Most weeds demand a strategy specific to them

Controlling weeds

With weed resistance to glyphosate becoming more common, strategies to improve fallow weed control become more critical. A better knowledge of individual weed behaviour and strategies to drive down seed banks is critical.

With weed resistance to glyphosate becoming more common, strategies to improve fallow weed control become more critical. A better knowledge of individual weed behaviour and strategies to drive down seed banks is critical.


Weed control demands particular approaches for species targeted. Some protocols exist, but none apply in all situations.


Richard Daniel, Northern Grower Alliance (NGA), an independent research group with GRDC funding, highlights that eliminating summer growing fallow weeds is no easy task.

Each weed has its particular features that make it near impossible for a given control strategy to work effectively on all weed types at a given time. 

For example NGA research shows where a “double knock”, using glyphosate followed a few days later with paraquat, is generally a good strategy against barnyard grass and liverseed grass, it is rarely effective against feathertop Rhodes grass.

Mr Daniel stresses that glyphosate resistance in these and other weeds is increasingly common and levels will increase unless other control strategies are used.

It is also important even when resistance is not present that best control occurs when weeds are young and fresh.

Sometimes there is a very small window to get the best from glyphosate, or other herbicides.

NGA research shows a big role for residual herbicides, both used in winter crops as well as fallows. Some promising results are occurring from products yet to be registered for fallow use.

As more data is available the potential for new registrations improves.

Several residual herbicides used in winter crops also show useful residual against important fallow weeds.

NGA research supports that periodic cultivation, as one of the many weed control tools, is likely to maintain a useful role.

Richard Daniel believes that the suitability of cultivation varies for weed species and the stage of its life cycle when targeted.

For example liverseed grass tends to have one main germination per summer fallow (therefore one strategic cultivation can effectively control a large portion of the seedbank).

However weeds like barnyard have multiple emergences per season with a single cultivation less effective.

Feathertop Rhodes grass has some unique features. It has a strong preference for emergence from shallow soil depth.

Queensland Department of Agriculture and Fisheries research has shown negligible emergence when the seed is deeper than two centimetres.

A single shallow cultivation can be very effective, particularly when a large seedbank exists on the soil surface.

However, repeat cultivations are unlikely to be as successful as they may simply return seed to the surface and negate much of the benefit.

Seed longevity is also short, so implementing aggressive management for 12-18 months can rapidly reduce the weed seed level.

Double knocks are useful for a range of fallow weeds, but are not simple or low cost. Generally two different herbicides are used in double knocks and herbicide choice varies by target weed. Application, product and logistics costs are increased, and the environmental conditions between the two applications can be critical. 

Another key component of managing fallow weeds is to focus on starting the fallow with as low a weed burden as possible.

Also high levels of crop competition will assist, where possible use narrower row configurations, use more competitive varieties if available and sowing when conditions suit fast crop emergence all help. 

Richard Daniel stressed it’s critical to adopt a range of alternative suitable weed management strategies or we will lose glyphosate because of herbicide resistance in many of our key fallow weeds. Overall Mr Daniel stresses that weed control is complex and requires careful paddock by paddock strategies often involving a number of control methods.

Crop rotations allowing a range of herbicides, winter and summer crops, all contribute. More details of NGA weed control research is available from its website   

Next week. Pasture species to boost production in areas subject to periodic waterlogging.

  • Bob Freebairn is an agricultural consultant based at Coonabarabran. Email or contact (0428) 752 149.

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