The Imtrader – Clethodim Update


Key Points:

  • ALWAYS READ THE LABEL, it is a legal document.
  • The recommended rate (up to 500mL/ha) is safe to use on canola up to 8 leaf phase.
  • Split applications (4 leaf and 8 leaf timing) of the maximum rate are safe to canola.
  • DO NOT apply rates of more than 1L/ha post the 4 leaf stage on canola, severe yield loss will result.
  • DO NOT apply clethodim post 8 leaf phase to canola, severe crop yield losses WILL OCCUR.


Clethodim Use:

Clethodim is a Group A herbicide that is widely used in pulse and oilseed broadacre cropping situations for the selective control of grass weeds. Clethodim has traditionally been used to control hard-to-kill annual ryegrass, brome grass, silver grass, barley grass and a host of other monocotyledonous weeds.

In recent years, as the price of clethodim has fallen, producers are increasing the chemical rate of clethodim in their spray program. This increase has been mainly due to the emerging resistance of various monocotyledonous weeds to Group A herbicides. Recently, the APVMA approved an increase of the spray rate to 500mL/ha for clethodim when targeting many of these weeds. Many producers, however, are using rates up to and in excess of 1L/ha in some situations.

The Issue:

Many growers and agronomists have reported varying amounts of yield loss in canola crops where clethodim has been used. This has primarily been identified as flower, stem, pod distortion and/or abortion post clethodim applications. In some cases the chemistry batch, manufacturer or additional tank additives have been blamed for the damage caused.

The Findings:

In a recent study by Zerner & Wheeler (2013) via SARDI, it was found that the application timing and spray rate had a significant effect on canola. Four application timings and three application rates were assessed for their effects on yield in three popular canola varieties. The results are summarised below:

  • Following assessment, it was found that applications of clethodim at the maximum label rate (500mL/ha) were safe to the three canola varieties when applied up to the 8 leaf stage.
  • Higher than recommended rates (1L/ha) were safe to canola when applied at the 4 leaf stage, but were severely yield limiting when applied at any later timing.
  • A split application of up to 500mL at each of the timings (4 and 8 leaf) was not yield limiting to the three canola varieties.
  • Any application rate at bud initiation caused very severe yield loss.