DISAPPOINTING weed control is a concern in Australian orchards and vineyards, with a growing incidence of glyphosate resistance – especially in annual ryegrass.
The current register of known cases of glyphosate-resistant annual ryegrass in horticulture includes 37 incidences in South Australia, Western Australia, New South Wales and Victoria, plus glyphosate-resistance in fleabane in both orchards and vineyards.
All have occurred where there has been continuous reliance on glyphosate for several years, with little or no other use of alternative mode-of-action herbicides or other weed-control practices.
Australian Glyphosate Sustainability Working Group (AGSWG) and Crop Life recommendations for growers to reduce the risk of herbicide-resistant weeds include:
- Rotating herbicide mode-of-action groups within and across years, and using herbicides with lower frequency of herbicide resistance.
- Using robust label rates for maximum, consistent control of target weeds.
- Using non-herbicide weed control to reduce weed-seed banks and prevent weeds going to seed – including cover cropping, mowing, mulching, strategic grazing.
- Good farm hygiene with only clean machinery, vehicles, stock and footwear allowed in the orchard.
Research and development specialist for Crop Care, David Hughes, said growers had successfully lowered the incidence of herbicide-resistant weeds by adopting those practices.
“Like most cropping enterprises across Australia, under-tree and under-vine weed control has depended heavily on glyphosate – which has provided excellent and cost-efficient weed control,” Mr Hughes said.
“However, growers are increasingly noticing glyphosate no longer effectively controlling particular weeds, especially annual ryegrass.”
“Growers notice a scattering of single plants or patches of plants surviving a glyphosate application,” Mr Hughes said.
“They might initially appear to be affected by the spray, but recover after a few weeks. Early and rapid control of these patches with an alternative mode-of-action will prevent them from setting seed and spreading the resistance problem further.”
He advised growers suspecting herbicide resistance to have surviving weeds tested.
Mr Hughes said Australia was the first country to classify herbicides in groups according to their mode of action, indicated by a letter code clearly marked on all product labels – enabling Australian growers and advisers to clearly understand herbicide groupings when making weed-control plans, and to avoid resistance development through repeated use of herbicides from the same group.
“Glyphosate is represented in Group M. The key to effectively controlling resistant ryegrass and minimising the seed set of resistant strains is to use effective herbicides from other mode-of-action groups. Rotating herbicides with different modes of action is also the key to preventing or minimising herbicide resistance in the first place.”
Mr Hughes said that three years of trials in South Australia had investigated herbicide options for controlling glyphosate-resistant ryegrass in vineyards and orchards.
“The results were heartening, even where the incidence of resistance was particularly high,” he said.
“Herbicides in the trials included Alliance (containing mode-of-action Groups L and Q), Shirquat (Group L) and Rifle 440 (Group D) – either alone or in combination.
“Effective control of a wide range of glyphosate-resistant annual ryegrass biotypes was obtained with Alliance, and with a combination of Shirquat plus Rifle 440.
“Alliance – combining Amitrol (the Group Q part) and paraquat (the Group L part) – has provided a very effective, alternative option for managing resistant weeds.
“Amitrole is a systemic herbicide like glyphosate. It’s quirky that this slow systemic has worked so well in combination with a fast burn-down like paraquat. The combination provides more rapid control than glyphosate – quickly killing green tissue on contact – and longer-term control resulting in fewer survivors or transplants.
“In the SA trials, control of other key orchard and vineyard weeds was assessed at the same time, with Alliance also providing good control of barnyard grass, fleabane, capeweed, prickly lettuce, sow thistle, marshmallow, medic, wireweed, radish, turnip and mustard, as well as resistant ryegrass.
“Its wide range of target weeds, flexible use-rate and spray-pattern, compatibility and rainfastness make Alliance suitable for use in a wide range of horticultural enterprises across Australia.”
For effective control of annual ryegrass, Mr Hughes recommended a higher spray volume (250L/ha and above) with nozzles delivering medium spray-quality droplets.
“Resistance can develop within 3-4 years of using products with the same mode of action,” he said.
With more than 35 weed species in Australia now resistant to at least one herbicide mode-of-action group, and known resistance to 13 different mode-of-action groups, it’s time for everyone in the industry to take more care.”
More information about identifying and managing resistant weeds is available on these websites:
- Copy supplied by Crop Care
