Effective herbicide use is a matter of timing

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Portrait of Dr Chris Preston

Dr Chris Preston addressing growers in WA in March at the Grains Research Update at Yuna.

PHOTO: Brad Collis

University of Adelaide scientists mapping the extent of herbicide resistance in annual ryegrass across southern Australia’s high-rainfall zone (HRZ) have concluded that pre-emergent herbicide use is essential.

Their finding comes from research sites across South Australia, Victoria, southern New South Wales and Tasmania. The GRDC-supported research, involving varying weather patterns, topography and soil types, indicates options such as Sakura® plus Avadex® Xtra or Sakura® followed by Boxer Gold® post-emergent can provide effective control of annual ryegrass in the HRZ.

However, pre-emergent management and time of sowing are important factors in this.

“For wheat, time of sowing is very important. You’re better off sowing early with a robust pre-emergent herbicide package than wasting time trying to get rid of all weeds prior to sowing and miss the ideal sowing window,” lead scientist Dr Chris Preston says.

Water solubility

Dr Preston says water solubility is a crucial factor in how effective the herbicide is alongside the cereal seed: “If you’ve received rain but it’s dry in the surface five millimetres, that’s when you are going to get failures, because herbicide products are not going to work,” he says.

He recommends the Pre-emergent Herbicide Use fact sheet – updated in December 2015 – as a guide.

He notes that most growers rely on rainfall, so it is important to know that Sakura® and Rustler®, for example, are low-soluble herbicides and after rain are likely to cause crop damage: “Timing is a critical part of weeds management – time of sowing, time of rain and time of herbicide application.”

The university’s research included random farm surveys across south-eastern Australia to map ryegrass herbicide resistance in the HRZ, and the findings from these surveys will be incorporated into a new national resistance-mapping tool developed by Bayer (see Weed resistance maps help navigate herbicide options).

There were some sites where a negligible or zero reading of herbicide resistance was recorded (Table 1).

 

 Herbicide used
Herbicide group (and subgroup)
 Populations with resistance (%)
Table 1 Extent of resistance to herbicides in annual ryegrass samples collected from cropping paddocks in Gippsland in 2014.
 TiriflurX® D 0
 Rustler® D 0
 Avadex Xtra®
J 0
 Boxer Gold®
J + K 0
 Sakura® K 0
 Oust® B (SU)
50
 Intervix® B (Iml)
5
 Hoegrass® A (Fop)
60
 Axial® A (Den)
20
 Select® A (Dim)
0
 Roundup UltraMax®
M 5

“At these sites, it means you can still use that herbicide. For example, trifluralin can be used in northern Victoria, Tasmania or the NSW southern plains,” Dr Preston says.

Other options include managing seed-set and being aware of the resistance status of paddocks to effectively rotate herbicides.

“We should be rotating herbicides in a sensible fashion – know what your resistance status is. In the cereal phase, use robust strategies that help reduce weed numbers in following years.”

In some places, two years of treatment will be required to get ryegrass under control.

“As a rule of thumb, it is necessary to reduce ryegrass populations by 97 per cent or more to keep the seedbank the same. Obtaining high control of seed-set is essential for breaking the seedbank of ryegrass.”

Trials in western Victoria’s HRZ using pre-emergent, post-emergent and seed-set management aim to determine replacement practices for clethodim.

In a three-year trial at Lake Bolac, Victoria, in collaboration with Southern Farming Systems (SFS), a variety of pre-sowing cultural options were tested along with three intensities of in-crop management.

Only intense in-crop treatments over three years kept the seed increase to the smallest value. These were Sakura® plus Avadex® Xtra in wheat, Boxer Gold® in barley and Rustler® plus Roundup Ready® (RR) and triazines followed by crop-topping in RR triazine-tolerant canola.

However, there were high weed populations because of a lack of seed-set control tactics in the trial.

Dr Preston presented the findings at a grower field day at SFS’s Bairnsdale, Victoria, trial site in October 2015.

 

Table 2 Pre-emergent results. Season-long control of annual ryegrass by pre-emergent herbicides in six trials conducted in high-rainfall zones across NSW, SA and Victoria.
 Herbicide treatment
 Reduction in annual ryegrass spikes (%)
 Mean Minimum
Maximum
Nil 0
0
0
TriflurX® 44 14
96
Boxer Gold® 78
43 92
Sakura® 90 63 96
TriflurX® + Avadex Xtra® 75 21 96
Boxer Gold® + Avadex Xtra® 82 60 97
Sakura® + Avadex Xtra® 92 83 96
TriflurX® pre + Boxer Gold® post 84 74 94
Sakura® pre + Boxer Gold® post 95
90
97

More information:

Dr Chris Preston,
08 8313 7237,
christopher.preston@adelaide.edu.au

GRDC Update papers

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Weed resistance maps help navigate herbicide options

GRDC Project Code UA00113, UA00121, UCS00020

Region South