Published on 31/03/2022
By Dr Kerry Harrington, Associate Professor in Weed Science at Massey University and leader of the NZ Herbicide Resistance Task Group.
The most common problem is that you get a build-up of tolerant weeds. An example would be if you always apply glyphosate and nothing else.
There are weed species that have never been well-controlled by glyphosate, such as white clover, mallows and tall willow herb. If you ever see lots of these three weed species growing together, it is often a sign of overuse of glyphosate.
By applying amitrole or glufosinate occasionally, you can stop this build-up.
Another problem you can get is a build-up of resistant individuals within a weed species that normally can be controlled. Mutations occasionally occur that can make a plant become resistant to a particular herbicide.
These mutants are seldom a problem. But if the mutation means the plant no longer dies when sprayed with glyphosate and if the only herbicide sprayed is glyphosate, these surviving mutant plants can produce seeds. The resulting plants may also be resistant and you can end up with a population of weeds that are resistant to glyphosate.
This has happened in many vineyards in Blenheim. Growers applied only glyphosate for a number of years and populations of ryegrass developed that are resistant to this herbicide. Normally glyphosate should kill ryegrass. These growers also have problems with tall willowherb and mallows.
But it is not just in vineyards where problems can occur. There are increasing problems with herbicide resistance developing in New Zealand, especially in arable cropping.
As many farmers and growers know, to overcome issues both with the build-up of tolerant weeds and the development of resistant weed populations, it is best to rotate herbicides. Use herbicides with completely different modes of action alternately wherever you spray.
Another possibility is to use mixtures of herbicides. When the mix includes two mechanisms of action, it will cut down the risk of resistance problems.
This is the case in situations like preparing to sow a crop. Glyphosate will always be the best herbicide to use, but it can be mixed with another herbicide, such as tribenuron, saflufenacil or carfentrazone, to help stop the problem of weeds not killed by glyphosate.
New mode of action classification
But how do you know which herbicides have different modes of action? Up till now, herbicides from different groups had different letter codes, which are shown on the herbicide label.
For example, clethodim is a Group A herbicide, whereas glyphosate is a Group G herbicide and atrazine is a Group C1 herbicide.
But the system of mode of action codes used in New Zealand was different to that used in Australia, and theirs was different to the one used in USA or Britain.
There were about four or five different systems used around the world for signifying what the mode of action of a herbicide is. For people looking on the Internet, it depends which country a website was prepared as to what mode of action code was shown.
In 2021, it was decided that all countries around the world should use the same mode of action classification for herbicides. This will eventually result in less confusion as any mode of action information seen on the internet will be the same no matter the country from which the information originated.
Over the next few years, this will probably cause confusion within New Zealand. Whereas the current system is based mainly on letters, the new system is based on numbers.
Using the earlier examples, clethodim will become a Group 1 herbicide instead of Group A, glyphosate will be Group 9 and atrazine will be Group 5.
It was decided to use numbers instead of letters as there are only 26 letters and we were rapidly running out of letters for new groups of herbicides.
But labels on herbicides can’t all be changed overnight. The mode of action code for each herbicide product will be changed over the next three years as old stock is used up and new labels are printed.
The full list of herbicide active ingredients available in New Zealand with their old and new mode of action codes can be found on the NZ pesticide resistance website. Go to resistance.nzpps.org then click on ‘Herbicides’ then on ‘Modes of action’.
Novachem editors note: You can also access the NZ pesticide resistance website via the Novachem website home page by clicking on ‘Resources & links.’ Alternatively, the old and new mode of action groupings are shown in the current printed copy of the New Zealand Novachem manual.
Some in the industry may feel that this change in mode of action classifications is not helpful as growers may become even more confused over how best to rotate herbicides than they already are.
However, the change was only made after consultation throughout the industry within New Zealand. It was generally felt there were more advantages to a harmonized system throughout the world rather than each country having its own system.
The old system was developed specifically for New Zealand problems many years ago, such as splitting Group O into sub-groups as nodding thistle was resistant to Group O1 herbicides but were controlled by Group O3 herbicides. Likewise, fathen in maize because resistant to Group C1 herbicides but not Group C2 or C3.
Although the new system will now merge these groups again, it has been recognised throughout the world that the mode of action system will always need extra advice for specific weeds.
This is a better approach than having localised modifications to the system for every weed resistance issue. The number of resistant weeds are much greater now than back when we only had a few such as nodding thistle and fathen to worry about.
If farmers and contractors rotate between the mode of action groups to control weeds overtime, it will stop many problems from occurring. Many people will require advice from agronomists or chemical companies to make sure the rotations they develop will work best for their system.