Altering the timing of clethodim application and using angled nozzles can improve annual ryegrass control in canola

By Heather Cosgriff and Georgina Moloney, SFS

Key messages:

  • Annual Ryegrass (AGR) resistance was not an issue in the trial as it was a bare paddock sown to AGR. Poorly timed application of clethodim reduced the chemical’s efficacy.  Improving the efficacy of spraying is key to increasing strike rate. Weather conditions, machinery and water quality also play an important role, not just nozzle selection and rates.
  • Clethodim can be sprayed from two leaves to tillered growth stages but applying at early growth stages will enhance control of annual ryegrass (ARG) populations significantly. Controlling ryegrass early will also reduce seedbank and enhance crop yields.
  • When ARG was left to spray at late tillering, shading of weeds by crop and cooler weather reduced herbicide coverage.
  • At early growth stages of annual ryegrass, no significant difference was observed when using flat vs angled nozzles. Using angled nozzles provided greater control of ryegrass populations at later growth stages by increasing coverage, which shortened time taken to kill ryegrass.
  • Spraying clethodim 1-3 days before or after frost occurrence will reduce the product’s efficacy.

Group A resistant annual ryegrass (ARG) is an increasing issue in stubble retained and cultivated farming systems.  Clethodim (Select®) is popular choice of Group A herbicide to tackle ARG in canola crops and it is important that it be applied correctly to avoid resistance.

It is well known that clethodim does not work well in cold conditions, but is the product less efficient at different weed growth stages?

SFS, as part of the GRDC Stubble Initiative (Project No. BWD00024), set up a trial to explore the timing of clethodim application along with the use of both medium and medium angled nozzles to see which combination provided the best control of ARG.


The control of ARG in canola using clethodim was investigated. Prior to sowing, a heavy amount of ARG was spread over the trial site to ensure a high and even ‘weed’ population.

On May 13, 2015, Thunder TT canola was sown at a rate of 60kg/ha at 10-15mm depth and 150mm row spacing.

Individual canola plots were sprayed with clethodim (500mL/ha rate) at different growth stages – two leaf, three leaf, early tiller or late tiller – using both standard (Airmix 015 at 1.5 Bar, medium droplet size) and angled nozzles (Syngenta angled 015 nozzles).

Clethodim was applied at 500mL/ha + 1% Hasten and 100L/ha of water.

The crop was inspected on September 8 and 30 to assess the control of ARG.

The canola crop was managed to best practices. The trial was not taken through to harvest.


Figure 1. Annual ryegrass counts per m2 in Thunder TT canola sprayed with clethodim at different growth stages. Counts assessed on September 8 and 30, 2015. Late Tiller LSD (P=0.05) = 1.2 plants/m2.

Figure 1. Annual ryegrass counts per m2 in Thunder TT canola sprayed with clethodim at different growth stages. Counts assessed on September 8 and 30, 2015. Late Tiller LSD (P=0.05) = 1.2 plants/m2.

Table 1. Ryegrass counts per m2 in canola plots sprayed with clethodim using medium and medium angled nozzles at different growth stages.
Medium Nozzles

ARG growth stage sprayed

Date sprayed Weed count: 8/09/15

Weed count:

2 leaf

27/07/15 3.44e


3 leaf

7/08/15* 28.88b


Early tiller

1/09/15 22.00c


Late tiller

18/09/15 39.19a


Medium Angled Nozzles

2 leaf

27/07/15 3.44e


3 leaf

7/08/15* 12.38d


Early tiller

1/09/15 32.31b


Late tiller

18/09/15 30.25b


Means followed by the same letter do not significantly differ LSD (p=0.05). *The minimum temperature recorded on the spray day was -4.50C.
Table two. Cressy Weather station weather records of weather recorded on days of spraying Select (clethodim). *not recorded. **minimum temperatures below 00C occurred either side of spray day.
Spray date

Min. temp 0C

Max. temp 0C Recorded temp 0C @ time  spraying Wind speed/ direction Weather

Field conditions



11.7 8.5 5km NNW fine




13.2 * 17 km SSW  *




14.8 9.7 9 km E fine

72% humidity



15.6 15.5 11 km N fine

50% humidity


Group A resistant ARG is an increasing issue in farming systems and applying herbicide correctly is key to avoiding resistance.

In Victoria, SFS asked agronomists to collect weed samples for herbicide resistance testing. Surprisingly, Select® (clethodim) came back with huge variance in resistance levels (3-20%) suggesting other factors could be lowering the chemical’s efficacy.

Not just correct rate and nozzle selection, but time of application, growth stages and weather conditions need to be taken into account when spraying. In this trial, the nozzle selection and time of application of clethodim on ryegrass in canola was explored. ARG was sown to a clean paddock to rule out resistance as cause of poor weed control.

Clethodim is a selective post emergence herbicide used to control annual and perennial grasses in broad leaf crops. It is a Group A herbicide that works by inhibiting acetyl coA carboxylase; an enzyme found in plant chloroplasts. Symptoms of ryegrass deterioration can be observed 7-14 days after the application of clethodim.


Applying Clethodim to early ARG growth stages will provide the greatest control (Figure 1). The herbicide attacks the younger leaves of the plant first and applying early enables the product to kill the ryegrass quicker due to a greater number of young leaves. In this trial, symptoms of the plant deteriorating were observed earlier in ryegrass plants with greater number of younger leaves. In these plots, ARG still present at time of assessment were observed to be purple in colour and dying off.

At the time of the first assessment, the late tiller treatment had not been applied as the growth stage had not been reached, hence the very large weed presence. Ryegrass plants had 10+ tillers at late tiller timing, and were thriving, and this treatment was the only one to have ryegrass survival. This might due to short time frame where the last assessment occurred just two weeks after spraying out (Table 1), leaving insufficient time for ryegrass to die out.

None the less, spraying conditions and growth stage played a large role in spray efficacy. 

Nozzle selection

The use of angled nozzles versus flat-fan nozzles was also observed in the trial.

The design of angled nozzles makes them ideal for spraying tall crops. With less drift and more uniform distribution, their use allows the herbicide to reach weeds at ground level.

Table 1 describes the number of ryegrass plants counted in canola plots sprayed with clethodim using medium and medium angled nozzles at different growth stages.

At two leaf stage there was no significant difference between flat and angled nozzles, this was due to the canola crop being also quite early in growth, only at emergence. Both flat and angled nozzles were able to provide sufficient herbicide coverage on weeds.

However, as ryegrass grew towards tillering, from three-leaf stage on wards, the canola crop was also growing in height and using an angled nozzle proved to significantly reduce ryegrass populations compared to flat-fan nozzles.

Figure 2: Ryegrass sprayed three-Leaf standard nozzle.

Figure 2: Ryegrass sprayed three-Leaf standard nozzle.

4.1.1 Ryegrass sprayed 3 leaf angled nozzle

Figure 3: Ryegrass sprayed three-leaf angled nozzle.

Using an angled medium nozzle was advantageous when canola plant shaded the finer vertical leaves of the ARG. The angled nozzles allowed herbicide to reach ground level, provided greater coverage and concentration of clethodim on ryegrass and shortened the time taken to kill ryegrass.

The significant difference between using angled nozzles and flat nozzles at late tiller demonstrated that it took longer to kill weeds sprayed with flat nozzles due to poor coverage (Table 1).

The late tiller application was the only treatment to have ARG plants survive to be counted at the second weed assessment date.

Temperature, wind and humidity

Weather conditions at time of spraying were observed (Table 2). Cold weather has been reported to have an adverse effect on efficacy. Spraying crops with clethodim within days of a frost occurrence hinders the control of ryegrass due to an inversion layer forming.

Clethodim was sprayed when the minimum temperature for the day at the Cressy weather station was -4.5°C (Table 1). Despite perfect spraying conditions at the time, high weed counts were still recorded on September 8 when plants were at three-leaf stage, despite being sprayed four weeks prior; ample time for the ryegrass to die off.

As ARG resistance can be ruled out due to sowing ARG into a clean paddock, the cold weather could be another reason poor control of ARG was observed. It may have created an inversion layer, preventing the chemical from acting as effectively.

At low temperatures, plants do not take up as much water and grow slowly. Temperatures for the August 7 spraying were quite low, resulting in less herbicide being taken up by the plant and spreading around it quickly.

Wind speed and humidity are other key factors that can disrupt spray efficacy. Wind speed at time of spraying was between the recommended 3-20km/hr. High wind speeds can cause spray drift. High humidity on August 7 (Table 2) will reduce the evaporation of droplets between the nozzle and ground. It can also increase droplet size, reducing spray drift.

In the trial, weather and nozzle selection has been discussed as a huge factor in spray efficacy. Machinery is another. Check nozzle technical guidelines for tractor speed, bar pressure and conversions for different nozzle spacing. Boom height should be 50cm above target plant, in this case ryegrass. Water quality can also disrupt efficacy, water should be checked for hardness, salinity and muddiness. Ammonium sulphate can be used to improve water quality and herbicide efficacy. Using an adjuvant can also improve target.

Canola is particularly susceptible to weed competition in early growth stages. Growers should be aiming to spray ryegrass at two leaf stage to avoid losses. Spraying early would also reduce the addition of ryegrass seeds to the soil seed bank and reduce weed issues for the following crop.

If growers are unable to spray ryegrass early or populations were persistent, using an angled nozzle will provide greater coverage than flat nozzles.

Avoid spraying clethodim when weather is below 0°C days leading up to and after spraying. Increasing the water rate and concentration of clethodim at later growth stages would be favoured to ensure eradication.

Resistance is not always to blame for presence of ryegrass. Always try to increase the spray efficacy using the tips above.


GRDCLogoStacked_TM_CMYKThis project was funded by the GRDC stubble project – maintaining profitable farming systems in Victoria and Tasmania with retained stubble (project number BWD00024).

Thanks to Wayne Richardson – Syngenta Australia for supply of angled nozzles.

Thanks to Pivot Incitec for supply of fertiliser used on-site.


Canola Council of Canada (2007) 12 tips for better spraying results [Online]. At: (verified 2007).

Grains Research Development Corporation (2009) Canola best practice management guide for South Eastern Australia [Online]. Avaliable at: (verified 2009).

Grains Research and Development Corporation (2008) Managing Spray Drift [Online]. Available at: (Verified 20th December 2008).

Grains Research Development Corporation (2015) Managing variation in spraying speed [Online]. At: (Verified 1st October 2015).

About BCG

Birchip Cropping Group Inc. (BCG) is a not-for-profit agricultural research and extension organisation led by farmers in the Victorian Wimmera and Mallee.
This entry was posted in Herbicide application in retained stubble systems, Weed management and tagged , , , , , , , , . Bookmark the permalink.

2 Responses to Altering the timing of clethodim application and using angled nozzles can improve annual ryegrass control in canola

  1. Pingback: 2015 field trials | The stubble project: Victoria and Tasmania

  2. Pingback: Break crops in retained stubble systems in south west Victoria | The stubble project: Victoria and Tasmania

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