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Double-knock: second knock is essential

Preventing weeds from setting seed is a central tenet of integrated weed management. The double-knock principle helps achieve this by using one tactic to kill the bulk of the weeds present and a different tactic to kill any survivors.

Michael Widderick, principal weeds research scientist, Queensland’s Department of Agriculture, Fisheries and Forestry, is a proponent of the double-knock, which he said originally used herbicide as the first tactic followed by cultivation.

“Cultivation was phased out with the advent of zero till farming and replaced with a second herbicide application,” he said.

“Now the first knock is a translocated herbicide, followed with the second knock contact herbicide. While the tactic involves two herbicides it is essential that they have different modes of action.

“Two applications of the same chemical is not a double-knock and is actually likely to increase selection pressure, hastening the development of herbicide resistance.”

The key to implementing the double-knock strategy is to understand that it is a two-phase tactic targeting weeds of the same generation. Therefore the double-knock tactic can be used several times during a fallow period, targeting different generations of weeds.

“Rainfall events are usually the trigger for a double-knock,” said Dr Widderick. “Soon after rain a new generation of weeds will germinate and the first knock application should occur while the weeds are small.

“The optimal time between the first and second knock depends largely on the weed or weeds being targeted,” he said. “The optimal timing can be as short as two days but is generally within one or two weeks of the first knock application.”

Glyphosate remains an effective first knock herbicide however with the increasing number of glyphosate resistant weeds being identified, research is underway to investigate the efficacy of alternative chemistry.

Michael Widderick

“Group A herbicides are being tested as an alternative first knock treatment for summer grass species, either on their own or mixed with glyphosate,” said Dr Widderick.

“The risk of Group A resistance developing is high so if these chemicals were introduced for fallow weed control it would be vital for any survivors to be controlled.”

For the second knock the options are limited to diquat for broadleaf weeds and paraquat or glufosinate for grasses.

“The double-knock tactic works well and can achieve very high levels of control when implemented correctly,” he said.

“Success hinges on the tactic being applied when the weeds are small. We have found that the first knock needs to achieve at least 70 per cent control so that the second knock does not have to work too hard and this is best achieved when the weeds are small.

“If the first knock is applied to large weeds the rate of control is likely to be low and then it is hard to achieve adequate coverage of the large survivors with the contact herbicide, making the second knock ineffective,” said Dr Widderick.

If some large weeds are present, Dr Widderick suggests looking for a non-chemical option for the second knock. “Chipping or strategic cultivation is likely to be more effective in removing large survivors than applying contact herbicide,” he said.

Dr Widderick said that herbicide resistance is commonly seen following regular use of glyphosate over a period of 15 years or more.

“If a grower has relied heavily on glyphosate in the past I would recommend they consider replacing one or two of their fallow sprays with a double-knock application,” he said.

“For some weed species the first emergence in the fallow is the biggest and so this would be the best generation to target with the double-knock.”

If glyphosate resistance has been identified then using glyphosate on its own is no longer an option in the affected paddock.

Double-knock using a different mode of action for the first knock can be highly effective and some minor use permits are in place for alternative herbicides, provided they are applied as part of a double-knock treatment.

“The use of residual herbicides as a mix partner for either the first or second knock is currently being investigated,” said Dr Widderick.

“If applied at the end of the fallow, a residual does not usually reduce the knockdown efficacy of the tactic and can provide useful residual control during the fallow and even into the following crop. However, careful planning would be required to ensure the residual did not affect future planting options.”

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