Rotating Pesticides – Weed-killers, Drenches, Insecticides etc.

It’s not about rotating ‘names’ – it’s all about rotating modes of action and maybe not actually rotating at all.

And is it effective for all pesticides?

Over the next few Smith & Georg AgChemNews editions we’ll consider some current wisdom gleaned from industry and industry support experts.

In the last edition of the Smith & Georg AgChemNews we looked at sheep drenches and we also looked at Rappers; in this edition we’ll have a look at herbicide rotation and weed resistance prevention and Keith Richards and Jimi Hendrix get a mention!

Here’s an extract from CropLife’s online ‘Herbicide Resistance Management Strategies


Moderate resistance risk

Resistance to Group D herbicides is known for an increasing number of populations of annual ryegrass and dense flowered fumitory.  Resistance has generally occurred after 10 -15 years of use of Group D herbicides.   

Where possible, avoid the use of Group D herbicides on dense ryegrass populations.  Consider using alternative methods of weed control to reduce weed numbers before applying herbicides.

To assist in delaying the onset of Group D resistance, rotate and or tank mix with herbicides from other modes of action. 

Use Group D herbicides at robust rates eg the maximum label rates to ensure high levels of weed control particularly when targeting annual ryegrass.

All the above recommendations should be read in conjunction with the Integrated Weed Management (IWM) strategies

Clearly when the body representing herbicide manufacturers is concerned about resistance then chemical users should be too.

The above extract talks about ‘Group D’, what’s this all about?

Table from GRDC

Herbicides are all classified into ‘Modes of Action’ (MOA) Groups, the table below shows the modes, risk of resistance to the actives in those modes and typical number of years of application for resistance to develop:

Using the same MOA herbicide, season after season will promote resistance – how is that so?

The best way to demonstrate this is with a chart:

This shows that year by year, as resistance to a specific herbicide (MOA) grows, resistancecontinued use of that herbicide, with no other use of other actions, will increase the population of the resistant (in red) weeds.

A general guideline for the rotation of chemical groups should consider:

  • consider IPM/IWM first – with due consideration an approach that minimises the use of chemicals may deliver results that protect the future use of chemicals, by avoiding encouraging resistance.
  • avoid continued use of the same herbicide or herbicides having the same site of action (MOA) in the same field, unless it is integrated with other weed control practices (IPM).
  • limit the number of applications of a single herbicide or herbicides having the same site of action (MOA) in a single growing season
  • where possible, use mixtures or sequential treatments of herbicides having a different site of action (MOA) but which are active on the same target weeds
  • use non-selective herbicides to control early flushes of weeds (prior to crop emergence) and/or weed escapes



IWM & IPM – Integrated (Weed or Pest) Management is the terminology used to describe an approach to dealing with pest problems that encompasses much more ipm-modelthan reaching for the nearest drum of pesticide and encourages utilising all the ‘tools in the tool bag’, and to continually review the process – participation in a Smith & Georg AgVet Chemical Accreditation Course will provide you with a foundation on which to develop your own IPM plan.

It’s probably worth re-visiting Herbicide (MOA) Groups – where to find this information?

Here I’ll use the information provided by the APVMA (remember them? They are moving to Armidale.) I’m not going to try to make this a label reading course, just simply point out where, on the label, you can find the Herbicide (MOA) Group and where to go to find out more on reading labels.model-pesticide-label

Find Section 5 on the above APVMA Model Pesticide Chemical Label – this section tells you what MOA group this pesticide is in. Don’t stop there; also look at Section 12. Here will be described any resistance warnings, with appropriate instructions to be followed to assist with the prevention of resistance.

Finding out more:

Smith & Georg provide comprehensive label reading tuition in their AgVet Chemical Accreditation course.

The APVMAUNDERSTANDING PESTICIDE CHEMICAL LABELS’ booklet also has useful information.

Here’s the actual Section 5 and Section 12 information from a product:


The Australian Herbicide Resistance Initiative (AHRI) wants us to treat herbicides ‘so they are like Keith Richards and not Jimi Hendrix’. Properly (?) looked after they can ‘live hard & die old’ not ‘live hard & die young’! Find out more here!keith-richards