Weed control during 2016 has been difficult, with many paddocks very likely to have some weed escapes. Of particular concern are grass weeds in break crops, where post emergent herbicide applications have not been applied or were not effective.

Poor control of grass weeds such as ryegrass, wild oats and brome grass can have long term implications for cropping rotations, resulting in an increase in herbicide resistance levels.

Break crops with grass weeds present, should be the main focus for late season or post-harvest weed seed management.

Despite the best efforts, it is inevitable that some grass weeds will escape this season. Therefore it would be wise to conduct a herbicide resistance test on each population of weeds, to assist with future management strategies.

There are a range of options available to control weeds, prevent seed set or destroy seeds. Following is a summary of the options available, as well as some commentary on their use.

Seed Set Control Options

Pre Harvest Spraying (Crop Topping)

Glyphosate over canola is the most common and effective
A good option for managing ryegrass
Paddocks need to be prioritised, as it can be difficult to get the timing right over large areas
May need to be applied by air in wet paddocks
Can bring canola crop quickly to maturity prior to direct heading
Likely to condense harvest period making harvest logistics difficult
Cannot be used where seed is going to be retained
Could allow the first summer spray to be delayed

Windrowing

Generally needs to be combined with another strategy
Potentially useful in barley to control ryegrass or to confine wild oats seed to a windrow for narrow windrow burning
It is risky in all crops except canola, in wet years
Not 100% effective in stopping seed set of grass weeds
Reduces harvest efficiency in cereals

Cutter Bar Spraying

Registered in canola only
Windrower needs to be equipped with boom and tank
Spray drift and dust sometimes cause radiator blockages
Most effective on ryegrass
Less effective on wild oats
Prevents regrowth of weeds after windrowing
Could allow the first summer spray to be delayed
Reduces windrowing efficiency

Narrow Windrow Burning

Works well in canola
A non-chemical means of weed control
Paddocks need to be prioritised, as it is very difficult to burn large areas in a timely manner
Will not work well in water logged paddocks, where the crop is uneven and difficult to windrow
Doesn’t work well in cereals - Can be difficult to completely burn the windrows - Often the whole paddock burns - Low harvest height reduces harvest efficiency - Weed seeds are concentrated in the windrow creating a dirty strip - Residual chaff in the windrow reduces the efficacy of pre-emergent herbicides

Hot Burn

Very effective in cereals
A non-chemical means of weed control
Works best when chaff is spread evenly across the paddock
Grazing of the stubble should be avoided, particularly in dirty paddocks. Grazing works the seed down to the soil surface where it is protected from the fire
Burning early results in a hotter fire that destroys more weed seeds
Creates an opportunity to maximise the efficacy of pre-emergent chemicals

Hay / Silage

A non-chemical means of weed control
Cutting should be early, before the target weeds have set seed
More suited to smaller areas on mixed farms
Can be difficult in wet years
For larger areas, some contractors will buy the standing crop. This could be less profitable but eliminates the risk and work for the grower
A knockdown spray is required after baling to prevent regrowth

Fallowing

Appropriate for smaller areas and patches
Either chemical or mechanical
Very effective
The long term benefit often outweighs the loss of crop yield

Late Season Weed Control in Crops