New to NZ – herbicide classifications change

Published on 02/08/2021

Dr Kerry Harrington, associate professor in weed science at Massey University, says up until now we’ve had a unique hybrid alpha-numeric MOA classification system for herbicides registered in NZ.
Originally this was created for good reason, he says, but 10 years on, it’s time to align our way of doing things with the rest of the world.

“When we first introduced an MOA classification system for herbicides in NZ, three different schemes were being used internationally – one in America, one in Australia, and one that covered Europe and virtually everyone else, which is known as the HRAC system, because it was devised by the Herbicide Resistance Action Committee.”

“Under all three schemes, certain MOAs were grouped together. But we had found that here in NZ, if you rotated chemistry within the group, you could control resistant weeds, so that’s why we modified the HRAC system to suit our conditions.”

A prime example of this is phenoxy resistance in nodding thistles. In NZ, plants with reduced susceptibility to MCPA and 2-4, D can still be controlled using clopyralid. But under all three existing overseas classification schemes at the time, MCPA, 2,4-D and clopyralid were part of the same MOA group, making it confusing for NZ users for whom the key message in managing resistance is to rotate between groups, not within them.

So that’s why clopyralid in NZ became classified as an O3 MOA, while MCPA and 2,4-D were classified as O1. Since then, the numeric HRAC system has become the dominant global classification for herbicide MOAs, Harrington says.

At the same time, more and more users turn to the Internet for guidance on resistance management, which raises the risk of further confusion when online overseas MOA references don’t match those on NZ labels.

Changing over will take time: “It’s going to be a bit confusing for the next three to five years.” But as farming continues to become more global, and farmers, agronomists and academics increasingly share information to stay both productive and sustainable, standardised herbicide MOAs will ultimately help the cause.

Harrington says the first step has been to add the ‘new’ HRAC classifications to the existing MOA list maintained by the NZ Plant Protection Society (www.resistance.nzpps.org).

Meantime his advice for anyone who uses or recommends herbicides is to always keep resistance in mind, because it’s not going away. “New research which is about to be published, for example, shows there is much more herbicide resistance in the arable cropping areas of Canterbury than first suspected, especially in crops like wheat and barley.”

The 2021/2022 publication of the Novachem manual includes the updated herbicide MOA chart.

Novachem New Zealand Agricultural Manual

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New to NZ – herbicide classifications change