Boosting knockdown efficacy against hard-to-kill weeds

This dominance has arisen primarily in response to no-till farming practices that have vastly reduced soil erosion and increased stored soil moisture to support crops in tough seasons.

Tony Lockrey, senior agronomist with AMPS Moree, says there is increasing pressure on the glyphosate-followed-by-paraquat tactic across the northern region.

“For most growers, glyphosate alone does not provide an effective knockdown treatment for fallow weeds, and where growers have moved to a double-paraquat use pattern, weed regrowth is becoming more common, including known paraquat resistance in tall fleabane,” he says.

If a single-pass knockdown tactic is the only practical option, Tony says that a 3-way mix of glyphosate (Group 9), picloram (Group 4) plus another Group 4 herbicide (e.g. MCPA, chlorpyralid or fluroxypyr), provides the best broadleaf weed control using systemic products. The choice of the second Group 4 will be dictated by proximity to sensitive crops, the spectrum of secondary broadleaf weeds and the plantback requirement for the intended following crop.

If a double-knock is possible, following the glyphosate mix with either paraquat or glufosinate would be the preferred option. If using paraquat, a night-time application increases control by 5 to 10 per cent and increasing the water rate with good droplet spectrum for coverage and compliance (e.g. 150–200 L/ha) has more impact than increasing the rate of paraquat above 500 g/ha of active ingredient. Using the correct adjuvants and rate is essential to achieve the best result.

For glufosinate-based sprays, Tony says growers and agronomists are still fine-tuning application techniques, but using sulphate of ammonia and oils appears beneficial, as well as spraying in daylight with high humidity (>60%).

When glyphosate-resistant grasses are also present in the paddock, the management decisions are more complex.

“For example, some of the Group 4 actives can enhance broadleaf weed control, and where permitted, Group 1 products can provide effective control of grass weeds,” says Tony. “Unfortunately, the Group 4 and Group 1 products can be antagonistic to each other, so it is often necessary to decide which segment of the weed spectrum needs to be prioritised for the spray job at hand.”

Remember that there is a regulated imperative to double-knock any Group 1 herbicide application in fallow, either with a desiccant, such as paraquat or glufosinate, or cultivation. This requirement is to stop weed seed set on any plants that survive the Group 1 herbicide treatment.

“The addition of a Group 14, such as pyraflufen-ethyl (e.g. Sledge), to paraquat can help to control grasses following a Group 1 application,” says Tony. “If using glufosinate as the second knock, it appears from our trial work that saflufenacil (e.g. Sharpen and a component in Voraxor) could have the best spike activity.”

If unable to apply a chemical double-knock, a full cultivation may be required to fulfil the ‘no seed set on survivors’ obligation on the Group 1 label.

“An occasional full cultivation is not a bad option as it also destroys disease inoculum and buries feathertop Rhodes grass, milk thistle and fleabane seeds to a depth that renders them unviable,” says Tony.

“Research on Queensland soils has shown that strategic and infrequent cultivation does not damage soil structure. For many growers, cultivation is a key strategy to managing these hard-to-kill weeds and manage herbicide resistance.”

The addition of a residual in the knockdown mix or within a double-knock, enhances efficiency of operations as it contributes to the success of the current weed control treatment and also suppresses weed emergence for a time.

A standout option in this situation, especially toward the end of the fallow, is terbuthylazine, a Group 5 residual herbicide that offers much greater rotational flexibility for growers across summer and winter crops.

“This strategy can make all the difference in seasons where frequent rain events add pressure to repeatedly cover the whole fallow area,” he says.

“Treating small, actively growing weeds will always give the best result when applying knockdown herbicides. Robotic spot spraying of small weeds has been very effective on northern region grains farms and given growers more time to attend to other critical operations.”

Having a plan to manage any survivor weeds is the best way to avoid future weed blow-outs and herbicide resistance.

WeedSmart extension agronomist, Paul McIntosh, says adding diversity to the herbicide program in the northern region can conserve soil moisture and nitrogen over summer and provide excellent early weed control that allows competitive crops to establish.

“Growers can access an array of residual herbicides that have different roles to play in a WeedSmart Big 6, integrated weed management program – in each of the fallow, pre-seeding and early post-crop emergence phases,” he says.

“Some products complement knockdown herbicides, and while others help set up a competitive crop. With reduced germinations at planting, there are fewer weeds to control in-crop, and the crop itself can do most of the work. There is always evidence of this wherever a gap occurs in a field, and the weeds flourish unabated.”

Check the label of all products before use and ensure that the plant-back requirements have been met before planting the next crop.

Other resources

Reinventing the double-knock in fallow

Can I win against ryegrass with dual-resistance to glyphosate and paraquat?

Reinventing the pre-seeding double-knock tactic

Residuals that don’t lock up your rotation in the northern region

Is one-time tillage a weed control option in a no-till farming system?