FEATHERTOP rhodes grass (FTR) is a major weed in chemical fallows that is notoriously hard to kill with glyphosate.
It is an aggressive weed that can establish in bare fallow situations and produce a large quantity of seed if left uncontrolled
Several biotypes of this species are resistant to glyphosate and can also survive a double knock of glyphosate followed by paraquat, particularly once the weed is larger than four to five leaf stage.
To give growers more options, a study was conducted to assess the potential of other herbicides and use patterns that can control large FTR plants or stop seed set.
University of Queensland Centre for Crop Science professor Bhagirath Chauhan said an integrated approach was essential to controlling feathertop Rhodes grass.
"In applying the WeedSmart Big 6 to FTR in a bare fallow situation we have identified some tank mix and herbicide sequences that growers could deploy to help manage this difficult weed and stop seed set," Dr Chauhan said.
In the study, adjuvants did not improve glyphosate efficacy on mature (40 to 50 leaf) FTR plants.
All the plants survived and produced seed after being treated with glyphosate, indicating that the population used in the study was resistant to glyphosate at this rate and weed growth stage.
In those glyphosate resistant populations, the second knock product was doing the heavy lifting when applied to large (8 to 10 leaf) FTR plants.
Dr Chauhan said glyphosate and the double knock tactic could often provide good control of resistant FTR plants if the herbicide is applied when the plants are small and actively growing.
"The traditional double knock of glyphosate (Group 9 [M]) or glyphosate + 2,4-D, followed by paraquat (Group 22 [L]) or glufosinate (Group 10 [N]), applied to older FTR plants (8-10 leaf) achieved increased phytotoxicity through improved mortality, reduced biomass or fewer seed panicles," he said.
"However, the double knock was no better than using paraquat or glufosinate alone when applied to 8 to 10 leaf FTR plants.
"FTR is not listed on glufosinate labels in Australia but is used to control other weeds in fallow situations at the rate (750 grams active ingredient per ha) tested in this study."
Clethodim and haloxyfop are possible suitable alternative herbicides to treat large, glyphosate resistant FTR plants, with excellent results achieved in the study in pot trials conducted in an open environment.
However, that would be more difficult to achieve in the field.
Clethodim and haloxyfop were tested on FTR plants at the 24 to 28 leaf stage.
Clethodim is registered for use against FTR in a number of summer crops, but without any crop competition many FTR plants survived the registered rate (90g ai/ha), although weed biomass and seed production was severely curtailed.
Whereas haloxyfop efficacy against FTR at this growth stage was 100 per cent at the registered rate of 80g ai/ha.
Dr Chauhan said a combination of these two treatments also resulted in 100pc control.
"The effective use of these two herbicides (both Group 1 [A]) relies on excellent coverage and application when the plants are actively growing," he said.
"This is difficult to achieve in field conditions, which is why the label recommendations are typically for younger weeds.
"These herbicides are known to readily select for resistant biotypes so when applied in a chemical fallow situation (with no competition), it is necessary to target small weeds with robust application rates and to apply a second knock with a contact herbicide, such as paraquat."
In the study, it also seemed there was a synergistic effect when isoxaflutole (Group 27 [H]; e.g. Balance) was mixed with paraquat.
Neither of these herbicides provided useful control of FTR at the 40 to 50 leaf stage when applied individually, however when mixed together, they achieved a higher level of weed mortality and prevented panicle production.
For example, a tank mixture of isoxaflutole 75g ai/ha, with paraquat 600g ai/ha, resulted in 92pc FTR mortality and no panicle production.
"Even at a paraquat rate of 300 g a.i. per ha mixed with isoxaflutole 75g ai/ha, only 17pc of the large FTR plants survived when the mixture was applied to both the plant and the nearby soil - allowing uptake through both the leaves and the roots," Dr Chauhan said.
"The benefit of this mixture may be reduced if the weed patch is dense, potentially reducing the amount of the isoxaflutole that reaches the soil.
"Even the prevention of seed set in large FTR plants is of significant value in managing the seed bank of this invasive weed, as FTR seed remains viable for less than 12 months."
Such a use pattern is not currently specified on product labels, although both products are registered for weed control in fallow situations.
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