers were likely to do a broadcast spray after harvest for their first pass, however having an optical sprayer available for the second pass could be a significant benefit.
Mechanical weed management
While there is reluctance to disturb stubble cover in paddocks Mr Congreve said in some cases the most effective option for growers faced with a high number of well established or resistant weeds, could be to mechanically manage these weeds through cultivation.
"Throwing high rates of chemicals at large or resistant weeds can be wasted money and while cultivation may be considered a last resort, it's normally effective," he said.
"Growers do have to consider management challenges that could result from cultivating, such as trafficability, loss of stubble cover and possibly erosion depending on soil type and follow up rainfall."
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