Taking an integrated approach to northern management

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Image of people at a field day
Dr Michael Widderick at a Queensland Department of Agriculture and Fisheries Hermitage Research Station field day, Warwick, Queensland. PHOTO: Adam McKiernan

Heat, moisture stress and increasingly herbicide resistance make weed control in summer crops and fallows a real challenge for northern region growers, but integrated weed management tactics can have a big impact on summer weeds

KEY POINTS

  • For maximum efficacy double-knock should be applied to small, unstressed weeds
  • Chaff tramlining is an easy and effective harvest weed seed control tactic for managing summer weeds in the north

Common summer weeds in the northern region include feathertop Rhodes grass (FTR), awnless barnyard grass, common sowthistle and flaxleaf fleabane. Each are considered difficult to control, in part due to herbicide resistance with all four species now having confirmed cases of glyphosate resistance.

To improve weed control, manage existing herbicide-resistant weeds and prevent further resistance development growers need to combine a variety of different weed management tactics including chemical and non-chemical options.

Preventing seed-set is the primary goal for sustained weed control. Uncontrolled weeds can add thousands of seeds to the soil seedbank; FTR up to 40,000 seeds per plant, awnless barnyard grass 42,000, common sowthistle 25,000 and fleabane 110,000.

While there are many different tactics that can be used as part of an integrated strategy for these key weeds, the two that are increasingly being implemented are the highly effective strategies of double-knock and harvest weed seed control (HWSC).

Double-knock

While growers generally think of double-knock as two sequential applications of herbicides with different mode of action groups, it could also be a herbicide followed by tillage, manual removal or grazing. The second tactic aims to control any survivors of the first treatment.

Typically, a herbicide-based double-knock for summer grasses and sowthistle has been glyphosate followed by paraquat (e.g. Gramoxone®) or for fleabane an initial treatment of glyphosate + 2,4-D (or similar Group I herbicide) followed by paraquat. Increasingly, growers are using Group A grass-selective herbicides in fallow to control summer grasses and while this is often followed with a second knock, the practice substantially increases the risk that these weeds will develop resistance to Group A herbicides.

Double-knock is widely used and effective on many weed species but for maximum efficacy it should be applied to small, unstressed weeds. Any survivors still need to be controlled using another tactic.

Harvest weed seed control

HWSC was developed in Western Australia to manage herbicide resistance in in-crop weeds by collecting and destroying weed seeds during harvest. Techniques include chaff carts, narrow windrow burning, the bale-direct system, seed impact mills and chaff tramlining. Northern region growers see some of these systems as quite expensive to adopt and others, such as narrow windrow burning, as removing too much crop residue through fire. Chaff tramlining, which directs harvested chaff and weed seeds onto permanent tramlines, is considered an easy and inexpensive form of HWSC for the northern region.

The chaff tramline creates an environment that can suppress weed emergence, and even if weeds do emerge in the concentrated tramline it can be treated as a separate zone in the field, reducing the area of herbicide application to less than 10 per cent of the whole paddock. Current GRDC research led by the University of Sydney is evaluating the impact of chaff tramlining in both summer and winter phases of the crop rotation, measuring weed seed survival and emergence under chaff lines.

GRDC has released a series of short videos on some of the tactics for managing these key summer weeds (see more information).

More information

Dr Michael Widderick, Queensland Department of Agriculture and Fisheries
07 4529 1325
michael.widderick@daf.qld.gov.au

See the videos on YouTube.